Wikipedia:Reference desk/Archives/Science/2009 August 14

= August 14 =

Gull-proof sack?
How does this work, exactly? --Kurt Shaped Box (talk) 00:05, 14 August 2009 (UTC)


 * Looks like they have an embedded weave of fibers in there that makes it more difficult for gulls to peck a hole in the bag. 71.236.26.74 (talk) 00:12, 14 August 2009 (UTC)


 * Tyvek, possibly, or something similar. --Tagishsimon (talk) 00:15, 14 August 2009 (UTC)


 * Is the market for specifically-seagull-proof woven garbage bags really that large, or is this just a special printed label on an otherwise generic animal-resistant sturdy trash bag? Nimur (talk) 01:10, 14 August 2009 (UTC)


 * Good question. I wonder if large gulls (they being the ones who generally raid bin bags in towns) have more powerful bites than, say housecats or foxes? It's actually quite possible, considering that the jaw muscles are bringing all the pressure of the bite to bear on a much smaller surface area than you'd find in the mouth of a mammal. --Kurt Shaped Box (talk) 01:30, 14 August 2009 (UTC)


 * In many British localities, the problem of seagulls attacking garbage sacks pre-collection and spreading their contents all over the street is very considerable. I recently read a long blog entry touching on this problem in Edinburgh (though it was more about the misapplied monitoring of those who compounded the problem by putting their sacks out too early): not sure if it's netiquette/Wiki-appropriate to post a link here, though. 87.81.230.195 (talk) 01:32, 14 August 2009 (UTC)


 * I agree the scale of the problem, but note that Edinburgh has suffered a garbage collection strike in recent times which will have exacerbated the problem for them. --Tagishsimon (talk) 01:48, 14 August 2009 (UTC)


 * I don't know if this is the case in Edinburgh but I've heard that in some places, once the gulls have strewn the rubbish all over the street, the bin men are all like "Meh, it's not our problem now - it's not in a bag/wheelie bin" and they'll leave it there to rot. --Kurt Shaped Box (talk) 01:53, 14 August 2009 (UTC)


 * Haha, you read Language Log? John Riemann Soong (talk) 14:09, 14 August 2009 (UTC)
 * Well spotted, John! Both read and occasionally comment on. 87.81.230.195 (talk) 16:48, 14 August 2009 (UTC)


 * A couple of weblinks for reference: East Devon council, BBC News. It's a re-usable bag, so presumably there's a normal polythene bag within; they're for users who do not have access to a wheelie bin, and Devon, at least, is charging a fiver for them. --Tagishsimon (talk) 02:01, 14 August 2009 (UTC)


 * For a sack?! Time to go into business:  Undercut them by 50% and you'll still be making a huge profit margin.  Tempshill (talk) 03:04, 14 August 2009 (UTC)


 * Am I the only person who squinted at the photo and concluded it was some kind of a sack for reusable seagulls? It would parse SO much easier if it had been written:

Reusable Seagull Proof Refuse Sack
 * Or maybe it's just time to get my eyeglasses upgraded :-)
 * --DaHorsesMouth (talk) 22:43, 14 August 2009 (UTC)
 * It just needs some punctuation: "seagull-proof" needs a hyphen and only the first word should have been capitalised. Although: "Reusable seagull?  Proof!  Refuse sack."...yeah, I'd refuse the sack alright. SteveBaker (talk) 02:30, 15 August 2009 (UTC)
 * I'd sincerely hope that you weren't intending on (re)using the seagull for evil. --Kurt Shaped Box (talk) 01:28, 17 August 2009 (UTC)
 * I'm having a hard time imagining a non-evil use for some kind of zombie re-used seagull - which (since they have proof) is the only reason I need to refuse the sack. I hope this clarifies the situation. SteveBaker (talk) 21:57, 17 August 2009 (UTC)

Sitting too close to a TV
Is it true that sitting too close to a TV is dangerous? And if so, then why is it ok to have a computer monitor so close to your face? ScienceApe (talk) 01:18, 14 August 2009 (UTC)
 * a) because it strains the muscles in your eyes


 * b) it's not okay to sit too close to the computer monitor for long periods of time, for the same reason.


