M. Arago's Biographical Memoir of James Watt. 275 



Newton himself, when repeating an experiment which had 

 been known for fifteen centuries, he discovered the composi- 

 tion of white light ? He attached to that experiment an in- 

 terpretation so natural, that now-a-days, it seems impossible 

 to find any other. AH that you obtain, says he, with the help 

 of any process whatsoever, from a pencil of white light, was con- 

 tained therein in its state of mixture. The glass prism has 

 no creative power. If the parallel and infinitely slender pencil 

 of solar light which strikes upon the one face issues from the 

 other divergingly, and with increased breadth, it is because the 

 glass separates that which, in the white ray, was by its nature, 

 unequally refrangible. These words are nothing more than a 

 literal interpretation of the well known experiment of the pris- 

 matic solar spectrum, an interpretation, however, which had 

 escaped the penetration of Aristotle, Descartes, and Eobert 

 Hooke. 



But, without departing from our present subject, let us come 

 to arguments which bear still more directly on this point. The 

 theory conceived by Watt concerning the composition of water 

 reaches London. If, according to the apprehension of the 

 time, it was as simple and as evident as it now appears to be, 

 the Council of the Royal Society would not have failed to 

 adopt it. But it was far from doing so ; its strangeness made 

 them even doubt the truth of Priestley's experiments: they even 

 laughed at it, says Deluc, as at the explanation of the dent d'or. 

 Again, a theory, the conception of which was attended with no 

 difficulty would certainly have been disdained by Cavendish ; 

 and yet, with what pertinacity, under the influence of that in- 

 genious man, did Blagden claim priority of discovery in oppo- 

 sition to Lavoisier. Priestley, upon whom a considerable share 

 of the honour attached to the discovery of Watt must natu- 

 rally fall, and whose affectionate regard for the celebrated en- 

 gineer cannot be questioned, wrote to him, under date of 29th 

 April 1783 : " You will examine with surprise and indigna- 

 tion the sketch of an apparatus with which / have undermined 

 to its very basis your beautiful hypothesis.'''' 



Upon the whole, then, a theory which was ridiculed at the 

 Royal Society, — which forced Cavendish out of his habitual 



