THE STOliY OF NINETEENTH-CENTURY SCIENCE 



air is not cooler because the dew is formed, but that the 

 dew is formed because the air is cooler having become 

 so through radiation of heat from the solids on which 

 the dew forms. The dew itself, in forming, gives out 

 its latent heat, and so tends to equalize the temperature. 

 This explanation made it plain why dew forms on a 

 clear night, when there are no clouds to reflect the radi- 

 ant heat. Combined with Dalton's theory that vapor 

 is an independent gas, limited in quantity in any given 

 space by the temperature of that space, it solved the 

 problem of the formation of clouds, rain, snow, and 

 hoar-frost. Thus this paper of Weils's closed the epoch 

 of speculation regarding this field of meteorology, as 

 Hutton's paper of 1784 had opened it. The fact that 

 the volume containing Hutton's paper contained also 

 his epoch-making paper on Geology, finds curiously a 

 duplication in the fact that Weils's volume contained 

 also his essay on Albinism, in which the doctrine of 

 natural selection was for the first time formulated, as 

 Charles Darwin freely admitted after his own efforts 

 had made the doctrine famous. 



IV 



The very next year after Dr. Weils's paper was pub- 

 lished, there appeared in France the third volume of the 

 Memoires de Physique et de C/iimie de la Societe d'Ar- 

 cueil, and a new epoch in meteorology was inaugurated. 

 The society in question was numerically an inconse- 

 quential band, listing only a dozen members. But every 

 name was a famous one : Arago, Berard, Berthollet, 

 Biot, Chaptal, de Candolle, Dulong, Gay-Lussac, Hum- 

 boldt, Laplace, Poisson, and Thenard rare spirits every 



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