Wikipedia:Reference desk/Archives/Science/2008 January 11

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January 11

Dysgraphia/Agraphia

Can someone with severe dysgraphia or agraphia still type normally? I've created a character with Gerstmann's Syndrome in a roleplay I'm doing and I wanted to know how that affects his computer use. I guess also finger agnosia and left-right disorientation might also make it harder to use a keyboard? Or is that something you can learn to cope with? There's always hunt and peck, of course... Kuronue | Talk 01:08, 11 January 2008 (UTC)

Geiger muller tube internal quenching

External quenching uses external electronics to remove the high voltage between the electrodes. Self-quenching or internal-quenching tubes stop the discharge without external assistance, and contain a small amount of a polyatomic organic vapor such as butane or ethanol; or alternatively a halogen such as bromine or chlorine. Ions will collide with quench gas molecules, and give up energy to them by causing them to dissociate.

The halogen tubes were invented by Sidney H. Liebson in 1947, and are now the most common form, since the discharge mechanism takes advantage of the metastable state of the inert gas atom to ionize the halogen molecule and produces a more efficient discharge which permits it to operate at much lower voltages, typically 400–600 volts instead of 900–1200 volts. It also has a longer life because the halogen ions can recombine whilst the organic vapor cannot and is gradually destroyed by the discharge process (giving the latter a life of around 108 events).

This pair of paragraphs states that the inert gas atoms (or organic molecules) are ionized and that this somehow quenches the GM tube signal but shouldn't the ions and electrons derived from this "quencher" also be accelerated towards the electrodes, exciting more molecules along the way? I fail to see how the quencher should act any differently from the other gasses, assuming that it is in fact ionized itself. If a diatomic halogen molecule dissociates, without either atom becoming an ion, I can see how this would quench, but is this how the organic vapour also quenches? ----Seans Potato Business 01:21, 11 January 2008 (UTC)

This should be asked on the relevant talk page not here.--TreeSmiler (talk) 01:30, 11 January 2008 (UTC)

The question is quite welcome here. Article talk pages are for discussions about editing the article. This page is to help people answer their questions about science—which this question certainly is. TenOfAllTrades(talk) 04:26, 11 January 2008 (UTC)

It may be welcome, but if the questioner wants an (proper) answer in a reasonable time, he is more likely to get it from editors of that page than those of this page, thats all.--TreeSmiler (talk) 16:16, 11 January 2008 (UTC)

I just happened on this question and I dont know much about GM tubes but this page may give a clue:[1] —Preceding unsigned comment added by TreeSmiler (talk • contribs) 16:29, 11 January 2008 (UTC)

Much clearer. I'll try to fix the article. Thanks. ----Seans Potato Business 21:51, 17 January 2008 (UTC)

Tremulating musssels

I no sum people who have tremulating mussels in there arms. what is it called/ —Preceding unsigned comment added by 79.76.136.192 (talk) 02:50, 11 January 2008 (UTC)

Do you mean Tremor? <suggested diagnosis deleted SpinningSpark 11:50, 11 January 2008 (UTC)> Graeme Bartlett (talk) 04:34, 11 January 2008 (UTC)

I suspect you mean fasciculation. - Nunh-huh 08:56, 11 January 2008 (UTC)

Poop smells good!(moved from entertainment desk)

how many colors of poop are there? 8 brown blue white pink red green yellow purple. it all depends on what you eat. —Preceding unsigned comment added by 208.102.38.113 (talk) 12:48, 10 January 2008 (UTC)

How many shades would be more accurate. Technically no two shades of poop are exactly the same leaving us with infinite shades of poop. Why not knock yourself out and catalogue some for us and report back? I await your results with baited breath. Lanfear's Bane | t 13:06, 10 January 2008 (UTC)

Depends what sort of animal and what it has been eating. Perhaps the questioner would enjoy our articles on poop and the Bristol Stool Scale.--Shantavira|^{feed me} 13:37, 10 January 2008 (UTC)

I'm not sure this is the right type of question for the entertainment section, unless poop has become entertaining all of a sudden.--Dlo2012 (talk) 14:29, 10 January 2008 (UTC)

