41 



periment it should seem either that charcoal contains hydrogen, or else 

 that the charcoal after distillation contained some oxygen." 



Speaking of the latter gas in this computation Cavendish uses all 

 the various terms, dephlogisticated air, pure air, and oxygen : I have 

 given in the Appendix a letter in which he assigns to Blagden his rea- 

 sons for disapproving of the introduction of a nomenclature entirely 

 new ; and unquestionably the term oxygen was open to the same ob- 

 jection with the old name of dephlogisticated air, as equally involving an 

 erroneous hypothesis : Cavendish shewed in his paper on air that oxy- 

 gen does not always acidify ; and it has since been proved that it does 

 not exist in all acids: he objected to the new language, that without 

 the advantage of being already understood, it involved, no less than the 

 old, theories which in the rapid progress of chemical discovery might 

 quickly be disproved : he thought the time for such a reformation of 

 terms not yet arrived : he was mistaken ; for with the new language a 

 new and more rigorous system of reasoning was introduced into com- 

 mon use, and by degrees chemists learnt from experience to correct 

 also their notions of nomenclature, and discovered that in expressing 

 undecompounded substances, the less significant is the name, the better 

 it serves the objects of science. 



Having now concluded my analysis of the chemical contents of 

 these MSS., I think it right to enable every one to judge for himself, 

 by lithographing the whole of Cavendish's experiments on the composi- 

 tion of water, and by giving a few extracts from those on other subjects. 

 I request the attention of M. Arago in particular, and of M. Dumas 

 who has adopted his views, to a letter addressed by Cavendish in Fe- 

 bruary 1785 to the editor of the Journal de Physique*, which will show 

 them how incapable he was of falsifying dates, or attempting to reap 

 fame from the mistake of a printer. It is to be regretted that Watt did 

 not take equal care to prevent the propagation of such mistakes. When 

 Dr. Black's lectures were published after his death, with a dedication 

 to Watt, he would have done well not to have allowed the history which 

 is there given of the discovery of the composition of water to pass 

 without correction; almost every statement and every date in that 

 history is a mistake, and the effect of those mistakes is to give prece- 

 dence to Wattf. Independent of other errors, the whole experiment 



* Appendix, p. 65. 



t The following is the account of this discovery given in Black's Lectures 

 as edited by Robison, with a dedication to Watt, in 1803. The errors are 

 printed in Italics. 



" Dr. Priestley was occupied with the examination of inflammable air, and 



