NOTES. 479 



that fixed air was an oxide of carbon. Whether it was an 

 acid oxide or not, could not be determined 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. 117, 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 profound 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 mis- 

 print (apparently) of friction for torsion, the other the con- 

 fining 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 distortion, I cannot comprehend, nor can the learned 

 Professor himself, whom I have consulted. I say nothing 

 of a similar charge respecting the Torricellian experiment, 

 except to observe, that my reference to it is most studi- 

 ously framed to exclude the very construction put upon it 

 by the critic, as the sentence beginning " unless" must 

 plainly show to any candid reader. 



Now I write with great and unfeigned personal respect 

 for the learned critic, who, had his work been given under 

 the sanction of his name, would have been more careful in 