Yes I suggest this whole thread is moved to science desk ASAP! (before it stinks the whole place out)--TreeSmiler (talk) 03:04, 11 January 2008 (UTC)

There's at least one entertaining book on the subject. -- kainaw™ 14:40, 10 January 2008 (UTC)

It's more relaxing than entertaining. Lanfear's Bane | t 16:20, 10 January 2008 (UTC)

In times past, my colon was an unhappy place due to ulcerative colitis. Over the years, I have observed poops ranging from tar black to white as the driven snow. I've pooped sky blue once, and blood red many times. Poop is a mystery wrapped in a conundrum wrapped in a fresh-roasted pita bread. ----

You may have pooped sky blue, but please note that if you pee turquoise, it means you've fallen into a short story.

Atlant (talk) 12:54, 11 January 2008 (UTC)

Did you look up faeces yet?--TreeSmiler (talk) 03:01, 11 January 2008 (UTC)

Don't unerestimate the effect of licorice or beetroot. Graeme Bartlett (talk) 04:31, 11 January 2008 (UTC)

For a nice forest green color I highly recommend "Fierce Grape" Gatorade.  :-P -- HiEv 04:20, 12 January 2008 (UTC)

Would a machine be affected by Chemical, Biological, or Radiological weapons?

Machines being vehicles, or robots. Affected in the negative sense. Would they impair it in any way? Malamockq (talk) 03:37, 11 January 2008 (UTC)

Depends whether the machine is susceptible to chemical biological or radiological weapons doesnt it?--TreeSmiler (talk) 03:40, 11 January 2008 (UTC)

That was kinda meaningless.. You basically rephrased my question. Malamockq (talk) 04:11, 11 January 2008 (UTC)

Yes the machine would not be usable by humans because it would be contaminated. The machine itself may be still functional, but it would turn into contaminated waste if treated with these weapons. Graeme Bartlett (talk) 04:17, 11 January 2008 (UTC)

If it were robotic (autonomous), it would be fine though? Malamockq (talk) 04:20, 11 January 2008 (UTC)

it could be, radiation can disturb electronic chips, particularly RAM. Also acids, or molten metals could damage a metallic robot. The US attacked Serbia's electricity systems with powdered graphite that short-circuited things out. Powdered aluminium could ahve the same effect. So the robot could stand the bio hazards. But not the attacks targeted at electronics or machines. EMP can wreck electronics too. Graeme Bartlett (talk) 04:28, 11 January 2008 (UTC)

FYI, it wasn't graphite powder, but graphite filaments. Here's a picture of them. --Sean 15:02, 11 January 2008 (UTC)

(ec) It depends. Most machines are going to be much less susceptible to all three types of devices than humans are. Nevertheless, high doses of ionizing radiation will harm modern electronics (military equipment is often 'hardened' to provide some protection; see radiation hardening for details). Some chemical agents may react with metals or plastics and damage hardware in that way; for modern nerve agents, the concentrations that do physical damage to hardware are hundreds or thousands of times higher than those that will kill human beings. TenOfAllTrades(talk) 04:36, 11 January 2008 (UTC)

Also on the biological side, there are bacteria which eat all sorts of weird things including petroleum, methane, and some metals. It would probably be difficult, but not impossible, to create bacteria which could consume essential parts of a robot or the whole thing. (A bacteria that consumed silicon would make it tough for many electronic components to function). Anynobody 04:48, 11 January 2008 (UTC)

Well, as a chemical weapon, a douse of water could be devastating to an electronic machine whereas it wouldn't do any damage to a human. On the other hand, robots don't breathe so wouldn't be phased by mustard gas or most toxic fumes (unless they were corrosive). HYENASTE 22:03, 11 January 2008 (UTC)

The article on radiation hardening says "commercial-grade chips can withstand between 5 and 10 krad" - that's 50 - 100 gray, and for gamma or beta radiation that would be 50 - 100 sievert, with 10 sievert killing a human nearly certainly (but I'm not sure about the actual exposure levels of humans vs. robots, it depends on the nature of the radiological weapon). Icek (talk) 11:37, 12 January 2008 (UTC)

What does antimatter look like?