 * Regards, --— Cyclonenim | Chat 01:44, 14 August 2009 (UTC)


 * No, it's not true. I've been sitting close to computer screens for close to 40 years - and aside from being a little astigmatic in one eye (requiring glasses for driving - but nothing else), I'm doing fine.  I had my eyes tested just a few weeks ago - and my eyesight hasn't changed in the slightest over the last 30 years.  Admittedly, I'm not a good statistical sample - but I've worked with an awful lot of people who also spend their entire waking hours glued to a screen - and I don't see any horrible eye injuries.  There have been many studies about this kind of thing and no clear results have ever been attributed to computer use.  Sure, you can get eyestrain - but that's not a permanent condition.  You eye muscles are like most other muscles - they get tired - you rest them - they recover.   It's a good idea to take a break from computer use for at least a few minutes in every 20 minutes (mostly so you get a change of posture and a break from typing) - and using that time to focus off into something in the distance will help to relax your eye muscles and avoid strain.  Almost all of the problems relating to computer use relate to bad sitting posture, poorly adjusted work-stations, lack of exercise and repetitive strain issues due to too much rapid typing - or excessive mouse use.  I'm pretty sure that the "Don't sit too close to the TV, you'll hurt your eyes." admonition that probably everyone's mother has delivered repeatedly has more to do with you blocking the adult's view rather than any evil deathrays emitting from the screen. SteveBaker (talk) 02:13, 14 August 2009 (UTC)


 * I think the best thing is to direct people to Myopia. SB's claim that 'many studies about this kind of thing and no clear results have ever been attributed to computer use' is not entirely accurate. As the article mentions, some studies have found evidence frequent near work (which includes computer use and sitting very close to the TV) may be a contributing factor, while others have not. In other words, it's a case of, we still can't be sure, rather the being definite either way (this doesn't exclude the possibility the evidence is far stronger in one direction). Edit: After some external discussions about this, I feel I should emphasise near work includes a lot of things besides normal computer use and sitting very close to a TV. It obviously includes stuff like reading a book. Also I've been wondering whether to mention this or it will just confuse things but I decided to go ahead. I believe one of the reasons why there is a fair amount of suspicion environmental factors may be at play is because of the greatly increasing apparent prevalence of myopia seen in some populations (especially East Asian I believe) that appear to be correlated to industrialisation and development. It's possible these are related to improved diagnosis and perhaps a greater demand for visual acuity but the trend appears to be quite strong and I believe several factors including some of the studies the article discusses suggest something else may be at play Nil Einne (talk) 04:22, 14 August 2009 (UTC)

Some screens emit radiations that can be harmful. —Preceding unsigned comment added by Quest09 (talk • contribs) 16:28, 14 August 2009 (UTC)
 * Can you point us to sources supporting your otherwise uncorroborated assertion? 87.81.230.195 (talk) 16:46, 14 August 2009 (UTC)
 * TVs do emit harmful "radiations," namely the horrible programming that rots the mind. But I think Quest09 is referring to radioactivity. I recall a study from the 1990's that did find detectable radiation from the face of CRTs, both computer monitors and TVs. The radiation was found to be from radon in the dust that accumulates on on the front of the class due to eletrostatic attraction induced by the charge buildup. The same effect probably occurs for plasma screens but not for LCDs. The correct mediation is to clean the screen, and of course to clean any other dust buildup in your house. -Arch dude (talk) 17:28, 14 August 2009 (UTC)
 * Found a ref: -Arch dude (talk) 17:32, 14 August 2009 (UTC)

More about it: (...) most surveys of cathode ray tubes in the literature were made while the units were energized and indicated low-energy x-rays, (...) See Moms were right after all, sitting too close to TV can harm you. --Quest09 (talk) 18:31, 14 August 2009 (UTC)
 * You might want to read that reference more carefully. That was not x-rays from the CTR beam. It was gamma radiation from the materials from which the tubes were manufactured. "Significalnly above background" is a term from the field of statistics, where "significant" means that the results are statistically higher than background, not "significant" as in harmful to health. Note that bricks, stones, and cinder blocks also emit radiation that is "significalntly above background," and fo the same reason: trace amounts of transuranics in the earth from which they are made. The radon/dust effect I mentioned above is a whole lot higher, at five times background, but is still not very unhealthy. -Arch dude (talk) 20:27, 14 August 2009 (UTC)


 * My reference is a reference about low energy x-ray radiation, even if its main topic is radioactivity in cathode ray tube. Quest09 (talk) 11:10, 17 August 2009 (UTC)

