Wikipedia:Reference desk/Archives/Science/2018 January 23

= January 23 =

What substance is responsible for a metallic taste/smell?
What substance is responsible for the taste/smell that people describe as "metallic"? (The smell I'm talking about is what you get when you have a stainless steel pot on a stove that has boiled dry.) --134.242.92.97 (talk) 00:01, 23 January 2018 (UTC)
 * Sometimes the metal taste is from, well, metal. See here and, just for the cool link alone, also lasagna cell. Sometimes the metal taste is because of a medical condition. See here and also at metallic taste, but it seems like you're referring to the former rather than the latter. Matt Deres (talk) 02:32, 23 January 2018 (UTC)


 * The smell may be trans-4,5-Epoxy-(E)-2-decenal or Oct-1-en-3-one (cause by iron in contact with skin). The smell of iron dissolving in acid is due to methylphosphine and dimethylphosphine which may happen if you boil a food dry. You can also read A 'metallic' smell is just body odour or https://chemistry.stackexchange.com/questions/7916/why-can-we-smell-copper to see this explained more. Graeme Bartlett (talk) 11:42, 23 January 2018 (UTC)


 * Metallic taste independent of eating anything can also be caused by higher radioactive exposure. -- Hans Haase (有问题吗) 18:42, 24 January 2018 (UTC)

Blue animals
When talking about animal skin, "blue" usually means blue-gray in color. Why is it so rare for animals to be the fully saturated blue?? Georgia guy (talk) 22:58, 23 January 2018 (UTC)
 * It's not just a rare case of animal coloration, but when it happens it usually happens by structural coloration rather than normal types of pigmentation. I don't know why it's rare or why it's not pigmentation, but those are some lead articles on the topic that could be expanded if you find more explanation. DMacks (talk) 23:04, 23 January 2018 (UTC)


 * Btw, there is a c:Category: Blue animals. This NPR article discusses the question, but the bottom line as to why, is "still a mystery".
 * —107.15.152.93 (talk) 00:19, 24 January 2018 (UTC)
 * There doesn't have to be a "reason", really. We humans like to look for teleology in everything, but evolution is undirected. It could simply be that the random walk of evolution on Earth didn't "come up with" blue coloration in animals often. Not that there's anything wrong with asking the question, of course. --47.157.122.192 (talk) 02:33, 24 January 2018 (UTC)
 * I don't think "a reason" implies teleology. A reason isn't the same as intention or purpose.  Arguably everything (barring some truly random things like individual acts of nuclear decay) happens for a reason. Even if animal coloration was initially random and happened not to be blue, that then means that anything that subsequently became blue due to random mutation would stand out distinctly, and so be more likely to get eaten, and so fail to pass on its blue genes. That (to me at least) could be describes as a "reason", despite no direction being involved. Iapetus (talk) 11:13, 24 January 2018 (UTC)
 * The locus coeruleus is not really very blue either. Tyrian purple is a fair try by animals to produce a true blue pigment, but, well, it's purple, not blue.  Indigo, closely related, was left to the plants, but is one of two pigments contained in Tyrian purple.  Any animal ought to be able to evolve such a pigment since it is produced (via indican) from tryptophan.  And yet ... they typically don't.  You might see blue diaper syndrome or blue Fugates, though it may not be true blue enough.  Curious... Wnt (talk) 03:04, 24 January 2018 (UTC)
 * The locus coeruleus is not really very blue either. Tyrian purple is a fair try by animals to produce a true blue pigment, but, well, it's purple, not blue.  Indigo, closely related, was left to the plants, but is one of two pigments contained in Tyrian purple.  Any animal ought to be able to evolve such a pigment since it is produced (via indican) from tryptophan.  And yet ... they typically don't.  You might see blue diaper syndrome or blue Fugates, though it may not be true blue enough.  Curious... Wnt (talk) 03:04, 24 January 2018 (UTC)

A few random comments. What is called "blue" in the English set of color names covers a huge swath of color-space, while "yellow" for example is extremely narrow, even though blue and yellow are generally considered opposites. This is misleading, because one can find shades of blue whose opposites are peach or brown.

