Wikipedia:Reference desk/Archives/Science/2008 February 25

= February 25 =

Alternate method of boiling
Would it be possible to boil food by lowering the pressure instead of raising the heat? I assume yes, but would the food produced have the same taste/texture as a conventionally boiled dish? I understand that meats need the heat to fully cook, but is the same true of rice/noodles?H YENASTE 00:27, 25 February 2008 (UTC)


 * Yes, you could boil food that way, but I don't think it would qualify as cooking it. I believe the cooking time is increased at high altitudes due to the reduced temp where boiling occurs (based on the lower air pressure).  But, if you don't add any heat at all, I don't think you would expect any of the benefits of cooking, like killing bacteria and breaking down difficult to digest foods. StuRat (talk) 01:16, 25 February 2008 (UTC)


 * dry pasta needs to be both rehydrated and cooked (as opposed to fresh pasta, which still needs to be cooked). If you wet pasta and then lowered the pressure you'd simply dehydrate it (or anything else you put under vacuum). Furmanj (talk) 01:52, 25 February 2008 (UTC)


 * Exactly, this would be a reveres of the effect of a Pressure cooker where cooking time is reduced by increasing pressure and therefore temperature. The section Pressure cooker is particularly relevant. -- Q Chris (talk) 11:45, 25 February 2008 (UTC)


 * You would boil the liquid, but the chemical changes involved in cooking pasta and rice (the breakdown of starchy grains) require the heat. Actually, you'd be more likely to freeze your food by doing this (boiling requires heat anyway, it's just that it happens more spontaneously if you lower the pressure). I've certainly hooked things up to rotary evaporators (a piece of chemistry apparatus) and developed a crust of ice on the outside through over-eager vapourising. Sockatume (talk) 01:55, 25 February 2008 (UTC)

basically, if you lower the pressure enough, it becomes freeze drying, which is not the same as cooking. Gzuckier (talk) 18:29, 26 February 2008 (UTC)

Electric Car
I'm considering getting a cheap electric car, maybe something that's been retrofitted from a gasoline-powered car. What sort of issues should I be on the lookout for? Safety, maintenance, anything that might help me find something practical. Black Carrot (talk) 00:58, 25 February 2008 (UTC)


 * Here are some issues:


 * 1) Range on a full charge.


 * 2) Acceleration.


 * 3) Top speed.


 * 4) Can it be recharged at home or only at certain locations ? Where are the locations near you ?


 * 5) How long does it take to fully recharge ?


 * 6) How long do the batteries last before they will no longer hold a charge ?


 * 7) How much will it cost per mile to operate ?


 * 8) How is electricity in your area generated ? If it's made by burning coal, then an electric vehicle used there won't help the environment, but if electricity comes from a nuclear plant or clean energy source, then it will help.


 * You might want to consider a gasoline-electric hybrid, as that is a nice compromise that can solve many of the issues that electric-only vehicles have. StuRat (talk) 01:07, 25 February 2008 (UTC)


 * The following articles may be useful: Electric car, Plug-in hybrid, Hybrid vehicle. MrRedact (talk) 01:25, 25 February 2008 (UTC)


 * Even if coal is used to generate the electricity it is likely it will still be better for the environment than an internal combustion engine. Electric motors are generally much more efficient than ICE's. Pollution is not in the city thus less smog. CO2 Emissions however would probably be about the same (guess).Shniken1 (talk) 03:06, 25 February 2008 (UTC)


 * But don't forget about the inefficiency due to electrical losses in the power lines and during storage in the batteries. StuRat (talk) 03:38, 25 February 2008 (UTC)


 * I haven't done the sums, but I suspect Shniken1 is right. Internal combustion engines are just utterly crap when it comes to efficiency and it is very difficult to limit the pollution from a vehicle. According to Electric power transmission, in the US average losses in transmission and distribution is only 7.2%. And don't forget something like a petrol-electric hybrid will suffer some of the same losses due to storage anyway Nil Einne (talk) 09:19, 25 February 2008 (UTC)


