234 The Rise of the Modern [lect. 



Black moreover made another discovery. Using as a test 

 for, the presence of fixed air the fact that it, when driven 

 through a clear solution of lime water, i.e. a solution of caustic 

 lime, caused a precipitation, in consequence of its combining 

 with the caustic lime and converting it into mild lime, he was 

 able to prove that fixed air was given off in fermentation, was 

 a product of the burning of charcoal and was present in ex- 

 pired air. 



He thus rediscovered the gas which van Helmont had 

 discovered more than a hundred years ago. This is what 

 he says, writing some years afterwards in his Treatise of 

 Chemistry : 



" I fully intended to make this air (fixed air) the subject of 

 "serious study.... In the same year, however, in which my first 

 " account of these experiments (on magnesia, etc.) was published, 

 " namely 1757 (sic), I had discovered that this particular kind of 

 " air, attracted by alkaline substances, is deadly to all animals 

 " that breathe it by the mouth and nostrils together ; but that 

 r if the nostrils were kept shut I was led to think that it might 

 "be breathed with safety. I found for example that when 

 " sparrows died in it in ten or eleven seconds, they would live 

 "in it for three or four minutes when the nostrils were shut 

 "by melted suet. And I convinced myself that the change 

 " produced on wholesome air by breathing it, consisted chiefly, 

 "if not solely, in the conversion of part of it into fixed air. 

 " For I found, that by blowing through a pipe into lime water, 

 " or a solution of caustic alkali, the lime was precipitated, and 

 "the alkali was rendered mild. I was partly led to these 

 " experiments by some observations of Dr Hales, in which he 

 " says, that breathing through diaphragms of cloth dipped in 

 " alkaline solution made the air last longer for the purposes of 

 " life. 



" In the same year I found that fixed air is the chief part 

 " of the elastic matter which is formed in liquids in the vinous 

 " fermentation. Van Helmont had indeed said this, and it was 

 " to this that he first gave the name gas silvestre. It could 

 " not long be unknown to those occupied in brewing or making 

 "wines. But it was at random that he said it was the same 