Lets say I had anti-iron. Would it look more or less like regular iron? 64.236.121.129 (talk) 14:43, 11 January 2008 (UTC)

Another question. If you had anti-iron, would it only annihilate against iron, or would it annihilate against any normal matter? 64.236.121.129 (talk) 14:45, 11 January 2008 (UTC)

It would look like normal iron, and it would be annihilated by contact with any normal matter, not just anti-iron. --Stephan Schulz (talk) 14:56, 11 January 2008 (UTC)

Except in sci-fi stories, where you seem to only annihilate anti-you and so on, a convenient (for the plot) fiction. (I'd love to refer to Let That Be Your Last Battlefield, but that doesn't seem to be an example of this.)

Atlant (talk) 16:59, 11 January 2008 (UTC)

(ec)Anti-iron would look like regular iron and anti-you would look like regular you - but I wouldn't try shaking hands with him.

You just gave me an idea for the ultimate star-crossed lovers story. :) --Sean 15:05, 11 January 2008 (UTC)

The positrons in anti-iron would annihilate against the electrons in any other regular matter, followed by the nuclei. SeeAnti-matter SpinningSpark 14:59, 11 January 2008 (UTC)

As a side note, there are ways in which matter and antimatter can combine to form exotic new forms, such as in Positronium. Of course, Positronium is unstable, and has a half life of 1 microsecond or less, but at least for a little while annihilation isn't the only option. --Bmk (talk) 15:17, 11 January 2008 (UTC)

Wouldn't an anti-you actually look like a mirror image you, because of CP-symmetry ? Gandalf61 (talk) 15:26, 11 January 2008 (UTC)

Possibly, if CP-Symmetry applied to macroscopic bodies, but I'm not sure that's trueSpinningSpark 15:59, 11 January 2008 (UTC)

So the consensus is that photon interaction with antimatter is identical to the situation for regular matter? Are you sure this does not violate some symmetry rule? I'm imagining the macroscopic case is "many photons bounce off a surface" and a microscopic equivalent is "a single photon scatters from a single anti-atom" - my gut instinct is that the scattering process will be quite different. Nimur (talk) 17:53, 11 January 2008 (UTC)

A photon is its own antiparticle and interacts with normal matter and antimatter identically. -- Coneslayer (talk) 18:03, 11 January 2008 (UTC)

This is why no one's ever managed to climb the Anti-Matterhorn Adambrowne666 (talk) 11:36, 13 January 2008 (UTC)

Do clones have introns?

As title. Weasly (talk) 14:48, 11 January 2008 (UTC)

Of course. By definition, a clone has identical DNA, introns, exons and all. -- kainaw™ 15:08, 11 January 2008 (UTC)

I was told by a reputable fellow that introns code for bugger all, and therefore are not used in the original DNA which is synthesized by using a modified bacterium. How, pray tell, do you explain this? —Preceding unsigned comment added by Weasly windom price (talk • contribs) 16:06, 11 January 2008 (UTC)

Perhaps your reputable fellow is confusing molecular cloning (copying of DNA sequences) with cloning of an organism. The original DNA in the embryo of an organism clone, such as Dolly the sheep, is taken intact from a somatic cell of the parent organism (in a process called somatic cell nuclear transfer) - it is not artificially copied. Gandalf61 (talk) 16:28, 11 January 2008 (UTC)

Indeed. I will take the matter up with him. Weasly (talk) 17:11, 11 January 2008 (UTC)

Many cloned genes do not have introns, as they are often not required for the purposes of cloning a gene molecularly, and its easier to clone the gene from cDNA (without introns), rather than genomic DNA (with introns). I suspect that is what your friend is referring to. Rockpocket 19:18, 11 January 2008 (UTC)

Inertia Theory of E Mach

In Albert Einstein: The Meaning of Relativity I read of a theory by E Mach which Einstein does not seem to support but agrees that the following three conclusions of Mach are in agreement with General Relativity;

1. Inertia of a body increases if other bodies are nearby

2. Acceleration of a body is induced by another body accelerating

3. A rotating hollow body generates a Coriolis field inside the hollow

Einstein then gives some equations in General Relativity tensors to support this claim which I shall not attempt to reproduce here because my wiki-math typing abilities are not up to it (besides, this is not the Mathematics Desk).