 * OK, but don't forget to add the energy loss from the charging procedure, energy loss from the batteries while waiting to be used, and energy loss when the batteries are discharged to power the car. Also, gasoline burns a lot cleaner than coal (especially high-sulfur coal).  Of course, you might also want to include the pollution generated when the gasoline is refined, but that is often done even farther away than the electricity generation plants, so will have a minimal effect on local pollution. StuRat (talk) 02:41, 26 February 2008 (UTC)

I'm not much concerned with the environmental impact. I'm sure it'd be negligible regardless. I'm more looking for cheap maintenance, in which budget I count gasoline. Also, electric cars are one step closer to flying cars. (We can only hope.) I've looked through our articles some, but they don't give a lot of actual advice. I like the list of points, though. Range and top speed had already ruled out those funny golfcart things. I'm looking for something near or under $10,000, which I think tends to rule out hybrids. Are there any safety problems I need to worry about - electrocution, stalling out, that kind of thing? Especially with something reworked from an old gas-powered car. Black Carrot (talk) 04:09, 25 February 2008 (UTC)


 * Also, don't forget Electric vehicle conversion, which points out some useful links, such as diyelectriccar.com forums.  You might want to post these questions there. Dforest (talk) 10:34, 25 February 2008 (UTC)

I don't see much hope than some retrofitted frankencar will save you money. Where would you take it for service? Cheap electrics are on the way soon-ish. If you want practical and cheap, look for a plain-old used economy car. As long as you don't want good top speed or acceleration, that is (but honestly, who cars about top speed as long as it's not impractically slow?) Friday (talk) 14:36, 25 February 2008 (UTC)


 * There are some rather small electric cars that aren't meant for highway travel, but just as a commuter vehicle in the city (think of the type of vehicles meter-maids sometimes drive). In that case, the top speed doesn't need to be more than, say 45 MPH.  Those who do want to drive on the highway will have to ensure that the vehicle can do a comfortable cruise speed for them.  In the US there are some 75 MPH speed limits, and many people might drive 80 MPH.  A top speed of 90 MPH would cover any emergency passing needs on such a road. StuRat (talk) 02:22, 26 February 2008 (UTC)


 * Racecar drivers. See, Tesla Roadster. On a side note, electric cars are far more efficient that ICE cars, or hydrogen cars. Issues of range and acceleration will be improved in the future. 64.236.121.129 (talk) 17:21, 25 February 2008 (UTC)


 * Uhh, the guy is looking for cheap basic transportation. If he was looking for high-priced exotic electric sports cars, I imagine he'd have said so.  Friday (talk) 18:31, 25 February 2008 (UTC)
 * I'm aware of that. I was illustrating a point that electric cars potentially can match the performance of ICE cars. 64.236.121.129 (talk) 20:15, 25 February 2008 (UTC)


 * For what it's worth, the article mentions the top speed is limited to 125. While it's a heck of a neat little car, it's not much of a top speed champ.  Once they get the transmission problems fixed it'll be quite formidable in acceleration, though.  Friday (talk) 20:27, 25 February 2008 (UTC)


 * Keyword is "electronically limited". It can go faster without the limiter. Considering its fuel efficiency equivilent is 135 mpg (most ICE cars are lucky to get 30 mpg), and its range 221 miles (ICE cars usually have a range of over 400 miles, so the Tesla isn't bad), it's definitely an impressive car that displays the potential of an electric car. 64.236.121.129 (talk) 16:33, 26 February 2008 (UTC)

For $10,000, you can just buy a used Prius. I found three used Priuses in my area for about $10,000 advertised on the Kelly Blue Book web site. They’re available now, they don’t require any retrofitting, parts and maintenance are readily available, their fuel cost is considerably less than with most cars, and you can quickly fuel up anywhere. MrRedact (talk) 21:39, 25 February 2008 (UTC)