Mineralogy of the Black Stone
Muslim legends state that the Black Stone fell from the heavens. Has it ever been analyzed to determine whether it's a meteorite? Neon Merlin  05:58, 14 August 2009 (UTC)
 * As of 9 months ago, at least, the black stone had not actually been scientifically analyzed, although one geologist (Zaghloul al-Naggar) was wanting to take a sample to do so. This page claims that “Western Scientists have confirmed that the black stone…is an outside meteorite object”, but the audio file that that claim links to, which is an interview with Zaghloul al-Naggar, does not actually support the claim that there has been an actual analysis of the stone.  Note that although Zaghloul al-Naggar has a PhD in geology, he hasn’t published any peer-reviewed research, his web site is all about promoting islam, and he has called Jews “devils in human form”.  So it’s questionable as to how unbiased he would be able to be, even if he actually did an analysis of the black stone at some point.  Red Act (talk) 07:31, 14 August 2009 (UTC)
 * P.S. This same web page also links to an audio file in which Zaghloul al-Naggar explains how through his careful interpretation of the quran, time descriptions in the quran can be shown to be consistent with the speed of light, accurate to three decimal places. With such a strong susceptibility to confirmation bias, and given that islamic tradition requires the stone to have fallen from heaven, it seems essentially impossible to me that Zaghloul al-Naggar would conclude that the black stone was anything other than a meteorite, if he ever does get his hands on a sample, regardless of what the black stone is actually made of.  And I can’t find anything about anyone else with any scientific credentials hoping to get a sample and do an actual analysis.  Red Act (talk) 08:15, 14 August 2009 (UTC)
 * I don't think anyone would be willing to do an actual analysis -- if he/she does and it turns out to be anything other than a meteorite, the Islamics would put a fatwa on his/her head (just as they did to Theo van Gogh (film director) for making an anti-Islamic documentary). Few scientists would be brave enough to risk that. 98.234.126.251 (talk) 00:16, 18 August 2009 (UTC)
 * I find it appaling that in the 21st century we are still in an age where practically the whole planet can be intimidated by the beliefs of people who worship a rock that fell from the sky.. Vespine (talk) 00:25, 21 August 2009 (UTC)

Direction of rotation of helicoptor blades
Which way do helicopter blades rotate? (i.e. clockwise or counterclockwise, when viewed from top) Is it standard? or does it vary across models? Is there any reason to prefer one way or the other? or is it arbitrary? --76.173.203.32 (talk) 06:07, 14 August 2009 (UTC)
 * American generally go counter-clockwise, the other type is French which go clockwise, other countries pick between the two. I believe clockwise is more common outside the US. It is pretty much arbitrary, like which side of the road you drive on. Vespine (talk) 06:17, 14 August 2009 (UTC)
 * From Helicopter rotor: “When viewed from above, the main rotors of helicopter designs from Germany, United Kingdom and the United States rotate counter-clockwise, all others rotate clockwise.” Of course, some helicopters have dual main rotors, in which case they rotate in opposite directions.  Red Act (talk) 06:32, 14 August 2009 (UTC)
 * You can usually tell just by looking at the rotor blades - the blades are thicker on one side and taper down to a knife-edge on the other - just like the wing on an airplane. The fatter edge of the blade is the "leading" edge - it meets the on-coming air-flow. SteveBaker (talk) 12:39, 14 August 2009 (UTC)
 * On most helicopters it is easier to look at which side the tail rotor is, right side for clockwise and left side for counter-clockwise. MilborneOne (talk) 16:36, 14 August 2009 (UTC)
 * Unless it has fantail design, like the Eurocopter EC 135, then you are back to SB's method. Googlemeister (talk) 16:42, 14 August 2009 (UTC)
 * I was going to mention the tail rotor - but then it occurred to me that it shouldn't matter which side it's on...so it's possible that this isn't a hard-and-fast rule (although I agree it's common). The pitch of the tail rotor can be adjusted to direct its thrust in either direction - I can't see why it would need to be on a particular side of the tail. SteveBaker (talk) 02:24, 15 August 2009 (UTC)

Electric power stations
how does a electric power station pay for itself. The esolar 5MW station has me wondering. --207.229.169.245 (talk) 07:13, 14 August 2009 (UTC)
 * The owners of the power station sell the electricity produced in the Electricity market. Apparently California (where I think the station you're asking about is located) has a wholesale electricity market, where "competing generators offer their electricity output to retailers. The retailers then re-price the electricity and take it to market, in a classic example of the middle man scenario." AlmostReadytoFly (talk) 07:23, 14 August 2009 (UTC)
 * As for "how do they build it in the first place" - they get a loan, which they later repay with money from selling the electricity. Dcoetzee 07:54, 14 August 2009 (UTC)