But he says " . . . their magnitude is so small that confirmation . . . by experiment is not to be though of". Since this book was first published in 1922 and last revised in 1956 this might possibly be no longer true but I have not seen Machs ideas mentioned anywhere else. Does anyone know if these effects have ever been proved? SpinningSpark 15:18, 11 January 2008 (UTC)

I know nothing about Mach's ideas, but I can say that a number of the very small, esoteric effects predicted by relativity have now been confirmed (at least indirectly). I believe the third point is a way of referencing the effect of frame dragging which is one of the most difficult predictions to study and is currently being worked on by Gravity Probe B (an experiment 40 years in the making). The second point is probably referencing gravitational radiation, for which there is indirect evidence from the study of neutron star binaries. I'm not sure about the first. Dragons flight (talk) 15:59, 11 January 2008 (UTC)

The end result in Einsteins discussion of this is three laws of motion. I will try to type in the first one which seems to be the relevant one to the first two effects. But don't hold your breath waiting - this could take me some time. SpinningSpark 16:56, 11 January 2008 (UTC)

Ok, here we go,

${\frac {d}{dl}}[(I+{\bar {\sigma }})\mathbf {v} ]=grad{\bar {\sigma }}+{\frac {\partial {\mathfrak {A}}}{\partial l}}+[rot{\mathfrak {A}}\times \mathbf {v} ]$

where the inert mass is now proportional to $(I+{\bar {\sigma }})$ (first law)

and the induced acceleration is represented by the ${\frac {\partial {\mathfrak {A}}}{\partial l}}$ term (second law)

Does this help anyone to answer my question? SpinningSpark 18:03, 11 January 2008 (UTC)

Thank you, Dragons flight, for pointing me at the frame dragging article. I now see that the second effect is "Acceleration Frame Dragging" (as opposed to "Rotational Frame Dragging") and has not been investigated. Not sure whether that is the same thing as gravitational radiation as you suggest, but I guess SOME sort of radiation must be involved. That leaves the first effect as the only one not nailed. SpinningSpark 18:42, 11 January 2008 (UTC)

As just a historical note, Einstein practically worshipped Ernst Mach's work when he was very young, as Mach was a hard-core believer that only phenomena which could observed, measured, and described were worth talking about, and an early rejector of constructs like the aether. That being said, Mach did not, in the end, support Einstein's theories of relativity, to Einstein's great disappointment, and Einstein himself became less and less Machian as time went by. (One could argue that by the time of his debates about quantum mechanics, Bohr was far more of a Machian than Einstein was.) --24.147.69.31 (talk) 19:25, 11 January 2008 (UTC)

"Einstien said nothing can accelerate to the speed of light, beause its mass would become infinite"

^That's a quote from the movie, K-Pax. Is that line true? If something tries to accelerate to the speed of light, its mass would become infinite? 64.236.121.129 (talk) 15:22, 11 January 2008 (UTC)

Yes, see Special relativity SpinningSpark 15:40, 11 January 2008 (UTC)

How would that work though? Would it become larger? 64.236.121.129 (talk) 15:57, 11 January 2008 (UTC)

As with most things in relativity, the answer varies by the observer's frame of reference. Someone observing an object at a relatively high speed (substantial fractions of c) will actually observe that its length has contracted. Mass in special relativity may be of assistance. As for the initial quote, I think the phrasing is poor -- I'd prefer to say that nothing can accelerate because it requires infinite force, and that infinite force is required because of infinite mass. On the other hand, f=ma is all in one equation, so it's not as though the K-Pax phrasing is wrong. — Lomn 16:07, 11 January 2008 (UTC)

No, the relativistic mass (which is different from the rest mass) is simply a measure of an objects' resistance to acceleration in the presence of external forces. So another way of stating Einstein's conclusion is: "As an object increases in speed towards the speed of light, the amount of effort required to accelerate it further becomes arbitrarily large." Nothing internal to the object physically changes, but outside observers would note needing ever increasing amounts of energy to deliver small increases in the objects speed. In the parlance of physics, if the amount of acceleration achieved for a given amount of force is decreasing than that is equivalent to saying that the object's effective mass has increased. Dragons flight (talk) 16:14, 11 January 2008 (UTC)

Hmm, I see. Thank you. Could we also say that the amount of energy required to accelerate it to the speed of light would be infinite? 64.236.121.129 (talk) 16:36, 11 January 2008 (UTC)

Yes. Someguy1221 (talk) 16:41, 11 January 2008 (UTC)

Another way to think of this is as follows: The equivalence between matter and energy means that as you accelerate an object and it acquires kinetic energy, at relativistic velocities, that kinetic energy becomes quite apparent as mass and you'll need still more force to further accelerate this additional mass.

