142 Royal Society : — 



have always been considered as essential elements in the determina- 

 tion of the question ; and it was the evidence which Mr. Brown 

 possessed in regard to these particulars, which, in his estimation, 

 "placed Cavendish's claims as the discoverer of the composition of 

 water beyond dispute." That evidence, however, was not derived 

 from any unpublished document, but formed part of a section of 

 Deluc's "Idees sur la Meteorologie," which although especially 

 entitled, — "Anecdotes relatives a la decouverte de FEau sous la 

 forme it Air" — appears entirely to have escaped the notice of those 

 who have advocated Cavendish's claims. It is the more conclusive 

 as coming from Deluc, the " ami zclc," as he justly terms himself, 

 of Watt, and who, in relation to this question, believed himself " a 

 portee d'en connoitre toutes les circonstances." 



The testimony of Deluc is as follows : — 



Vers la fin de I'annee 1/82 j'allai a Birmingham, oil le Dr. Priest- 

 ley s'etoit etabli depuis quelques annees. II me communiqua alors, 

 que M. Cavendish, d'apres une remarque de M. Warltire ; qui avoit 

 toujours trouve de Feau dans les vases ou il avoit brule un melange 

 d'air inflammable et d'air atmospherique ; s'etoit applique a decou- 

 vrir la source de cette eau, et qu'il avoit trouve, "qu'un melange 

 d'air inflammable et d'air d^phlogisti^ne en proportion convenable, 

 etant allume par I'etincelle clectrique, se convertissoit tout entier en 

 eau." Je fus frappe au plus haut degre de cette decouverte*. 



The italics and inverted commas are Deluc's own. 



In this communication made by Cavendish to Priestley the theory 

 of the composition of water is clearly indicated. The two gases 

 (known to have been hydrogen and oxygen) were mixed together in 

 due proportion^ and by means of, the electric spark were entirely 

 converted into water. Referring to one of Cavendish's experiments, 

 as recorded in his journal. Lord Jeffrey, the most candid and 

 judicious of Watt's advocates, has said: "if he [Cavendish] had 

 even stated in the detail of it, that the airs were converted, or 

 changed, or turned into water, it would probably have been enough 

 to have secured to him the credit of this discovery, as well as to 

 have given the scientific world the benefit of it, in the event of 

 his death, before he could prevail on his modesty to claim it in 

 public t'" The evidence which this distinguished critic and judge 

 regarded as sufficient to establish Cavendish's claim is now afforded, 

 not by a note in his private journal, but by the testimony of the 

 zealous friend of Watt, who states that it was communicated to 

 Priestley towards the end of the year 1 782, that is to say, several 

 months before Watt drew his own conclusions from Priestley's 

 bungling repetition of Cavendish's experiments. It was, moreover, 

 published to the world, and suffered to remain uncontradicted, while 

 all the parties were alive and in frequent intercourse with the author 

 and with each other. 



I have only further, in Mr. Brown's name also, to do an act of 

 justice to the memory of Lavoisier, by relieving it from the obloquy 



* Idees sur la Meteorologie, tome ii. 1787, pp. 206-7. 

 t Edinburgh Review, vol. Ixxxvii. p. 125. 