 * Besides that, there are subsidies, depending on the type of power station and the country or state. Icek (talk) 11:42, 14 August 2009 (UTC)


 * By changing power suppliers, I recently changed from a standard residential rate of 11.34 cents per kwh to 8.50 cents per kwh. The power comes to me from the same grid and through the same pole transformer. There were no wiring changes at all. To add insult to injury, the original supplier (PennPower) still bills me, but passes the money to the new supplier (Dominion People Plus). No doubt PP gets a small fee for doing the billing. I live in NW Pennsylvania. —Preceding unsigned comment added by 98.21.108.45 (talk) 15:55, 14 August 2009 (UTC)
 * Why is consolidated billing an insult or an injury (i.e., does it really matter where you mail your check)? As you said, "The power comes to me from the same grid and through the same pole transformer. There were no wiring changes at all." That infrastructure probably has significant startup investment and ongoing maintenance costs. Even if you mailed Dominion directly for the actual poewr generation, you are still using services of PP to get the power, so you would expect Dominion to tack additional charges to be passed back to PP anyway. DMacks (talk) 19:58, 15 August 2009 (UTC)

Magnetism
Is molten iron still attracted by a magnetic field? I know that heat erases magnetic properties of a magnet, so molten iron can't be a magnet, but I don't know if it can be subject to external magnetism. --151.51.54.231 (talk) 10:16, 14 August 2009 (UTC)
 * Above the Curie temperature (the temperature at which you "erase magnetic properties"), iron no longer behaves ferromagnetically but paramagnetically. It is still attracted towards stronger magnetic fields, but the attraction is weaker - see Curie's law. There are also diamagnetic substances which are repelled by magnetic fields (but most only weakly so, except for superconductors). I don't know exactly how iron behaves in the molten state, but this article might be useful (if you have access to it). Icek (talk) 11:33, 14 August 2009 (UTC)

Positioning sound
I was having a dispute yesterday about the extent to which our perception of where sound comes from is based on what we see. It seems to me that with only 2 ears, you could only place the source of a sound on a plane - that is, you could not tell whether it is in front of, above, behind or below you. Is there any way of discerning these without rotating your head while listening, or would a blind person not be able to tell whether an instantaneous click sound was produced behind his head or in front of it? Insperatum (talk) 10:22, 14 August 2009 (UTC)
 * Sound localization is not only done by differences in the sounds from the 2 ears, but also by different dampening of frequencies at different angles by the pinna. This can in turn be used to create the illusion of 3D sound with speakers arranged in a plane. Icek (talk) 11:40, 14 August 2009 (UTC)


 * Yep - basically, we can do simple stereo localization by the obvious tricks of comparing the delay between the sound waves entering our two ears. But because our heads are not symmetrical - a sound coming from above passes through a different mass of bone and brain than one coming from below.  That causes different delays, reflections and absorption of different frequencies in the sound.  We're able to detect that change in the quality of the sound and use it to estimate that third dimension...albeit imperfectly.  There is an audio recording system from a company called Sennheiser which uses a pair of microphones embedded into the ears of a fake plastic head which is designed to transmit, delay and reflect sound just like a real human head.  If you listen to a sennheiser recording on a pair of in-ear headphones, then you can clearly hear that third dimension of sound.  There was a while in the 1970's when you could buy sennheiser recordings of all sorts of music.  Unfortunately, you have to listen to them on headphones - the effect is largely destroyed if you use loudspeakers.  There are some computer software techniques that crudely simulate the effects of all of that 'stuff' that happens in your head and applies that to sounds used in computer games.  The effect is moderately successful - but again, it only works well when you are using in-ear headphones. SteveBaker (talk) 12:27, 14 August 2009 (UTC)
 * I seem to recall that there's some debate about the mechanisms of 3D acoustic perception; it seems likely that the brain fuses in some visual cues into the mix, in addition to the stereo-ear-channel processing for delays and frequency-dependent amplitude variations. Nimur (talk) 14:44, 14 August 2009 (UTC)
 * How do ear muff-style headphones compare to in-ear style? --Sean 16:17, 14 August 2009 (UTC)
 * In this case, not well. You really need the speakers to be in roughly the same position in your ears as the microphones in the fake head.  I believe there have been some successes at making synthetic 3D audio using cup-type headphones. SteveBaker (talk) 02:21, 15 August 2009 (UTC)