 * I'd be a bit worried that the batteries might be near the end of their life, though. Once they die, you might do best to total the car rather than pay to replace them. StuRat (talk) 02:22, 26 February 2008 (UTC)


 * Of the three used Priuses currently being advertised via Kelly Blue Book in my area for $10K, one has 66,350 miles on it, one has 95,584 miles on it, and one has 103,800 miles on it. Toyota says that the battery is designed to last about 180,000 miles.[http:// www.toyotapriusbattery.com/faq.html]  If the battery does need replacing (and it's not supposed to), it will cost about $3,000 to replace it, which isn't cheap.  But even if that does happen, I think a total of $13,000 for a used Prius with a brand new battery and lots of usable milage left on the car is still a not unreasonable price for it.  MrRedact (talk) 05:04, 26 February 2008 (UTC)


 * Sounds good, but two thoughts: Is mileage the only thing that matters for battery life, or do age and other factors matter, too ? Also, is 180,000 miles when the batteries quit completely ?  If so, they may only hold a greatly reduced charge for a long time before they finally die. StuRat (talk) 06:04, 27 February 2008 (UTC)

Why did the use of Greek fire stop?
Following on from "boiling oil" above, and Gwinva's ref to Greek fire, why did it fade away and how was the formula kept secret? (or should this go to the humanities desk?)Julia Rossi (talk) 01:43, 25 February 2008 (UTC)


 * I imagine it was a less effective weapon than cannons, which can be used at a far greater range. StuRat (talk) 03:08, 25 February 2008 (UTC)


 * Cannons? Cannons have a completely different role as artillery. It's not comparable at all. I asked this question on the humanities page, and the consensus was that the technology to use it had been lost. 64.236.121.129 (talk) 17:02, 25 February 2008 (UTC)


 * If your opponent had cannons and you were to rely on Greek fire, you'd never get close enough to use it, as their cannons would sink you first. If your weapons have a shorter range than the enemy, you're at a severe disadvantage.  There may have been other technologies between the two, as well. StuRat (talk) 02:12, 26 February 2008 (UTC)


 * What's stopping you from using both? Your argument is in line with saying that tanks are useless since attack helicopters can destroy them. They serve a purpose, that's why they are used. Greekfire logically served a useful purpose which was not in the artillery role. 64.236.121.129 (talk) 16:24, 26 February 2008 (UTC)


 * Tank columns are relatively useless against an enemy which has enough attack helicopters to render them ineffective (like when the Iraqis tried to use tank columns against the US in 2003). However, since many enemies don't have enough attack helicopters, tanks can still be effective against them.  The difference with ships equiped with cannons is that there is never any question of whether there are enough ships equiped with cannons to render the Greek fire ineffective, as a single ship with cannons (the target of the Greek fire) would be entirely capable of sinking the attacking ship using cannons long before it came into range to use the Greek fire.  The older technique of ramming ships in order to sink them is also no longer used for many of the same reasons that Greek fire can't be used (it requires getting closer than modern warfare allows and could damage the attacker almost as much as the target).  Now, if Greek fire could be kept on the ship with no negatives, like the risk of it exploding when struck, and didn't take up valuable resources like space and crew to operate, then it might make sense to have a "backup plan" in case enemy ships did get into range.  However, decreasing the number of cannons to make room for Greek fire would be a rather poor decision that would lose you more battles than it would win. StuRat (talk) 05:38, 27 February 2008 (UTC)


 * You missed the point with my tank example. I don't want to get into it, because then we would be going off topic, but there are other ways of destroying or immobilizing tanks which are just as effective. There are other examples of the basic point though. Using rockets instead of cannons. Using destroyers instead of cruisers. Using high explosive bombs instead of incendiaries. They serve a purpose that others can not. That's the point. The rest of your point goes into a lot of untested theory. You don't have any citations to back it up. The prevailing theory agreed on the reference boards, that is in line with the facts, is that the ingredients for Greek Fire was simply lost. 64.236.121.129 (talk) 15:51, 28 February 2008 (UTC)


 * Darn good question. Since we don't even know what the stuff was, it's hard to even speculate.  My own supposition is that it was probably almost as dangerous to those using it as to those it was being hurled against.  If it spilled, or if the "syringe" it was shot with burst, you'd be in big trouble!


