Wikipedia:Reference desk/Archives/Science/2017 March 7

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March 7

Containment building

If the outward pressure from steam in a limiting accident is the dominant force, containments tend towards a spherical design, whereas if weight of the structure is the dominant force, designs tend towards a can design. Modern designs tend towards a combination.

What exactly is meant by "the outward pressure from steam in a limiting accident" in this caption? What do "outward" and "limiting" refer to?--Hubon (talk) 02:40, 7 March 2017 (UTC)

• Looking at the PWR section of the article, the "limit" appears to be the pressure caused by the steam explosion that occurs when an accident causes the PWR to be suddenly depressurized. Other reactor types do not have this failure mod and therefore have lower (or at least different) containment constraints. A sudden massive increase in internal pressure is best handled by a large-volume spherical enclosure. -Arch dude (talk) 04:41, 7 March 2017 (UTC)

A limiting accident is also called a Design Basis Accident (DBA). Basically the DBA for each system is the accident or set of accidents that have the most severe consequences for the onsite workers or the general public. The DBA is an accident that could possibly happen, but is also not likely to happen during the entire lifetime of the nuclear facility. The designers look at the DBA and then design the building or tanks or pipes etc to still prevent leaks of significant (as set by regulation) radioactive material. The DBA should "bound" or be more severe than any other accident.

In the picture, for a PWR the limiting or bounding accident is a steam explosion and so the containment vessel must be able to stay intact in the case of overpressure inside. If you aren't worried about pressure blowing the top off your facility, but want it to be lighter (and cheaper) so you don't have to worry about collapse (from earthquakes etc) then you can go with a flat design.Tobyc75 (talk) 19:18, 7 March 2017 (UTC)

Puncturing a hovercraft

The hovercraft article speaks of their utility in covering difficult ground, such as swamps. But wouldn't this risk punctures, e.g. from trees and other pieces of wood significantly larger than in #Puncturing an automobile tire above? Maybe I don't understand the concept well enough, but if the thing depends on an air-inflated skirt, it seems that unintentional deflation would cause significant effects. Nyttend (talk) 12:21, 7 March 2017 (UTC)

The article doesn't discuss deflation, but given how the thing works, it sounds like it would take a catastrophic compromise of that skirt to disable the craft. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:44, 7 March 2017 (UTC)

The skirt is completely open at the bottom (hence skirt) and the "inflation" engine runs continuously at high power to lift the hovercraft. Small punctures would not do much harm. In many designs sides of the skirt aren't even air-tight. Jahoe (talk) 12:54, 7 March 2017 (UTC)

"The skirt is made of a bespoke material - neoprene coated nylon which is resistant to salt water, UV and tearing and has excellent wear characteristics. It's amazing stuff, very, very resilient..." Frequently Asked Questions about buying a Hovercraft Alansplodge (talk) 11:27, 8 March 2017 (UTC)

Hovercraft are characterized by having a very large lifting area - typically equally to the total area within the perimeter of the vehicle. Therefore the pressure difference relative to atmospheric pressure required to lift the weight of the vehicle is surprisingly small. Consequently, hovercraft perform satisfactorily despite their skirts not providing air-tight seals with the ground (or water) over which they are operating. Dolphin (t) 12:19, 8 March 2017 (UTC)

Our article mentions it, some hovercraft don't have skirts at all, but apparently such designs only perform well on flat surfaces. Someguy1221 (talk) 12:24, 8 March 2017 (UTC)

Strict definition of mutagenesis

Regarding the word "mutagenesis", can gene editing by inducing homology-directed repair and changing one specific nucleotide to another be called mutagenesis?

