478 NOTES. 



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 

 Eeviewer's statement, " 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 Lavoi- 

 sier, who showed fixed air to be carbonic acid, or a mixture 

 of carbon and oxygen.' 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. 

 Wh ether he understood 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 combination with them. 

 Black's own words are, ' These considerations led me to con- 

 clude that the relation between fixed air and alkaline sub- 

 stances was somewhat similar to the relation between these 

 and acids ; that as the calcareous earths and alkalis attract 

 acids strongly, and can be saturated with them, so they also 

 attract fixed air, and are in their ordinary state saturated 

 with it.' (' Experiments upon Magnesia Alba,' &c., p. 50.) 

 The whole page might be quoted. Nothing could be more 

 satisfactory to a chemist than this statement. The modern 

 definition of an acid is ' a substance which neutralizes bases, 

 and by combination with them, forms salts.' Power to affect 

 vegetable colours, or sour taste, the vulgar attributes of an 

 acid, are wanting in many of the most powerful of them : 

 for example, in silicic acid. The Reviewer's reference to 

 Lavoisier is quite meaningless. The French chemist showed 



