THE POUR ELEMENTS. 427 



tion, by some authorities which are not however so nu- 

 merous as might be supposed ; that you will not refuse 

 to remark how few authors in the present day refer back 

 to original sources ; how disagreeable they find it to dis- 

 turb the dust of libraries ; and, on the contrary, how con- 

 venient to feed on the erudition of other people, to reduce 

 the composition of a book to the mere work of editing. 

 But the promise that I hold of your confidence in me, 

 has appeared of more weight. I have pressed into my 

 service a number of printed memoirs, tlie whole of a 

 voluminous authentic correspondence still in manuscript ; 

 and if after fifty years I come to claim for Watt an 

 honour that has been inconsiderately granted to one of 

 his most illustrious countrymen, it is because it has 

 seemed to me beneficial to show that in the heart of 

 academies, truth is sure to shine out sooner or later, and 

 that in matters of discovery, there is never any pre- 

 scription. 



The theory of the four pretended elements, fire, air, 

 water, and earth, the varied combinations of which were 

 to produce all known bodies, is one of the numerous lega- 

 cies that have come down to us from the brilliant phil- 

 osophy which, through several centuries, dazzled and 

 misled some of the noblest intellects. Van Helmont 

 was the first who shook, though slightly, one of the prin- 

 ciples of this ancient theorem, by pointing out to the at- 

 tention of chemists, vai'ious permanently elastic fluids, 

 several sorts of air, that he called gases, the properties 

 of wliich differed from those of common air, from those 

 of the elemental air. Boyle's and Hooke's experiments 

 raised more serious difficulties still : they prove that com- 

 mon air, necessary to respiration and to combustion, un- 

 dergoes remarkable changes in the course of tliose two 