 * As to the secrecy of the formula, I'm not sure, either, but if Coca-Cola and Kentucky Fried Chicken can manage it even in far-flung multinational corporations with millions of employees, presumably some clever Greeks could figure out a way, too. —Steve Summit (talk) 03:57, 25 February 2008 (UTC)


 * I think these "secrets" are more or less myths spread by the marketing division - I think every good food scientist could come up with some stuff with taste and flavor sufficiently similar to Coca-Cola so that nobody will note the difference (and not every Coca-Cola tastes the same to begin with; see also OpenCola). Icek (talk) 14:49, 25 February 2008 (UTC)


 * Hmm, their secrets aren't even mentioned in the articles – which dovetails nicely with "boiling oil" : ) Julia Rossi (talk) 04:24, 25 February 2008 (UTC)


 * Knowledge doesn't have to deliberately kept secret for it to be lost, of course. Few people these days know much about (for example) thatching or hedge laying, yet that would have been common knowledge in days gone by.  People find new ways of doing things, and the old are lost.  You can imagine the scenario: "Let me teach you how to make Greek Fire, my boy."  "Grandfather! Don't be so silly!  No one uses that anymore.  It's old fashioned and dangerous, and utterly pointless.  But let me show you what I learnt at school today..." New technologies were better (or were perceived to be better).  Certainly, as Steve says above, it must have been pretty dangerous to work with.  Gwinva (talk) 04:40, 25 February 2008 (UTC)


 * Hi Gwinva – I rather had in mind that at the time it was being used, others didn't seem to have the weapon (just going on the article) because the formula was a secret. Like your generation example, though, the Khitan Queen Shu Li seemed to trust in cavalry technology so flamethrowing didn't catch on for some time and when it did, followed the gunpowder fuse solution. Julia Rossi (talk) 05:41, 25 February 2008 (UTC)


 * Well, and we've discussed a few times on here in the past that flamethrowing is difficult to do well in anything but a purely defensive or naval role. The range is very much a factor of the size of the apparatus before you have things like propane. There's a kind of naive belief floating around that flamethrowing is some sort of super technology, but compared with something like, say, the crossbow, it is not really all that effective (and far more dangerous to handle) and too hard to mobilize en masse. --98.217.18.109 (talk) 15:10, 25 February 2008 (UTC)
 * No one said they are a super technology, nor have I heard anyone ever say they were. They can be superior weapons to crossbows, depending on what flamethrower you are using. Flamethrowers were used during WW2 while crossbows were not, and for good reason. 64.236.121.129 (talk) 17:02, 25 February 2008 (UTC)
 * Crossbows most certainly were used in WWII. Its a neat commando weapon, silent, but better range than a knife. Sp in ni  ng  Spark  18:19, 25 February 2008 (UTC)
 * Source? 64.236.121.129 (talk) 20:13, 25 February 2008 (UTC)
 * Bah. Such a comment is unnecessarily inflammatory, particularly in light of all the other claims made in this section.  Can we just note that crossbows-to-flamethrowers is apples-to-oranges anyway and move on? &mdash; Lomn 20:16, 25 February 2008 (UTC)


 * There's nothing inflammatory about requesting a source to a claim made by someone. If you make a claim, burden of proof is on you. 64.236.121.129 (talk) 16:24, 26 February 2008 (UTC)
 * I felt the manner, if not the substance, of the request was inflammatory (and granted this can be quite subjective). "Source?" reads quite brusquely to me, and a brusque request coupled with your own lack of sourced claims above led to my conclusion.  If you really want a source, though, the first Google hit for "ww2 commando crossbow" is all you need -- surprisingly simple to find. &mdash; Lomn 20:26, 26 February 2008 (UTC)


