relative to Black, Watt, and Cavendish. 117 



coveries when almost within his reach to escape his penetra- 

 tion, did not attempt to repeat the exhibition, or seek to pur- 

 sue the project any further." 



If you are dissatisfied with Leslie's version of your anecdote, 

 let me refer you to other authorities. In one of those articles 

 of the Encyclopedia Britannica which are stated to have been 

 composed or revised by Professor Miller, Dr. Muirhead, and 

 Sir David Brewster, the circumstance is thus narrated : — " In 

 the year 1766 Mr. Henry Cavendish ascertained the weight 

 and other properties of this gas, determining it to be at least 

 seven times lighter than atmospheric air. Soon after which 

 it occurred to Dr. Black that perhaps a thin bag filled with 

 hydrogen gas might be buoyed up by the common atmo- 

 sphere." 



I hope I have now illustrated sufficiently the value of the ca- 

 non of criticism which you have laid down for these delicate in- 

 quiries, — that nothing is so necessary as to "preciser I'epoque." 



" Cavendish," you say, " only more precisely ascertained 

 the specific gravity of inflammable air ; and showed, what 

 Black coidd not be ignorant of, that it is the same from what- 

 ever substance it is obtained" Now, in the first place, inflam- 

 mable air is not the same from whatever substance it is obtained. 

 This was the error into which Priestley fell when he attempted 

 to repeat the experiments by which Cavendish had discovered 

 the composition of water ; this was the error under which 

 Watt laboured till after the publication of Cavendish's paper 

 in 1784, and which nullified the researches of the one and 

 the speculations of the other. But supposing you to mean 

 "that inflammable air is the same, whether obtained from zinc 

 or iron," why do you say that Black could not be ignorant of 

 that? How do you think he was to know it? How did 

 Cavendish know it? He tells you that he learnt it by having 

 ascertained by experiment that the specific gravity of the gas 

 from either material was the same. Had Black ascertained 

 this ? Had he any test whatever by which he could know that 

 these gases were the same ? 



But Cavendish ii only more precisely ascertained the specific 

 gravity of inflammable air." If any person conversant in the 

 history of pneumatic discoveries were to be asked to enume- 

 rate the most important of the early advances in that branch 

 of science, he would certainly name — 1st, the discovery of the 

 weight of the air by Galileo ; 2nd, the discoveries of its law 

 of compression, and of the factitious gases, by Boyle ; 3rd, 

 the theory of the fixation of gases by chemical attraction, 

 propounded by Newton ; 4th, the discovery of specific and 

 elective affinities in one of those gases, by Black ; 5th, the 