Atlant (talk) 17:03, 11 January 2008 (UTC)

The rest mass of a photon is zero. Any mass that you try to attribute to it (hf/c^2)is purely due to its energy which is hf. If you try to accelerate a photon, its energy increases but not its speed. Conversely if you try to slow down a photon, its energy is reduced (red shift) but its speed remains the same. There is no problem for the photon in travelling at c. In fact it must do so.

The wave version of light travels at the speed of em radiation in the medium (vacuum) and is sqrt (μ/ξ). no amount of pushing is going to speed up the photon. Any energy applied in trying just increases the frequency (or energy)--TreeSmiler (talk) 17:08, 11 January 2008 (UTC)

Also, in K-Pax, I think he mentioned that an object already moving faster than the speed of light can be accelerated to many times the speed of light. He may have been referring to the tachyon. That would be an understatement, as there's nothing keeping you from accelerating the tachyon to the point where it reaches its destination the exact moment it leaves, or even earlier (though the latter would be equivalent to a tachyon going the other direction). In fact, from some point of reference, any tachyon is moving instantaneously or back in time. — Daniel 23:15, 11 January 2008 (UTC)

Yes, because mc²/√1-v²/c² where m is the mass of the object and c is the speed of light and v is the velocity of the object. this formula tells you the kinetic energy which approaches infinity at light speed. read the book "Relativity" by Albert Einstein.

Three Phase 600V systems in Canada

I would like to know why Canada seems to be in the only country in the world that uses 600 volts in medium voltage systems?

The "myelectrical" wiki shows that only Canada out of hundreds on countries uses 600V systems. Was there a Canadian company that standardized to 600V in the history of the Canadian Electrical Systems, or some other factor that drove this use of 600V?

Any answers, thoughts, comments, or discussion is greatly welcomed.

Gleask (talk) 21:20, 11 January 2008 (UTC)

in the US, at least, 600v is an upper limit for one level of insulation. Edison (talk) 03:31, 13 January 2008 (UTC)

We like the smaller conductor diameter in Canada.

Physics of playground swing

Why does "pumping" your legs create a faster and faster swing on a playground swing? 96.224.41.170 (talk) 18:35, 11 January 2008 (UTC)

Half of the answer is at Resonance#Examples, which explains why the pumping tends to push you gradually higher & higher. As for why the leg-swinging makes you go, here's how I would picture it. As you're swinging, and you come to a stop at the back of the swing, you throw your legs forward. What this does is move your center of mass a little higher, so you've effectively lifted yourself a bit higher. The opposite happens at the front, and each time, your energy increases a bit. That's certainly not the most technical description, but it seems to make sense to me. jeffjon (talk) 18:41, 11 January 2008 (UTC)

Hmmm. The more I think about it, the more I think that the "center of mass" bit of my answer might be fishy. Anyone? jeffjon (talk) 18:42, 11 January 2008 (UTC)

I think a good way to think about it is to imagine the swing-person system as a double pendulum. As you use your muscles to swing the lower pendulum (your body), that energy couples into the upper pendulum and eventually excites its motion. It's a bit of a simplification, but that's the general idea.. As Jeffjon mentioned, this is only practical at the larger pendulum's resonant frequency. --Bmk (talk) 19:31, 11 January 2008 (UTC)

Try: http://www.phy.cuhk.edu.hk/phyworld/articles/swing/swing_e.html Dragons flight (talk) 19:40, 11 January 2008 (UTC)

Looking for Standard/Info on Food vacuum packaging

Hi. I'd like to not have to pay a Food Technologist for this information if that's okay:

I'm looking for info on food preservation by vacuum packaging. Is there a standard available?

Food gets preserved by removing oxygen which causes mould and fungi and allow microbes to reproduce.