 * Lomn, you are trying to create a problem out of nothing. Given our history, this appears as you trying to harrass me. Please stop it. I do not have to look for a source, because I didn't make the claim. If someone makes a claim, then it is up to them to provide a source, not me. Please drop this issue, because you are bordering on trolling. 64.236.121.129 (talk) 14:43, 28 February 2008 (UTC)


 * Um, my comment was specifically and carefully written to indicate that I was talking about pre-modern flame throwers. It's obviously a different game when you're talking about ones of a modern design. As for a "super technology," I've seen flame thrower/Greek fire questions again and again on here with people wondering why they weren't being used constantly. That's all I'm referring to. They weren't used constantly because technologically speaking they had severe limitations (and still do, to some degree, which is why they aren't being used constantly in warfare these days either). --98.217.18.109 (talk) 22:46, 25 February 2008 (UTC)


 * Not quite. You didn't specify what you were referring to exactly. In any case, like I said before, the non-use of greek fire has been attributed mostly to the loss of the technology to create it, rather than the percieved lack of usefullness of it. 64.236.121.129 (talk) 16:24, 26 February 2008 (UTC)
 * But similar things have been invented and re-invented many times over the course of history. If there was a technical need for it, it certainly could have been re-invented again, it was not exactly unknown, and though we like to mythologize Greek fire itself, making flammable substances and liquids is something humans have been able to do for quite some time. --98.217.18.109 (talk) 13:51, 27 February 2008 (UTC)
 * They were re-invented again. During WW1 and WW2. It took them that long to make something just as good, and practical. The technology for a practical flamethrower was obviously out of their reach until then, otherwise they would have made them. During the Civil War, Greek Fire was threatened actually, but never used. Whether the threat was credible or not (they had the technology), is unknown. 64.236.121.129 (talk) 15:56, 28 February 2008 (UTC)


 * I wondered what 64.236.121.129 and 98.217.18.109 were referring to with these allegedly frequent references (since I'd never heard of Greek Fire at all, let alone here on the RDs), but they're right, it has come up time and again. If anyone else is curious, try this google search. —Steve Summit (talk) 23:27, 25 February 2008 (UTC)


 * It's not super frequent but this is the third time in basically as many months, so it stuck in my mind. --98.217.18.109 (talk) 00:44, 26 February 2008 (UTC)

earthworms
1.Uses of earthworms to the soil? 2.Importance of earthworms? 3.What nutrients it can give? —Preceding unsigned comment added by 121.1.14.122 (talk) 10:58, 25 February 2008 (UTC)
 * Check out the Earthworm article, in particular, Earthworm. Dforest (talk) 11:12, 25 February 2008 (UTC)


 * The tunnels they make help loosen the soil. Their waste products are also full of nutrients which are good for plants. 64.236.121.129 (talk) 17:18, 25 February 2008 (UTC)


 * You might enjoy our Vermicompost article.


 * Atlant (talk) 13:52, 26 February 2008 (UTC)

Purchasing Explosives
Hello, this is a totally serious question, although it might seem stupid to some.

I am a theatre technician (UK), who often detonates small pyrotechnics on stage, often Lemaitre pyro pods. However, I am becomming increasingly interested in the area and science behind theatrical explosions, and would like to detonate something slightly bigger (not onstage obviously).

How are the big explosions achieved in films, and how can I buy these explosives? I know they don't just ignite a barrel of fuel, there's cables and firing devices and timecode involved. But how do they actually purchase the explosive material to start with? A google search provides me with no information.

