512 NOTES. 



in 1 766. My reason for so stating was my distinct recollection 

 of Dr. B. having in his lectures shewn us the experiment of 

 pouring fixed air out of a receiver on a candle, and his having 

 given this as a property originally known to himself when he 

 discovered the gas, though it is very true that the published 

 lectures do not decide either way the question of his early 

 knowledge. His not mentioning Mr. Cavendish or any one 

 else as having first taught it him is with me, who well knew 

 his scrupulous exactness in such matters, quite decisive of his 

 having himself observed it. 



I shall only cite further my correspondent's note on the 

 Reviewer's statement, Sf that I was wrong in ascribing to Dr. 

 Black the discovery that fixed air has acid properties." 

 (p. 110.) "The Reviewer adds that 'the acidity of fixed air 

 was indicated for the first time by Priestley and his fellow- 

 labourers, and only completely established by Lavoisier, who 

 shewed fixed air to be carbonic acid, or a mixture of carbon 

 and oxygen. 5 His Lordship is quite right, and the Reviewer 

 doubly and egregiously wrong. Priestley did not indicate for 

 the first time the acidity of fixed air. Whether he under- 

 stood Black's views concerning it does not appear, but he 

 expressly disclaims the discovery as his own. His words are, 

 ' It is not improbable but that fixed air itself may be of the 

 nature of an acid, though of a weak and peculiar sort. Mr. 

 Bergman of Upsal, who honoured me with a letter upon the 

 subject, calls it the aerial acid; and among other experiments 

 to prove it to be an acid, he says that it changes the blue 

 juice of tournesole into red. ('Phil. Trans/ 1772, vol. Ixx., 

 p. 153.) It does not appear whether Black was aware of the 

 reddening action of fixed air on vegetable colours, but he was 

 abundantly aware of the functions of fixed air as an acid ; that 

 is, of its power to neutralize bases, and to form salts by com- 

 bination with them. Black's own words are, ' These con- 

 siderations led me to conclude that the relation between fixed 

 air and alkaline substances was somewhat similar to the rela- 

 tion between these and acids j that as the calcareous earths 

 and alkalis attract acids strongly, and can be saturated with 



o j -' 



them, so they also attract fixed air, and are in their ordinary 

 state saturated with it.' ('Experiments upon Magnesia 