Any help is appreciated
Rfwoolf (talk) 20:49, 11 January 2008 (UTC)

What country are you interested in? I saw on your user page that you are South African, but if you are actually in Britain you might be interested in BRC/IoP Global Standard - Food Packaging and other Packaging Materials. The acronyms (for those that don't know) stand for British Retail Consortium and the Institute of Packaging. SpinningSpark 21:46, 11 January 2008 (UTC)

Thanks SpinningSpark. I'm in Australia for a while, but the standard doesn't have to be for a particular country because this is not necessarily about complying with legislation. Any particular advice in getting the standard you mentioned? Thanks for the help Rfwoolf (talk) 08:01, 12 January 2008 (UTC)

Check out their website at this page[2]. There may be a charge for a copy though. SpinningSpark 10:21, 12 January 2008 (UTC)

Nature is funny—"everyone knows" removing oxygen prevents growth of stuff. But oops, what about all the stuff that is anaerobic, like Clostridium botulinum? DMacks (talk) 22:23, 11 January 2008 (UTC)

Nimur (talk) 22:46, 11 January 2008 (UTC)

The question itself appears to state this as a common-knowlege premise. DMacks (talk) 05:41, 12 January 2008 (UTC)

Yes I'm aware that a low-oxygen environment can cause certain bacteria to grow which can be potentially dangerous. One would hope this would be outlined in a standard, or a study that uses the said standard. In vacuum packaging, the "plastic" bag has a low oxygen transmission rate, but still actually permeates oxygen, but what OTR (Oxygen Transmission Rate) would prevent anaerobes like Clostridium botulinum? Should one use a bag that has an extremely low OTR which would strongly assist long shelf life but also encourages anaerobic organisms? These are relevant questions Rfwoolf (talk) 08:08, 12 January 2008 (UTC)

No, I don't think any noticeable amount of oxygen gets into a vacuum-packed bag. It wouldn't be a vacuum anymore, would it? As to what keeps C. botulinum from growing, my guess would be lack of water -- vacuum-packed foods are ordinarily dehydrated as well, which means you don't really have to protect them against microbes (nothing grows without moisture) but only against chemical degradation such as oxidation. --Trovatore (talk) 04:41, 13 January 2008 (UTC)

Hmm, actually the article on vacuum packing suggests it's sometimes used even for non-dry foods. That's a little surprising. But in that case I suppose you'd want to take the same precautions as in canning -- either make sure it's too high in acid for C. botulinum to grow, or else heat it to a temperature sufficient to kill all the spores, and then package it before any more spores can get in. --Trovatore (talk) 05:01, 13 January 2008 (UTC)

...and I guess I had the time sequence wrong there; according to canning the heat needs to be used after the food is sealed in the container. I guess that makes sense; otherwise spores might get on the food at the last minute and remain alive after the can is sealed. --Trovatore (talk) 05:18, 13 January 2008 (UTC)

A quick search [3] came up with this ancient study [4] & this newer one[5]. Perhaps this is the best[6] which mentions a variety of means including cold storage, heat treatment and chill storage etc Nil Einne (talk) 23:03, 15 January 2008 (UTC)

Even though I've asked for a standard on food packaging, your search strategy (Botulinum) and its subsequent results are nonetheless very helpful. Thank you very much. I would still be open to any other findings on food packaging standards, but for now this helps a great deal. I am still trying SpinningSpark's suggestion but alas you have to register and they haven't activated me yet :( . CheersRfwoolf (talk) 13:38, 16 January 2008 (UTC)

Academia and standardization buzzwords

In academia and standardization, one often encounters the following groups:

• technical committee
• council (standardization)
• working group
• task force (standardization)
• possibly more (can you name them?)

