Stahl's theory of phlogiston still held the field and enthralled 

 the minds of chemists. Black found that when mild lime was 

 burned, and caustic lime was formed, fixed air was given off. This 

 led up to the important test of Black, viz., that when this fixed air 

 is passed through a clear solution of lime-water a precipitate of what 

 we now know as carbonate of lime is formed, i.e., caustic lime 

 combines with fixed air to form mild lime (1757). 



He found that fixed air is given off during fermentation and in 

 the expired breath, and is formed when charcoal is burned. He 

 had rediscovered the gas sylvestre of Van Helmont. 



Black's remarks on Fixed Air are worthy of being quoted. 



" Here a new and boundless field seemed to open before one. We know not how 

 many different airs may be thus contained in our atmosphere, nor what may be their 

 separate properties. This particular one has evidently very curious and important 

 ones. ... I fully intended to make this air and some other elastic fluid the subject 

 of serious study. A load of new official duties was laid upon me [i.e., he was elected 

 Professor of Medicine and Chemistry in Glasgow]. In the same year, however, in which 

 my first account of these experiments was published -namely, 17571 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 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 twelve 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 vinous fermentation. Van Helmont had indeed said this, 

 and it was to this that he first gave the name gas sylvestre." (Treatise on Chemistry, 

 Vol. II., p. 87, 1803.) 



We have already referred to the work of Boyle, and following the 

 story of "fixed air" we next come upon the work of the HON. 

 HENRY CAVENDISH (1731-1810). 



" The great majority of the distinguished chemists of Great Britain have sprung from 

 the middle or lower ranks of the people, but two of the most famous of them, the Hon. 

 Robert Boyle and the Hon. Henry Cavendish, were men of illustrious lineage, and 

 Cavendish was much the more high-born of the two." 



"Twelve years after the publication of Black's paper, in 1766, Cavendish pub- 

 lished the first essay on Factitious Airs. He took up the investigation of fixed air 

 where Black and his pupils had left it, and examined in particular its properties when free, 

 on which Black had published scarcely anything." 



" The operations of his intellectual powers exhibit a degree of caution cavendo tutus 

 is the motto of the family almost unparalleled in the annals of science, for there is 

 scarcely a single instance in which he had occasion to retrace his steps or to recall his 

 opinions. (Three Papers containing Experiments on Factitious Air, Phil. Trans. 17G6, 

 p. 141.) It had been observed by Boyle, that some kinds of air were unfit for respiration ; 