 * What if you had a 7 speaker home entertainment system? John Riemann Soong (talk) 15:11, 14 August 2009 (UTC)
 * Well - if they are positioned all around you (including above and below), I presume that suitable recordings could be made for them that would reproduce 3D moderately well. SteveBaker (talk) 02:21, 15 August 2009 (UTC)


 * "Visual capture" or the "ventriloquism effect",  is the phenomenon of sight overruling auditory localization. When 16 mm movies were the common mode of audiovisual presentation, sometimes the speaker was on the projector, or was somewhere other than behind the projection screen. But during the movie, the audience was able to perceive the speech as coming from the actors' images on the screen. With only 2 ears, absent head movement such as tilting the head, we can localize sounds in the lateral dimension but not in the vertical dimension. It would take 3 ears, not in a line, to truly localize in 3 dimensions. Edison (talk) 15:17, 14 August 2009 (UTC)
 * It is certainly possible to tell, with the eyes closed, and without head movement, whether natural sounds in the environment are coming from in front or behind. With binaural sound from earphones, this does not seem to be the case. This includes "binaural" recordings made with mics inside an artificial head. I did not note front/rear localization in listening to such a recording through earphones. Perhaps as stated elsewhere the influence on sound quality of the head aids in this for normal environmental sound. As for "visual capture," I can remember watching a movie, with the speaker behind me on the 16mm projector, when the projector lamp went out. The sound source location seemed to "fly" swiftly, but not instantaneously, from the movie screen, in front, to the projector, behind. Edison (talk) 22:20, 14 August 2009 (UTC)
 * One common reason for not hearing the 3D effect is if you have your earphones swapped into the wrong ears. I found the stennheiser recordings pretty convincing. SteveBaker (talk) 02:21, 15 August 2009 (UTC)


 * Off-topic, but the McGurk effect is an interesting case of sight overruling sound. --Sean 16:17, 14 August 2009 (UTC)


 * It seems to me that the need for visual queues is easily debunked. I know two blind people rather well.  They are better at localizing a sound than I am.  They have no visual queues.  So, they are doing it strictly with two ears. --  k a i n a w &trade; 17:16, 14 August 2009 (UTC)


 * Not directly relevant to the question, but it may be interesting to know that barn owls localize sound in two dimensions by having one ear tilted up and the other tilted down -- this causes amplitude and time differences to carry indpendent information. Looie496 (talk) 17:34, 14 August 2009 (UTC)


 * It is difficult to locate the source of a single-frequency (sinewave) tone without moving the head. I have also observed that my cat snoozes contentedly while surrounded by all these household noises that it has become used to: radio, TV, traffic, door slams, kids yelling, etc. However a firework overhead upsets pussy greatly. It is clearly not the volume but the unexpected direction of the sound that causes alarm. Cuddlyable3 (talk) 18:25, 14 August 2009 (UTC)
 * That's not surprising though - the brain is relying on different transmission properties of different frequencies within a natural sound source. A simple, pure, sine wave wouldn't provide enough cues. You'd probably do better with 'white noise' or more natural sounds. SteveBaker (talk) 02:21, 15 August 2009 (UTC)

←Slightly off-topic, but holophonics may be of interest, as may this sound clip (use headphones). Fribbulus Xax (talk) 22:12, 14 August 2009 (UTC)

toxicity of crown ethers
Is it me or would a tiny amount of crown ether be potentially very toxic if ingested? I was wondering, because there aren't many mentions of safety precautions / MSDS stuff on our articles that cover them. John Riemann Soong (talk) 14:12, 14 August 2009 (UTC)
 * They are moderately toxic http://dohs.ors.od.nih.gov/pdf/Crown%20ethers%20REVISED.pdf page 5.
 * Long term effects are unknown.83.100.250.79 (talk) 14:19, 14 August 2009 (UTC)


 * John, it seems silly to even have to say this, but several of your previous questions seem to suggest that you plan to eat the results of your chemistry experiments. Please, don't do this.  There's no way that the Reference Desk, or the publishers of the MSDS, or any other source, can know what is actually in your concoction.  If you're so interested in food science, maybe you can consider interning in a lab or a company which specializes in chemistry for food applications.  That way, you can play around with some expert supervision.  Like many scientists, I'm sure you're excited about homebrew lab experiments, but whether you're working with chemicals, explosives, pressurized gases, (whatever) - the rules of lab safety don't disappear just because you're unsupervised.  Nimur (talk) 14:51, 14 August 2009 (UTC)


