A HISTORY OF SCIENCE 



ly original genius, John Dal ton, afterwards to be known 

 as perhaps the greatest of theoretical chemists, took the 

 question in hand, and solved it by showing that water 

 exists in the air as an utterly independent gas. He 

 reached a partial insight into the matter in 1793, when 

 his first volume of meteorological essays was published ; 

 but the full elucidation of the problem came to him in 

 1 80 1. The merit of his studies was at once recognized, 

 but the tenability of his hypothesis was long and ar- 

 dently disputed. 



While the nature of evaporation was in dispute, as a 

 matter of course the question of precipitation must be 

 equally undetermined. The most famous theory of the 

 period was that formulated by Dr. Hutton in a paper 

 read before the Royal Society of Edinburgh, and pub- 

 lished in the volume of transactions which contained 

 also the same author's epoch-making paper on geology. 

 This " theory of rain" explained precipitation as due to 

 the cooling of a current of saturated air by contact with 

 a colder current, the assumption being that the sur- 

 plusage of moisture was precipitated in a chemical 

 sense, just as the excess of salt dissolved in hot water is 

 precipitated when the water cools. The idea that the 

 cooling of the saturated air causes the precipitation of 

 its moisture is the germ of truth that renders this paper 

 of Hutton' s important. All correct later theories build 

 on this foundation. 



" Let us suppose the surface of this earth wholly 

 covered with water," said Hutton, "and that the sun 

 were stationary, being always vertical in one place; 

 then, from the laws of heat and rarefaction, there would 

 be formed a circulation in the atmosphere, flowing 



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