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



processes, and such changes in its properties, as to imply that 

 it is a compound body. The numerous observations of Hales ; 

 the successive discoveries of carbonic acid gas by Black, and 

 of hydrogen by Cavendish ; of nitrous acid, of oxygen, of mu- 

 riatic and sulphurous acids, and of ammonia by Priestley, gave 

 the finishing blow to the antiquated notion that air vi^as simple 

 and elemental, and dismissed it as one of the rash and almost 

 always false conceptions, which proceed from those who have 

 the hardihood to consider themselves called not to discover, 

 but to divine, the footsteps of Nature. 



In the midst of these remarkable discoveries, water had al- 

 ways maintained its character of an element. The year 1776, 

 however, was distinguished by one of those observations which 

 necessarily led to the overturn of this general belief ; whilst, at 

 the same time, we must avow, that from the same period are to 

 be dated those strange attempts which chemists for a long time 

 made to resist the consequences which naturally flowed from 

 their experiments. The observation to which I here particu- 

 larly allude, was one of Macquer's. This judicious chemist 

 having placed a white porcelain saucer over the flame of some 

 hydrogen gas, which burned steadily as it issued from a bottle''s 

 mouth, observed that the flame was not accompanied by any 

 smoke properly so called, and that it deposited no soot. The 

 portion of the saucer upon which the flame struck — or, to use 

 his own words, which it lapped — was soon covered with vex'y 

 conspicuous drops of a liquid similar to water, and which, upon 

 trial, was found to be pure water. Here, assuredly, was a sin- 

 gular result, well worthy of attention : it was in the midst of 

 flame, in that portion of the saucer, as Macquer said, lapped 

 by the flame, that watery drops were deposited ! This chemist, 

 however, did not seize upon the fact ; — he was in no degi-ee 

 astonished at what was so wonderful ; he simply states it, with- 

 out the slighesl comment ; and failed to perceive that he had 

 touched upon the very threshold of a grand discovery. 



In the sciences of observation, then, is genius to be reduced 

 to the mere capability of saying at the fitting moment, — Why ? 



In the physical world, we consider as volcanos, mountains 

 which never have had more than a single eruption ; and in the 

 intellectual world, in like manner, there are men who, after one 