I thought that maybe mutagenesis would imply a lack of such control so you'd maybe get an A, C, G or T but site-directed mutagenesis suggests to me that this is not the case and mutagenesis can mean any change at all, intended or unintended. --129.215.47.59 (talk) 16:47, 7 March 2017 (UTC)

Mutagenesis is the generation of a mutation. Definitions of "mutation" are broad for example, "the changing of the structure of a gene, resulting in a variant form that may be transmitted to subsequent generations, caused by the alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes." By that definition I would say what you are describing is mutagenesis. 204.28.125.102 (talk) 19:10, 7 March 2017 (UTC)

• This is more a language question than a science question. "Mutagenesis" derives from "mutate", which historically has a connotation of accidental change as opposed to deliberate change. However, in the English language, word meanings mutate, and we have no centralized authority to define word meanings: meanings are what users make them, and "mutagenesis" has mutated to include some instances of deliberate change. So, to answer your question: since people are calling it that, yes, it can be called that. -Arch dude (talk) 03:34, 8 March 2017 (UTC)

wikt:Mutation is simply change. It doesn't matter how that change is procured - it's a mutation. If you start with a cloned bacterium, you ship it from one lab to another, grow it up for many generations, and you find a change in the sequence, it's a mutation -- even if you have no idea how, where, or why that change came about. This is the normal situation - actually knowing how the mutation was produced is a rare luxury. Wnt (talk) 13:21, 8 March 2017 (UTC)

Are certain hominids going extinct because their natural habitat is destroyed by climate change and agriculture?

Some hominids live in the rainforest. Will these rainforest dwellers go extinct or be forced to assimilate themselves into the human society? Are they really human? Are they able to reproduce with humans? 107.77.192.78 (talk) 18:10, 7 March 2017 (UTC)

Wikipedia has an article titled Hominidae. That article answers most of your questions. Of special note, however is that only homo sapiens are considered to be human, other human species (all members of the genus Homo) have gone extinct. There are no known human hybrids with other hominids, though some mythical attempts to do so are noted at the article titled humanzee. Other questions related to the legal status of the Great Apes are noted at Great ape personhood. --Jayron32 18:29, 7 March 2017 (UTC)

Modern humans are hybrids with other hominids, namely Neanderthals and Denisovans. StuRat (talk) 23:45, 7 March 2017 (UTC)

I assume Jayron meant, there are no human hybrids with hominids outside of homo. Though there is also doubt that those two you mention were actually different species, which our articles mention. Someguy1221 (talk) 23:55, 8 March 2017 (UTC)

I do believe at least one hominid, probably the subspecies known as troll, has not only escaped from its habitat but learnt to use wiki. Greglocock (talk) 18:36, 7 March 2017 (UTC)

'Learnt to use wiki' I would say is a relative term. Infinite monkey theorem--Aspro (talk) 23:35, 7 March 2017 (UTC)

Trolls are not natural but magic. They may seem human like but they are not. Extinctions are unclear in most cases. Also many species simply have evolved and thus live on. In these cases extinction may be a missleading term to describe what became of what they where befor. --Kharon (talk) 00:54, 8 March 2017 (UTC)

The trolls have it. To begin with, populations of the Western chimpanzee have fallen 80% [1], leaving open the possibility they may soon only survive in captivity, which for some recognized performing chimps is "assimilated into human society" much more than some human prisoners. And as Jayron noted, efforts toward "great ape personhood" imply a legal recognition of human status. I don't know if any jurisdictions currently recognize their right to pursue interspecies relationships, but it seems a logical enough deduction to make from that, and when raised in captivity I would not be surprised if chimps come to find humans attractive. (Some say dogs do...) Pregnancy from these relationships is not guaranteed (then again it never is), but it certainly seems plausible (not to mention fun) to find a way to increase the odds by technical means. Wnt (talk) 12:58, 8 March 2017 (UTC)

Help identifying caterpillars

Hi, I would appreciate help identifying the following two caterpillars:

• 
  1
• 
  1
• 
  1
• 
  2
• 
  2

Both pictures were taken yesterday at Beit Guvrin National Park in Israel. Thanks in advance, --SuperJew (talk) 18:18, 7 March 2017 (UTC)

It doesn't seem to match any of the little beasties here. You might try a Google image search like this, though I didn't immediately see anything promising. Matt Deres (talk) 03:20, 10 March 2017 (UTC)

Thanks for the site Matt Deres! Looks maybe like 1 is Ocnogyna loewii and 2 is Simyra dentinosa? --SuperJew (talk) 10:50, 10 March 2017 (UTC)