I'm 22, but naturally no way to prove it, please do not answer if you fear I'm a terrorist etc :) Radiofred (talk) 11:44, 25 February 2008 (UTC)


 * A lot of it is just petrol (or gas in USA), they first aerosolise it with a small explosive and then light it, thus producing a large fireball without an explosion. It is therefore safer to use 'near' people than high explosives.--Shniken1 (talk) 12:07, 25 February 2008 (UTC)


 * Vanishingly minor quibble: "gasoline" is more usual where you have "gas". I don't understand it myself. Our cars do run on "gas", not so much "gasoline", but an arsonist will have used "gasoline" as an accelerant and not "gas". Just a tip for the Brits and the rest. --Milkbreath (talk) 15:07, 25 February 2008 (UTC)


 * Thankyou, I never knew that. But I did know the word was modelled after Vaseline.   So if you use that product on your hair would one be said to have Vassed ones head, but if a nefarious villain then sets it alight would he be said to have vaselined the victim?  Sp in ni  ng  Spark  15:36, 25 February 2008 (UTC)


 * I don't think so; the weirdness seems to be only for the one word. My gut understanding (as a native speaker of American), which of course I've never needed to verbalize previously, is that "gasoline" is a substance whereas "gas" is a (literal, conceptual, or metaphorical) fuel &mdash; never mind that they often chemically coincide.  A sports team that stops playing well toward the end of a game might be said to be "running out of gas", but it would make no sense at all to say that they were "running out of gasoline" unless the fluid were actually somehow part of their strategy.  You would certainly be understood if you said you were putting gasoline into a vehicle (though it would probably be interpreted as noting that the vehicle did not use diesel fuel), but you would never say that to accelerate you needed to "give it [the engine] some gasoline", despite the fact that you are literally doing so.  --Tardis (talk) 17:00, 25 February 2008 (UTC)
 * unless the fluid were actually somehow part of their strategy LOL! 199.67.16.60 (talk) 14:39, 29 February 2008 (UTC)
 * Since when was 'American' a language............you speak English! —Preceding unsigned comment added by 84.67.199.152 (talk) 20:56, 26 February 2008 (UTC)


 * I took it as tongue-in-cheek. I think the gas/gasoline thing is mainly an issue of context. When we refer to "putting gas in the car" it is clear we are talking about gasoline; in other contexts, it might not be so clear. Dforest (talk) 10:30, 27 February 2008 (UTC)


 * You know you need a licence for what you are doing - right? See here if you are storing stuff in London, or if not you should contact your local authority.


 * Didn't have any trouble getting hits on Google myself - try this search string  Sp in ni  ng  Spark  13:23, 25 February 2008 (UTC)


 * Oh, and they will probably want to see some kind of qualification (NVQ or such like) before they allow you to have any of the big-boys stuff. Can't give you any real guidance with that though.  Sp in ni  ng  Spark  13:35, 25 February 2008 (UTC)


 * My impression on the gas/gasoline dichotomy is that it comes from "gas"'s extreme similarity to the word "gas", which is obviously unrelated. In cars, at least unless people are dumb enough to go for hydrogen power, there's no confusion and we can afford to use the abbreviated form exclusively, even to the point of building idioms around it as Tardis pointed out. In this application, though, there's the real possibility that someone might be igniting a gas, so it's important to keep the distinction. Black Carrot (talk) 20:11, 25 February 2008 (UTC)

• The police link and the google search are both appreciated!! Thanks! In all honesty I'm far too poor to purchase any now, but am very interested in researching the field. Thanks. Radiofred (talk) 00:14, 26 February 2008 (UTC)

There is a likelihood that when someone experiments with explosives and starts scaling up cute little explosions, the result will be death, blindness, severe burns, or loss of fingers. Edison (talk) 05:26, 26 February 2008 (UTC)

• Luckily, I know the difference between 'experiments' and 'responsible, controlled research'. Even small stage pyros have taught me the proper procedure with handling, storage, arming and firing. But thanks for your concern, especially for other less sensible people reading this. Radiofred (talk) 12:17, 26 February 2008 (UTC)


