270 M. Arago's Biographical Memoir of James Watt. 



of its hydrogen, but still united to its latent heat and light ? 

 If light be only a modification of heat, or a simple circumstance 

 of its manifestation, or a component part of hydrogen, oxygen 

 gas will be water deprived of its hydrogen, but combined with 

 latent heat." 



This passage, so clear, so precise, and logical, is taken from 

 a letter of Watt's, dated 26th April 1783. The letter was 

 communicated by Priestley to several of the scientific men in 

 London, and was transmitted immediately afterwards to Sir 

 Joseph Banks, the President of the Royal Society, to be read 

 at one of the meetings of that learned body. Circumstances, 

 which I suppress as being foreign to the present discussion, 

 retarded the reading of the letter for about a year, but it re- 

 mained the while in the archives of the Society. It appears in 

 the 74tli volume of the Transactions, with its true date, April 

 26. 1783. It is there to be found embodied in a letter from 

 Watt to Deluc, bearing date 26th of November 1783, and is 

 distinguished by inverted commas, supplied by the secretary 

 of the Royal Society. I ask not indulgence for this profu- 

 sion of details, because it is clear that the minute comparison 

 of dates can alone bring the truth to light concerning a dis- 

 covery which confers the highest honour upon the human in- 

 tellect. 



Among those who put in their claims to be the authors of 

 this most pregnant discovery, we shall presently see appear- 

 iiii^ the two greatest chemists of whom France and England 

 c; u boast. As every one will anticipate, I speak of Lavoisier 

 a..d Cavendish. The date of the public reading of the memoir 

 iu which Lavoisier gives an account of his experiments, whereby 

 he explained his views upon the production of water by the 

 combustion of oxygen and hydrogen, is later by two months 

 than the deposit of Watfs letter already alluded to, in the 

 archives of tho Royal Society of London. 



The celebrated memoir of Cavendish, entitled " Experiments 

 upon Air," is still later, being read on the 15th of January 

 1784. It could not fail to be a matter of astonishment, that 

 facts so well authenticated should become the subject of an 

 animated controversy, were I not immediately to bring under 

 notice a circumstance to which I have not hitherto alluded. 