What it means to be molecular

I was listening to a science podcast which was using the phrase "molecular nitrogen". From the context, I'm guessing that this is just gaseous nitrogen and not liquid or solid. Am I correct? †dismas†|^(talk) 18:41, 7 March 2017 (UTC)

Our nitrogen article uses the phrase "molecular nitrogen" to indicate N₂, nitrogen in pure-but-molecular form (as opposed to either N, which does not exist freely in normal conditions, or NO, which is molecular but not solely nitrogen). It does not appear to be restricted to the gaseous phase. — Lomn 18:57, 7 March 2017 (UTC)

To expand a bit, I've heard it used before with other elements. So, I guess I'm asking if it has a general meaning among scientists. †dismas†|^(talk) 20:04, 7 March 2017 (UTC)

Right, the general sense is to distinguish it from bound forms, or rather, form when it is bound in compounds with non-N elements. At nitrogen we have "Free nitrogen atoms easily react with most elements to form nitrides". "Free" means unbound, so e.g. N2 gas is not "free", but it is "molecular". N2 can be referred to as "molecular" whether it is liquid, solid, or gas. Phase of matter doesn't come in to it. The idea is that when we say "molecular X", we mean that only X occurs in the molecule. Here [2] is the Chemspider entry for "molecular nitrogen". I cannot yet find a concise RS definition of this sense of "molecular", but I'll post it if/when I can find it. SemanticMantis (talk) 20:34, 7 March 2017 (UTC)

"Molecular" means it is in the form of or composed of discrete molecules. The exact meaning can be hard to pin down because different fields of study have slightly different definitions of "molecule" or "molecular substance". But it's often used as an adjective by way of contrast to individual non-bonded atoms (the nitrogen examples mentioned above), and sometimes even as a distinction of covalent bonding rather than ionic. By that division, sucrose would be a molecular substance whereas sodium chloride is a collection of individual ionic atoms and graphite is a network solid. DMacks (talk) 20:42, 7 March 2017 (UTC)

Nitrogen is almost always in a molecular state as we experience it. Even when liquid or frozen to a solid it will contain molecules. Under extreme conditions it may be in an atomic nitrogen state (when very hot say in a star), or in a covalent network solid, when under extreme pressure. See solid nitrogen for more detail and references. Graeme Bartlett (talk) 01:17, 8 March 2017 (UTC)

• Just for some quick context (from a career chemistry teacher): a substance's atoms can be organized many different ways. They can be arranged as:

• single atoms (see noble gas)
• as molecules (which are discrete particles composed of atoms held together with covalent bonds)
• as ionic compounds (which are networks of positive and negative ions, like sodium chloride)
• as network solids (which are networks of atoms held together by covalent bonds, rather than discrete molecules. See graphite for example)
• as metals (which are networks of atoms held together by metallic bonding.)

• That probably covers most forms of what we call "normal matter". There are few exotic types of matter (Bose-Einstein condensate, neutron star, etc.) that are different yet, but you'll never hold that stuff in your hand. When we say "molecular nitrogen" we mean nitrogen in the form of discrete N₂ molecules; most nonmetals form diatomic molecules in their native, gaseous state. --Jayron32 12:51, 8 March 2017 (UTC)

• And to add on, the other "forms" nitrogen could take would be the rare gaseous atom, the nitride ion, the azide ion, or parts of molecules or polyatomic ions with other atoms, such as the nitrate ion or nitrogen dioxide, etc. --Jayron32 13:24, 8 March 2017 (UTC)

• If you're going to include anions, better include cations like like pentazenium also. Don't forget tetranitrogen (neutral or cationic). If four nitrogens is not enough, how about hexazine? Octaazacubane is a fun (though currently only theoretical) case. @Andy Dingley: what are the chances that two unrelated RD/S items make references to such an unusual structural form? DMacks (talk) 22:07, 8 March 2017 (UTC)

• The examples, as should have been obvious, was meant to be representative rather than exhaustive. A complete list of different moieties that are known to involve nitrogen would go on for volumes and would quickly overwhelm this entire page. Thanks for adding a few random more examples as well. --Jayron32 03:45, 9 March 2017 (UTC)