Is there some sort of logic behind what is what? I'd be great if there was a wiki article about this topic in general, and/or articles for the specific groups. Thank you, --Abdull (talk) 22:13, 11 January 2008 (UTC)

Consortium [7]? Nimur (talk) 22:47, 11 January 2008 (UTC)

I suggest moving this to the language refdesk.BrainyBabe (talk) 13:26, 12 January 2008 (UTC)

I'd say the folks here at the science reference desk are more involved with this topic than the one at the language desk. --Abdull (talk) 14:14, 12 January 2008 (UTC)

The Acidity of stomach acids in other animals

I have several questions relating to the potency and acidity of the digestive fluids in animals other than humans. Do animals like snakes, eels (or other fish), or mollusks, use the same acids as humans? Are they more or less acidic? or do they even use acids to break down their foods at all - do they use other methods? The article on stomach only refers to a human stomach, and the articles on the aforementioned animals don't mention any details about their systems. So I'm wondering if the RD community can shed light on the subject. — Kjammer  ⌂ 22:47, 11 January 2008 (UTC)

Someone correct me if I'm wrong, but I believe that the stomach doesn't secrete any particular acid; it merely secretes H+ ions, and Cl- happens to be the major counterion present. The variance between species would be in the concentration of H+, not in the "type" of acid. I think that the use of proteases is pretty universal, although some more primitive organisms might not use acid. 18.96.7.74 (talk) 00:28, 12 January 2008 (UTC)

You are correct, as Gastric acid explains. Rockpocket 00:55, 12 January 2008 (UTC)

Normally, mice have a higher gastric pH than humans and leopard sharks (3.1-4.5 vs. 1.5-3.5) [8] [9]. Humans, and rats and leopard sharks have all been shown to secrete gastric acid continuously. In comparison dogs, nurse sharks and Burmese pythons cease acid secretion when not feeding (presumably this is something to do with the fact these animals eat less frequently). In bony fish, lower gastric pH is observed in carnivorous species (5.3 and 5.5), while omnivorous species the gastric pH ranged from 5.5 to 7.0. Mollusks vary, but typically have a mildly acidic gastric sac. Rockpocket 01:23, 12 January 2008 (UTC)

7.0? I always thought stomach acid had to be, you know, acidic. How does it digest anything? — Daniel 02:00, 12 January 2008 (UTC)

Catla catla has a stomach pH of 7.02., though the oesophagus and parts of the intestine are slightly more acidic (~5). I guess it doesn't do much acid digestion in the stomach (indeed very low levels of acid protease was found there). Its stomach had higher levels of amylases and cellulases, probably indicating the function of gastric digestion in this species. Rockpocket 02:27, 12 January 2008 (UTC)

Great info, Rockpocket. Slightly off topic, but I remember reading as a kid about a carnivorous fish (possibly a type of snake mackerel), which digests the fish it eats almost instantaneously. Any idea what fish this was and what type of pH its stomach has, if the fish exists? --Wirbelwindヴィルヴェルヴィント (talk) 21:56, 12 January 2008 (UTC)

Sorry, Wirbelwind, I can't find anything on this specifically. A lot of carnivorous fish can digest food rapidly (probably because their prey consists of mollusks or micro-crustaceans, which are relatively easy to digest). For example, some fish have been estimated to fill their stomachs up to 20 times per 24hr period, which of course suggests they can empty the stomachs the same number of times. However, thats still some way away from instantaneous. According to the literature the fish with the fastest digestion rates are the migratory tuna, such as the Northern bluefin tuna. They appear to have a nifty adaptation that permits them to specifically increase their stomach temperature to around 10 degrees (C) above ambient, which aids rapid digestion (Carey, F. G., J. W. Kanwisher, and E. D. Stevens. 1984. Bluefin tuna warm their viscera during digestion. J. Exp. Biol. 109:1-20.) Rockpocket 03:39, 14 January 2008 (UTC)

Minimum input conditions for human life

What is the minimum amount of food and water necessary for human life (even a rather unpleasant and under-nourished one)? I'd assume that the food would be something like enough to maintain the basal metabolic rate of a person without any fat reserves, but that's just a layman's guess. What about water? Note: this is not at all a "medical question", I'm just curious. And I know, of course, that these things will vary, I'm just looking for averages, standard figures, etc. --24.147.69.31 (talk) 22:55, 11 January 2008 (UTC)

You will find some figures in the Survival skills article. As you say, it depends hugely on conditions: far more water is required in a hot desert: more calories are required if heavy manual labour needs to be maintained. SpinningSpark 23:38, 11 January 2008 (UTC)

Thanks—that's actually just sort of context I was thinking about. --24.147.69.31 (talk) 16:46, 12 January 2008 (UTC)