Which one of the following is correct? If both - what is the difference?

  • In an optically thin gas ...
  • In optically thin gas ...

For example:

  1. We consider radiative equilibrium in (an) optically thin gas.
  2. In (an) optically thin gas the condition of radiative equilibrium reads...

'In 'an'optically thin gas' implies a definable though at-this-stage unspecified gas'. 'In optically thin gas' would cover any gas which is 'optically thin'.

  • Nice succinct distinction. – FumbleFingers Nov 3 '13 at 16:17
  • Not sure it's true, though. Any gas which is optically thin, specified or unspecified, would, I think, nevertheless take an article. I think it is more an issue of countable and uncountable nouns. – Michael Broder Nov 3 '13 at 23:31
  • @MichaelBroder How is it that the same noun is countable in some senses but not in others? 'A cold wind from the north was blowing' versus 'cold wind makes me ill'? – WS2 Nov 5 '13 at 7:32
  • @WS2 Thanks. Regrets, I can't vote it up so far... – magnetar Mar 10 '15 at 16:44

To some extent this may be a case of the accepted usage in the field in question, which I assume is chemistry, or perhaps physics.

I say this because, just based on my ordinary understanding of the noun, "optically thin gas," I would say you do in fact need the indefinite article ("an") in both of your examples. And I would say that, if you take out the "an," you want to make "gas" plural ("gases").

So what I'm saying is, based on my non-specialist understanding of what these sentences say, you would always want to say "a" or "the" with the singular ("optically thin gas") and omit the article with the plural ("optically thin gases").

Another way of putting this is that it depends on whether, in your field or context, "gas" is being used an a countable or uncountable noun. If it's a countable noun, then you'd want to use an article with the singular, and no article with the plural. If it's an uncountable noun, you would omit the article in the singular. Sometimes nouns can be both countable and uncountable, with subtle differences in meaning. If this is a case like that, then when you use the article, you are referring to the countable noun (a specific gas, one of many gases that can be counted). When you omit the article, you are referring to the uncountable noun (gas in general, like "love" or "money" or "truth" or "beauty" or "justice").

  • Here you seem to accept the point that some nouns can be both countable and uncountable, as per my example in the comment above. – WS2 Nov 5 '13 at 11:30

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