• The point here is not "involve nitrogen" but that are just nitrogen (allotropes). Octaazacubane is exactly "molecular nitrogen". DMacks (talk) 08:31, 9 March 2017 (UTC)

Thank you, everyone!! †dismas†|^(talk) 20:11, 8 March 2017 (UTC)

Washing produce

I am trying to research the topic of washing ones fruits and vegetables before human consumption. From simple google searches, the results from "reliable?" sources are contradicting. Is it important to wash produce? From my research the contradictions are related to the preponderance of pesticides, harmful bacteria, and dirt found in produce. Some say these have no meaningful effect on human physiology, some say they do. (like how there is lead in chocolate or polonium in produce but no one cares because the poison is in the dosage). (I do live in northern california, which is where my produce comes from, if that helps at all) Second question: Are fruits/vegetables more contaminated in some areas than others? (Is some fruit cleaner than others based on geographical location?) 64.170.21.194 (talk) 23:29, 7 March 2017 (UTC)

One should, must, wash all produce if only to avoid Escherichia coli. Look it up. There are many thousand of out-breaks a year, even in developed countries with strict food hygiene regulations. You can die a horrible death from the more virulent strains.--Aspro (talk) 23:46, 7 March 2017 (UTC)

From our E. coli article: "commonly found in the lower intestine of warm-blooded organisms", not on the surface of fruit. Of course, anything is possible, but other contaminants, like insecticides, seem far more likely. Veggies which grow on the ground are another matter. StuRat (talk) 23:52, 7 March 2017 (UTC)

That's (once again) a shared brain fart. See e.g. 2006 North American E. coli O157:H7 outbreak in spinach.--TMCk (talk) 00:21, 8 March 2017 (UTC)

You do know, Stu, that vegetables and fruits are grown in soil treated with manure which was once in the lower intestines of warm-blooded organisms, at one time. Shit goes in dirt. Strawberries go in dirt. Shit gets on strawberries. Shit gets in you. --Jayron32 02:36, 8 March 2017 (UTC)

And now Stu has, of course, altered his comment after the fact as if... ah well, the usual...!--TMCk (talk) 03:28, 8 March 2017 (UTC)

That is not an alteration of comment that matters. Bus stop (talk) 14:39, 8 March 2017 (UTC)

Of course it was. TMCk had already replied when the comment was altered giving a spinach outbreak as an example. StuRat's original comment was replying to a discussion about fruit and vegetables and they gave no indication they were making a distinction about fruit vs vegetables other than not mentioning vegetables; nor did they give any indication they were making a distinction between how close to the ground the product grew. TMCk gave an example of a vegetable contamination which seems quite relevant to what StuRat had said in light of the thread. StuRat then modified their comment without mentioning this, making it seem like, when TMCk had replied giving a vegetable example, StuRat had already indicated that they were not mentioning vegetables and had already suggested stuff growing closer to the ground is more likely to be contaminated. Nil Einne (talk) 14:50, 8 March 2017 (UTC)

The editor added a distinction between "fruit", many of which grow in trees, and vegetables that grow on the ground. Strictly speaking, that is all they did. My concern was about a perceived instance of bullying. Maybe that is a problem in my perception. But every infraction does not have to be pounced upon. Bus stop (talk) 15:00, 8 March 2017 (UTC)