 * While we're on the subject of crown ethers and toxicity, has anyone tried to use them as antidotes for mercury poisoning? I know they're good chelating agents, but would they work for chelating mercury out of a person's body? 98.234.126.251 (talk) 04:45, 16 August 2009 (UTC)
 * I don't know of any published info about that. Would be more likely with a crown thioether (sulfur seems to be a better ligand for mercury than oxygen is). Would want to put some sort of hydrophilic chain attached, since crown ethers are good for solubilizing ions in non-ionic solvents (compare structures of given treatements in the mercury-poisoning article), and the goal is to make the mercury excretable rather than stay in any lipophilic environment. Also, crown-ethers are good and specific chelating agents for cationic metals, but only work well if the metal atom fits inside the crown. For example, 12-crown-4 holds Li+, 15-Crown-5 holds Na+ and K+, and 18-crown-6 holds K+. Given the amount of such ions in the body, need something with a pretty high and specific affinity for mercury. Mercury is large, so would need a pretty large ring to get decent binding I would think. Although interestingly, our 15-Crown-5 article says that one has  good selectivity for lead. DMacks (talk) 16:11, 16 August 2009 (UTC)
 * Water solubility increases with ring size, so that may also help solubility. John Riemann Soong (talk) 16:44, 16 August 2009 (UTC)
 * So, a hydrophilic-derivatized 15-crown-5-thioether could work for lead (and thus prob'ly for mercury too)? That's a good idea... where can I get the funding and the starting materials?  DMacks, are you interested in this project? 98.234.126.251 (talk) 00:26, 17 August 2009 (UTC)
 * I just looked up the MSDS for 15-crown-5: it's got a health hazard rating of 2 and a REALLY big LD-50, so toxicity problems will likely be relatively minor (after all, the antidotes in use now, like British Anti-Lewisite are also moderately -- or in some cases not-so-moderately -- toxic). A bigger issue at this time is, will it work like we want it to? 98.234.126.251 (talk) 02:53, 17 August 2009 (UTC)
 * Guess we'd better start with lit-search. Turns out synthesis and use of crown-thioethers for chelation of mercury ions with apparently good selectivity is known. among other refs don't even require advanced academic search engines to find. I didn't narrow down to physiological uses. DMacks (talk) 06:34, 17 August 2009 (UTC)
 * Great -- someone else has beat us to our idea AGAIN!!! It's hard to be an inventor these days, ain't it -- you come up with some great idea, and then you find out someone else has already put it on the market.  Kinda sucks, don't it?  Well, let's you and me keep coming up with more ideas till we come up with something nobody thought of yet. 98.234.126.251 (talk) 23:08, 17 August 2009 (UTC)
 * Thioethers have sharper bond angles, don't they? Won't that change the geometry a bit? John Riemann Soong (talk) 04:59, 17 August 2009 (UTC)

blood in the water
Some species of sharks are known to be able to detect blood in water at great distances. Are there any bony fishes that have this ability? Googlemeister (talk) 16:39, 14 August 2009 (UTC)
 * Sorry to nitpick and not answer, but it would be more precise to say sharks detect blood in water at very low concentrations. 152.16.15.144 (talk) 17:39, 14 August 2009 (UTC)
 * I don't quibble with the OP's words "detect blood at great distances" but if anyone can give a figure for a typical distance, can they say how long it takes for the blood to travel that distance? Cuddlyable3 (talk) 17:57, 14 August 2009 (UTC)
 * The diffusion rate of the blood would depend on the temperature of the water and local currents. My hunch is that the diffusion rate is going to not be uniform in all directions and vary significantly. The shark article gives the unsourced figure for sensitivity "as little as one part per million of blood in seawater". You can make a circular cow type approximation as to how far away the shark can sense blood while ignoring the time factor by assuming X amount of blood was spilt and assuming it is evenly mixed at the concentration of one part per million in a sphere of water. The radius of the sphere represents the distance the shark will sense blood. 1 mL of blood (several drops) mixed in 999,999 mLs of water gives 1ppm blood. 1 mL of anything is 1 cm3. 1,000,000 cm3 corresponds to a sphere with a radius of 62 cm. According to blood, "the average adult has a blood volume of roughly 5 liters" or 5000 cm3. Adding in the blood you get a volume of liquid of 5,000,000,000 cm3. That corresponds to a sphere with a radius of ~ 1060 cm or 10.6 meters. 152.16.15.144 (talk) 01:22, 15 August 2009 (UTC)
 * some predatory bony fish without the lateral line (the organ which sharks detect the struggles of fish and blood) they can detect it but in higher concentrations. around 10,000 p.p.m —Preceding unsigned comment added by The mophead (talk • contribs) 18:29, 14 August 2009 (UTC)
 * The lateral line detects motion (with little hairs, like your inner ears), and sometimes electrical impulses, but how could it detect blood in the water? --Sean 20:59, 14 August 2009 (UTC)
 * I'm fairly sure it can't. They taste/smell (the difference is minimal) the blood. --Tango (talk) 21:45, 14 August 2009 (UTC)
 * Catfish probably can although I'm not sure they would be looking for blood specifically. According to this book, catfish are very sensitive to amino acids. A channel catfish can sense alanine at 10-9 to 10-11 M. If I did my math right that corresponds to a minimum sensitivity of roughly 1 to 100 parts per trillion. 152.16.15.144 (talk) 01:45, 15 August 2009 (UTC)
 * Catfish thrive as bottom feeders in muddy rivers, so scent is a main means of finding prey. No visibility to see the food. Edison (talk) 19:15, 15 August 2009 (UTC)

