NOTES. 513 



Alba,' &c., p. 50.) The whole page might be quoted. Nothing 

 could be more satisfactory to a chemist than this statement. 

 The modem definition of an acid is e 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 power- 

 ful of them; for example, in silicic acid. The Reviewer's 

 reference to Lavoisier is quite meaningless. The French 

 chemist shewed that fixed air was an oxide of carbon. 

 Whether it was an acid oxide or not, could not be deter- 

 mined by analysis. That problem could be solved only by 

 ascertaining whether or not it formed salts by combining 

 with bases. That is the only method possible at the present 

 day, and was the one Black followed." 



So very easy is it for ill-informed and inaccurate writers to 

 launch charges of ignorance and inaccuracy and carelessness 

 against others ! M. Arago will no doubt be fully sensible of 

 this truth, though he will furnish no example of it in his own 

 person or in his defence of himself. 



As for the mysterious passage in p. 11*7, which states that 

 the critic had prepared a commentary on my account of 

 Mr. Cavendish's experiment regarding the density of the 

 earth, but that, possibly through pity towards a fellow crea- 

 ture, he suppressed it, giving, however, as the result, that 

 it would show " the most ingenious and entire distortion, not 

 merely of nearly every step in the process itself, but of nearly 

 every principle involved in it," I can only, with all humility, 

 but with all comfort, mention, that the passage is none of my 

 own, being taken very closely from the work of a most pro- 

 found mathematician, professor of the science in one of our 

 Universities; and that, in borrowing it, I find that I have 

 avoided two errors in the original, one the misprint (appa- 

 rently) of friction for torsion, the other the confining the 

 comparison to the time of the oscillation, whereas I make 

 it general, including therefore both the length and the 

 duration. I wrote the account at a distance from Mr. 

 Cavendish's paper, and therefore took it at second hand. If 

 friction is intended, and not torsion, in the account which 

 I copied, it is an omission certainly. How it can be called a 



2 L 