Yes but that distinction was not there when existing reply which gave vegetables was provided. Therefore, someone reading the thread will be mislead to believe that the respondent ignored the fact that StuRat had already made a distinction between vegetables which grow in the ground and fruit, when they could not because that wasn't part of what they read. This infaction is in fact far worse then the normal crap StuRat pulls. When they just say whatever crap comes to their mind without sources, that's annoying but at least everyone can see that even if non-regulars may not be aware they should not trust what StuRat said. When StuRat makes it sound like people didn't properly read what they said, that's a serious problem. StuRat themselves should know that it's a serious problem since they complained when an IP did it elsewhere, although in that case the clarification wasn't needed anyway since a simple well understood term was used, but StuRat replied without knowing what the term meant. Still the edit shouldn't have happened in the way it did and StuRat was right to point out it happened, even if they were wrong that it was needed. (Another key difference there is that it was a question and not a response, so it's easier to understand an OP's concerns when they've made a simple clear question but they're getting irrelevant replies because people are responding when they don't understand the terms used in the question but think it best to just reply anyway. By comparison if you're responding to a question which discusses both fruit and vegetables, you should make clear if you think there is a distinction between fruit and vegetables and your reply only addresses fruit. StuRat didn't do this initially but then tried to fix this which would be fine if they had done so without being misleading.) Nil Einne (talk) 23:25, 8 March 2017 (UTC)

No, TMCK's comment was not relevant to mine, since I was talking about fruit and she was talking about veggies. I added to my comment, without altering anything, to make this clear. StuRat (talk) 15:18, 8 March 2017 (UTC)

It is not relevant if it's spinach as everything grown on the ground, and that includes fruit, can be contaminated.--TMCk (talk) 15:23, 8 March 2017 (UTC)

As I said, anything is possible, but contamination with E. Coli is far more likely to be a problem with items which directly come in contact with manure, than a fruit like apples, which grows well above the ground. I don't mean to blame your comment, more everyone else who "jumped on the bandwagon" to attack me after. There's a history of scapegoating me here, where everyone attacks me for anything which can possibly be misinterpreted as a mistake. StuRat (talk) 15:31, 8 March 2017 (UTC)

Apples? Is that so or is that just another brain fart to share? "Are there certain foods that increase the risk of getting E. coli?" "...commonly associated with E. coli: -unpasteurized apple juice or cider;" and also: "-contaminated raw fruit and vegetables that are not cooked...".--TMCk (talk) 20:09, 8 March 2017 (UTC)
Let's add a Listeria, Salmonella and E. coli Prompts Granny Smith Apple Recall just in case this retracted but neither-the-less preposterous demand is repeated.--TMCk (talk) 16:57, 11 March 2017 (UTC)

As TMCk has said, this example also seems to illustrate why it's always wise to check refs before replying unless it's a subject you definitely know (and not just one you think you know). Maybe it's true that vegetables have a higher risk of contamination. This study only look at a few different things [3] and only found 1 sample which would be called fruit, Cantaloupe, contaminated with general E. coli. And leafy vegetables seemed worse although leafy herbs even worse and green onions are unclear. (Leafy herbs being worse than vegetables seems definite in this study, but given the sample sizes, green onions, canteloupes and berries are not clearly better or worse than leafy vegetables or each other. Tomatoes which I presume we agree are fruit for this discussion were definitely better than leafy vegetables.)

However even if the claim is true, your response seems to now go further in suggesting it's not a concern with apples. But apples and particularly unpasteurised apple juice and ciders seem to be one particular case where E. coli contamination is a concern. In case you're thinking the contamination happens during the production of the juice, this [4] is a machine under development in 2002 to try and detect such contamination of the fruit. [5] suggests to possible routes are fallen apples and fruit flies. This [6] is another study (it's not used in the earlier source) which looked at contamination of apples by fruit flies. Of particular relevance to this discussion is they found that washing did not generally eliminate the contamination. This doesn't of course mean you shouldn't wash. Note one reason why outbreaks in juices and ciders may be more likely to be known, other than in growth in the juice/cider, is because it's more likely to cause a mass outbreak. If 1 in whatever low number of apples causes the person eating to get sick this is difficult to detect (the best you can really do is look at contamination rates as that earlier study did). For various reasons, vegetables seem more susceptible to causing mass outbreaks but that doesn't necessarily mean the risk of general harm is higher.