acid question
If you place a gold alloy in acid that is not aqua regia (90% AU, 10% Cu) will the acid dissolve only the copper leaving you with a somewhat porous remainder? Googlemeister (talk) 16:49, 14 August 2009 (UTC)
 * The principle is known (see Raney Nickel), but only 10% copper will probably not provide a connected network of voids for this to happen, (try 20%) (also copper doesn't dissolve in many acids...)83.100.250.79 (talk) 17:56, 14 August 2009 (UTC)
 * Copper will dissolve fine in pure nitric acid while gold will not. That may work.  Also, if you grind up the alloy into a fine powder, you may be able to get enough surface area to get more of the copper out.  -- Jayron  32  04:57, 15 August 2009 (UTC)

chameleons: will they eat off your hand?
i have a pet yeman chameleon and im not sure if they will eat of my hand. or will it count my hand as competition for food? —Preceding unsigned comment added by The mophead (talk • contribs) 18:25, 14 August 2009 (UTC)
 * This thread says that they can and will bite, depending on the temperament of the animal. --Sean 20:34, 14 August 2009 (UTC)


 * Are you asking whether they will eat food out of your palm, or if they will devour your hand itself? The grammar of "eat off" is ambiguous. --98.217.14.211 (talk) 21:59, 14 August 2009 (UTC)


 * I have a bird that tries to eat off my fingers every time I put my hand in the cage, but it rarely breaks the skin. Edison (talk) 22:22, 14 August 2009 (UTC)


 * Is it a parrot of some denomination? They tend to do that. Unless the bird is obviously scared of your hand or annoyed by your presence, it's likely a substitute for the playful(ish) beak sparring that they engage in with others members of the same flock. --Kurt Shaped Box (talk) 00:01, 15 August 2009 (UTC)


 * I also enjoy the ambiguity of the hand competing with the chameleon and with the food. :) --Sean 23:45, 14 August 2009 (UTC)
 * May I remind respondants that per Reference_desk/guidelines we are specifically told: don't poke fun at a poorly-written question. SteveBaker (talk) 02:07, 15 August 2009 (UTC)
 * I'm honestly just confused about what they are asking. I'm not sure I understand what the bit about competition for food is meant to imply. --98.217.14.211 (talk) 04:31, 15 August 2009 (UTC)


 * There is a difference between "They eat off your hand" and "They eat your hand off"... Dauto (talk) 21:45, 15 August 2009 (UTC)


 * I had a pet chameleon when I was a kid, living in Kenya (I have no idea what kind of chameleon). It was a very small specimen - maybe just about an inch long with it's tail curled up.  It would stand on the tip of my finger while I carried it around the room looking for flies and other insects for it to feed on.  Because it had a 2" tongue (pretty remarkable for a 1" animal!) - you'd just have to get it within range of it's victim and it would do the rest! However, it only seemed interested in live insects - so "hand-feeding" in the conventional sense was impossible.  It never bit me...but it was pretty tiny. SteveBaker (talk) 02:14, 15 August 2009 (UTC)

Calculating my daily calorie intake needed to lose weight
If I know my current weight and my height, what's the best way of estimating how many calories I should eat each day to stay at that weight? (The idea being that I then reduce that amount so that I gradually lose weight). Thanks 78.146.206.220 (talk) 18:46, 14 August 2009 (UTC)
 * There are numerous estimation calculators online, but you probably should not put a great deal of faith in them as metabolism (baseline calorie usage) can vary a great deal among different people who weigh the same. The US government usually assumes people eat 2,000 kcal a day when they provide nutrition values.  Googlemeister (talk) 18:49, 14 August 2009 (UTC)