 * I reiterate, speaking from experience: if a chem book or other source says (actual chemicals redacted) "Soak a crystal of - in, let it dry, and the resulting  will be set off by the slightest contact or even a puff of wind", do not succumb to the temptation to make a larger amount at one time. Edison (talk) 22:51, 26 February 2008 (UTC)

• I have no interest in making them, I wish to purchase explosives and use electrical firing systems. I'm not a chemist, I'm not gonna kill myelf in my kitchen trying to make my own explosives. Radiofred (talk) 10:54, 27 February 2008 (UTC)


 * You may be interested in this extremely dangerous little book. --169.230.94.28 (talk) 21:34, 27 February 2008 (UTC)

color
The pattern on a piece of cloth consist of red triangle on a blue background what colors would appear onthe cloth in red and in green light —Preceding unsigned comment added by 205.217.233.30 (talk) 20:12, 25 February 2008 (UTC)


 * Under red light, the cloth would appear as a red triangle on a black background. Under green light, the cloth would appear solid black.  MrRedact (talk) 21:45, 25 February 2008 (UTC)


 * Think about this, because the point of this question was to make you think, not just to give the right answer. What does it mean when something is red? Does the red triangle absorb red light, or reflect it? What does it do to other coloured light? What about the blue background, why is it blue? Does it absorb or reflect red light? If you don't understand how to work this out, ask because you will need to be able to do this yourself in future. 130.88.140.122 (talk) 09:57, 26 February 2008 (UTC)


 * The answer to the question depends on how saturated (narrowband, monochromatic) the light sources are and how narrowband is the color filtering of the cloth swatches.


 * Atlant (talk) 13:51, 26 February 2008 (UTC)


 * Yeah, I figured someone would come along with a more precise answer. Under red light, the background would likely appear as more of a dark red rather than pure, pitch black.  And both colors would likely appear as more of a dark green under a green light, rather than pitch black.  How close those colors come to pure black would depend on properties of both the light source and the fabric pigment, as Atlant points out.


 * Furthermore, under green light, the two reflected colors may well differ in how close to black they appear, such that it’d be possible to visually distinguish between the triangle and the background. The triangle is likely to appear lighter than the background in this case (because the responsivity spectra of M human cone cells overlaps with that of L cells more than that of S cells), but again, it’s impossible to say that for sure without knowing more precisely what you mean by “red”, “green”, and “blue” in your question.  Two kinds of red light, for example, that appear identical when reflected off of a white piece of paper, can appear quite different when reflected off of different colored surfaces.  Similarly, two shades of red paint that appear identical under sunlight can appear quite different when illuminated with a different light source.


 * The original answer I gave would be exactly correct according to 3D modeling software like OpenGL or Direct3D, in which “pure red”, “pure green,” and “pure blue” have an unambiguous meaning for both light sources and surfaces. But the real world is a little more complicated than the approximate reality modeled by the software.  MrRedact (talk) 15:45, 26 February 2008 (UTC)

Role of Urea in concentrating uring
The wiki article on the Thick Ascending Limb of the nephron says something that seems contradictory to me.

1: The medullary thick ascending limb remains impermeable to water. 2:Urea which remains in the loop creates a solute potential that prevents water completely osmosing out into the interstitial space. This means that while almost all the ions are reabsorbed, there will still be some water in the urine, and hence, the concentration of the filtrate in the loop is decreased here. (If only ions were present, and a certain amount of ions were reabsorbed, one would expect the same amount of the water to be reabsorbed too, and hence the concentration would remain the same, but this is not true.)

How can 2 be true if the TAL is impermeable to water? What is the role of Urea here?

Many thanks Horia (talk) 20:17, 25 February 2008 (UTC)


 * Its role in concentrating urine can be found in these articles: Urea and countercurrent exchange.  Wisdom89  ( T |undefined /  C ) 00:07, 26 February 2008 (UTC)


 * that doesn't solve the contradiction between 1 and what's in italics in 2. I'll just assume it's wrong and probably change it soon.Horia (talk) 22:51, 26 February 2008 (UTC)