[7] while I'm not sure how good the source is, does quote someone they are calling an expert and raises concerns about contamination from other customers handling the products. It also interestingly suggests one in a third of 'food poisoning' could be from produce. This is one area where local norms could also be a big factor. Here in NZ for example, a lot of fruit including apples and others eaten with the skin on are handled and packed by the customer. By comparison, spinach, various bok choy variets etc are generally sold in a premeasured quantities sometimes in a simple bag by the store or whatever. (To be clear, I'm referring to a simple often open bag, not the sealed variety.) Other times they're just tied in a bundle but not bagged. Lettuce, cabbage etc are generally not bagged although some people will follow the advice and discard the outer leaves. In the cases where they are bagged, the risk of customer contamination would potentially be lower. Point being, it's possible the risk here is higher for fruit than for vegetables.

Nil Einne (talk) 00:20, 9 March 2017 (UTC)

The simple fact is, if you felt it was clear you were only talking about fruit, and were explicitly excluding vegetables which grew on the ground, despite the fact the whole discussion was about both, you would not have modified your response after the reply. At most, you would have offered clarification in a follow up reply. Or at least made it clearer that you modified your reply after it had received a response. You did not because you felt it okay to misleading people into thinking a person had ignored what you said when they didn't because you didn't actually say it. Nil Einne (talk) 23:18, 8 March 2017 (UTC)

For vegetables you cook thoroughly before eating, particularly by boiling, that would provide some cleaning and sterilization. For fruits where you discard the peel, there's even less concern. Fruits and vegetables eaten raw, with the peel, like grapes and tomatoes, are the area of most concern. Also note that rinsing in water may not be enough, as oil-soluble and wax-based chemicals may not rinse off easily. StuRat (talk) 23:56, 7 March 2017 (UTC)

It’s Time to Stop Eating Bagged Salads. Modern mass packaging insure that instead of one person or family becoming ill – every one consuming that contaminated batch becomes ill. --Aspro (talk) 00:13, 8 March 2017 (UTC)

Ignoring sensationalism resulting from the public scares about exceptions to general safety trends, bagged salads have more stringent guidelines than loose-leaf greens, loose-leaf greens are more likely to be contaminated by other shoppers, and more people are eating salads as a result of bagged salads. Even non-sensationalist sources that say that there are more risks still grant that bagged salads are safe. It's ultimately personal preference. Ian.thomson (talk) 03:01, 8 March 2017 (UTC)

• It is important to note that any fresh produce bought in an American grocery store will already have been washed very thoroughly, much more thoroughly than you can wash it at home unless you are really obsessive. So it comes down to a question of whether any harmful substances may have been deposited on it after it was washed. In most cases the likelihood of that is pretty low. Looie496 (talk) 14:25, 8 March 2017 (UTC)

I feel it is important to note that "fresh" means "non-packaged." Many packaged vegetables have "fresh" on the packaging. They are washed before packaging (at least they SHOULD be), but are not washed after that. There have been cases where they were washed with contaminated water before packaging. 209.149.113.5 (talk) 16:27, 8 March 2017 (UTC)

It depends on the product. TBH, I've never bought either celery or leeks that didn't have some dirt in it; to the point where if it didn't have at least a little dirt near the root end that needed to be washed off, I'd be suspicious that the product had been over handled. A lot of e coli contamination comes from repeated handling (which can lead to increased risk of cross-contamination). Generally, food is safer the less steps it takes to get from the source to your mouth. Here is an article on the problems with food safety and the global food supply chain. --Jayron32 15:58, 9 March 2017 (UTC)

Quote: “much more thoroughly than you can wash it at home unless you are really obsessive”. Is it obsessive, to just wash them under a running tap? Look at the guidance for food baggers Guidance for Industry: Guide to Minimize Microbial Food Safety Hazards of Fresh-cut Fruits and Vegetables. Look at all the weak links in the chain from field-to-table. Explain why the incidence of food poisoning is now increasing with bagged veg as suspect number one in the majority of cases reported. How can one even suggest that a running tap at home is less effective? Another editor said "Ignoring sensationalism resulting from the public scares". He hasn't obviously had serious food poisoning... because it makes you feel like you'd rather die than than suffer an hour longer. Others that have suffered this will back me up on what is is like. So to my mind is not scare mongering but alerting the populace, that ready to eat and what is printed on the bag are relative terms and caveat emptor--Aspro (talk) 17:53, 9 March 2017 (UTC).