 * See Schofield equation and Harris-Benedict equation. For more in-depth information, see this.  Red Act (talk) 19:14, 14 August 2009 (UTC)


 * There are calculators online that take in a lot of info about your height, weight, and activities, that try to give you an indication of what your baseline is. Google "Base metabolic rate". --98.217.14.211 (talk) 19:31, 14 August 2009 (UTC)


 * It's all a matter of having calories consumed greater than calories coming in. There are some estimations online of how much each activity uses, and you can calibrate that to yourself by, say, walking on a treadmill that you can calibrate. I would say the easiest way to lose weight is physical activity rather than dieting; just eat a healthy diet (veggies, fruit, some dairy, some protein, etc.) and spend a lot of time playing games outside, walking, etc. Awickert (talk) 04:36, 16 August 2009 (UTC)
 * Come to think of it, when I was more athletic, I used to burn ~6000 calories per day. Of course, that's a little on the intense end, but (since I enjoy food and exercise) I would say that more playing outside and working out is much easier (for me) than less eating. Awickert (talk) 04:47, 16 August 2009 (UTC)
 * The National Weight Control Registry consists of data about a set of people who have successfully lost a minimum of 30 pounds, and kept it off for a minimum of a year. Of the people in the NWCR, 98% modified their food intake in some way to lose weight, and 94% increased their physical activity.  So it is quite unusual to succeed at weight loss in the long term without both modifying food intake and increasing physical activity, but the data suggests that of the two, modifying food intake is the most crucial.  Red Act (talk) 08:33, 16 August 2009 (UTC)

Gene Identification
Besides the "knockout" technique what other techniques are used to identify a gene's purpose i.e. genes such and such are for hair and genes xyz are for the color or texture? —Preceding unsigned comment added by 70.136.158.98 (talk) 21:26, 14 August 2009 (UTC)
 * Probably the most commonly used method is to analyze the gene's DNA sequence similarity to other genes whose functions are known. Looie496 (talk) 21:41, 14 August 2009 (UTC)


 * You're basically asking for the methods of genetics, molecular biology, biochemistry, cell biology, and physiology. It's a bit too much for one post, but here are some other examples: Genomics studies the organization of the gene and regulatory sequences (this is much easier now with the completion of various genome projects).  In the good old days you would clone the cDNA from a gene expression library but this is rarely needed anymore, since most genes have been cloned at this point.  Still, it can be interesting to clone alternative splice forms of the gene from different tissues.  Gene expression analysis using northern blots, in situ hybridization, or DNA microarrays can show where the gene is expressed and give some idea of what tissues it is important in.  Western bloting can be used to study the protein and whether it is post-translationally modified.  Various biochemistry techniques are used to study protein-protein interactions.  Immunohistochemistry can be used to detect the intracellular location of the protein, which gives some hints about what it does.  Cell culture is used to study the gene in vitro.  Depending on the initial clues about what the protein does, there will be more specialized methods specific to a given type of protein.  People use comparative genetics and model organisms to see if the gene is conserved during evolution which would suggest a fundamental function.  Often a gene's family member can be studied in a more simple organism before learning what it does in humans.  Transgenic animals use gene overexpression to get information complementary to the knock-out.  Ok, that should get you started.  --- Medical geneticist (talk) 22:58, 14 August 2009 (UTC)

What about if we had an idea that the genes to make hair color like blonde for example if we knew that it was on chromosome 9, could they with any success study 1000 people who are natural blondes and look for genes that are the same (mostly) to try to uncover the genes involved in creating the blonde hair color? —Preceding unsigned comment added by 70.136.158.98 (talk) 04:31, 15 August 2009 (UTC)


 * Yes, you can do exactly that type of thing using families, with a technique called linkage analysis; or in larger groups of unrelated individuals using genome-wide association studies. However, one limitation is that although you may find a genomic location that is associated with a trait in a family or in a population,  you would still need to do additional work to identify which gene in that area was responsible for the association, and then even more work to prove that the association represents a causal relationship between the gene and the disease/trait.  Also, if a trait or disease is complex, the linkage or association studies may only reveal part of the story. --- Medical geneticist (talk) 17:08, 15 August 2009 (UTC)