Also, blue is very much like yellow, or a black and white pattern: either aposematic, warning of predators (think skunk, coral snake, blue-ringed octopus and kraits) or meant for sexual attraction (mandrill). Pure blues don't work very well for camouflage, so they are rare in general. Hence an animal is really going out of its way to produce a blue pigment, and again, most blue feathers produce the effect by iridescence, not pigmentation. μηδείς (talk) 03:54, 24 January 2018 (UTC)
 * That's not peach colour. It's just the colour you see when you look at a peach. A marketable peach, anyway. InedibleHulk (talk) 01:34, 25 January 2018 (UTC)

I should add that Larry Niven's Beowulf's Children, sequel to Legacy of Heorot, is a hard-science fiction novel based on several different ecological concepts in the colonization of a biologically highly advanced world (think earth w/o the KT event) where the most intelligent indigenous creature is an amphibian that as an adult eats its young (based on a recently described African frog tht lives off the tadpoles that eat the algae in its pond) and key species, symbiosis, ecological degradation, and the climactic discover of native aposematism, which saves the colony. μηδείς (talk) 07:23, 24 January 2018 (UTC)


 * This is speculation on my part, which someone might be able to refute or corroborate with, like, actual references, but is it not the case that blue chemicals and substances tend more than those of other colours to be poisonous to at least terrestrial metabolisms? {The poster formerly known as 87.81.230.195} 94.0.128.132 (talk) 01:13, 25 January 2018 (UTC)


 * My understanding is that blue pigments are not so much poisonous, as metabolically expensive. Keep in mind that the red end of the spectrum is low energy, so absorbing and emitting red is not very costly or damaging.  But blue light is much more energetic, so it causes more damge, and re-emitting it is also costly as a matter of upkeep.  (That is, things that reflect blue bleach easily under a yellow sun.)  Also, the sun emits "white light" but it is white to us because it fully saturates our capacity across the visual spectrum.  Nevertheless, yellow is more brilliant, given the sun's light is brightest in the yellow spectrum.


 * There's also the fact that red tends to look black rather quickly as one descends deeper in the see than blue does. Given all animal phyla I am aware of originated in the sea, a tendency toward blue for showiness at shallow depths (krait & octopus) implies we are incentivized by the color of our star Sol to produce blue only when the investment is worth the relative cost. μηδείς (talk) 03:23, 25 January 2018 (UTC)
 * The Sun actually emits most strongly in green. We don't perceive this because our photoreceptors are more sensitive to other wavelengths of light, and blue light is scattered by the atmosphere, so the Sun appears to have a yellow tint on a clear day. This, of course, also means that most plants reflect away the largest part of sunlight; the reason why is another little evolutionary puzzle. --47.157.122.192 (talk) 20:22, 25 January 2018 (UTC)
 * This may be a sidetrack, but ... wow. I mean, I know that the Sun's color is properly black because it radiates almost as a blackbody.  And it is white because, duh, it defines white light as we know it.  And it is yellow because, um, it looks yellow most of the time.  But now NASA it is blue-green because it radiates most in those frequencies?!  That is new to me.  Now to be sure, NASA points out that some of that is absorbed in the atmosphere; OTOH not sure if that matters .  But there's another wrinkle I see in that graph they're using which is philosophical.  The scale is in W/m^2, which is great.  But how much light does the Sun put out at one single monochromatic frequency?  Well, zero, I think.  Or one photon, with zero probability.  Because even spectral lines have a width from motion or some other reason, so if you stretch the rainbow far enough, you have nothing.  Which means that data on that graph is binned somehow, presumably in terms of fixed increments of wavelength.  If you look at the Sunlight article and specifically File:Solar irradiance spectrum 1992.gif, you'll find one where they bin in fixed increments of frequency, and by that standard the Sun's spectrum peaks at 10000 cm-1, i.e. 1 um = 1000 nm is Infrared-A (at least, our article uses that term; I'm not familiar with it).  Redder than red.  If you can think of another philosophy by which to mark your x-axis you can make it some color in between, I suppose.  Sail this world, see the sunset, see the green flash, the White Sun of Russia, the Darkness at Noon.  Sunlight is every color all on top of each other, but only those who have at least once tasted the forbidden fruit of the psychedelic art and science will ever truly understand that. ;) Wnt (talk) 21:28, 26 January 2018 (UTC)
 * That's a great theory, except ... a blue pigment is that which reflects or transmits blue light while absorbing the other frequencies, which is where you would think the damage would occur. There are lots of common pigments like the indigo I mentioned and synthetics that people use on clothes that manage to function for years without needing to be refreshed or recharged.  (To be sure, emission of blue light for bioluminescence does cost more energy ... funny thing is, there are many examples of that) Wnt (talk) 21:36, 26 January 2018 (UTC)
 * You don't seem to be contesting my claim that blue light is more energetic than red light, and hence blue pigments are more easily bleached or most importantly more costly to manufacture and replace than red pigments. μηδείς (talk) 02:46, 27 January 2018 (UTC)
 * Well, I can't spend much time to evaluate that idea because I'm busy trying to prove that Russell's teapot crashed to earth on Oak Island. But I did do a Google search and found one article  which seems to suggest that blue pigments once were expensive because they were rare in nature.  I also know barns are painted red because iron tends to be red and it is the most stable nucleus coming out of a supernova.  Can you provide a source that explains what you mean? Wnt (talk) 13:43, 27 January 2018 (UTC)