116 Rev. W. V. Harcourt on Lord Brougham's statements 



sentaient a l'esprit de Cavendish. Ceci doit etre regarde 

 comme une tres grosse omission [a most material omission*)." 

 Nothing indeed can be more unfounded than this animadver- 

 sion. In the passage to which you refer, the words of Caven- 

 dish are these: — " All the foregoing experiments on the ex- 

 plosion of inflammable air with common and dephlogisticated 

 airs, except those which relate to the cause of the acid found 

 in the water, were made by me in the summer of 1781, and 

 mentioned by me to Dr. Priestley, who in consequence of it 

 made some experiments of the same kind, as he relates in a paper 

 printed in a preceding volume of the Transactions." Now you 

 need only have referred to the volume of the Transactions 

 which Cavendish quotes, to have found the " epoch" which you 

 wanted. Priestley's paper was printed in March 1783; and 

 therefore Cavendish's communication of his " conclusive" ex- 

 periments was anterior to Watt's speculations in April, as well 

 as to Lavoisier's experiments in June of the same year. 



But though this " most material" or in M. Arago's transla- 

 tion, this "grosse" omission turns out to be none, you ought, 

 I repeat it, to have remembered your own demand for pre- 

 ciseness of dates, when you ascribed to Black a prior know- 

 ledge of the distinguishing properties of hydrogen gas. In 

 proof that Black knew before Cavendish that this gas is *.* so 

 much lighter than atmospheric air" you allege, that " as early 

 as 1766 he invented the air-balloon, showing a party of his 

 friends the ascent of a bladder filled with inflammable air : 

 Mr. Cavendish only more precisely ascertained its specific 

 gravity." 



As early as 1766? — Are you not aware that Cavendish's 

 paper on factitious airs was published in this year ? Is it not 

 a " most material omission" that you have forgotten to li pre- 

 ciser I'epoque" of Black's experiment with the balloon, so as to 

 show whether it was before or after the publication of Caven- 

 dish's paper ? Professor Leslie tells the story of the balloon 

 somewhat differently from you. "The late most ingenious 

 and accurate Mr. Cavendish, in 1766, found, by a most nice 

 observation, this fluid to be at, least seven times lighter than 

 atmospheric air. It therefore occurred to Dr. Black of Edin- 

 bro', that a very thin bag filled with hydrogen gas would rise 

 to the ceiling of a room. He provided accordingly the allan- 

 tois of a calf, with a view of showing at a public lecture such 

 a curious experiment before his numerous auditors; but owing 

 to some unforeseen accident or imperfection it chanced to fail, 

 and that celebrated Professor, whose infirm state of health 

 and indolent temper more than once allowed the finest dis- 



* Historical note, Life of Watt, p. 383. 



