LIFE of Br IlUt -'ION. 63 



firft law, the temperature, the humidity, and the power of con- 

 taining humidity, in the mixture, being all arithmetical means 

 between the fame quantities, as they exifted previoufly to the 

 mixture, the temperature produced would be exactly that which 

 was required by the humidity to preferve it in its invisible 

 form. If the fecond law took place, the moifture actually con- 

 tained in the mixture would be lefs than the temperature was 

 capable of fupporting ; fo that inftead of a condenfation of hu- 

 midity, the air would become drier than before. 



If, on the other hand, the third law be that which takes 

 place, after the mixture of two portions of air of different tem- 

 peratures, the humidity will be greater than the temperature is 

 able to maintain, and therefore a condenfation of it will follow. 

 Now, the experience of every day proves, that the mixture of 

 two portions of humid air of unequal temperatures, does indeed 

 produce a condenfation of moifture, and therefore we are autho- 

 rifed to conclude that the laft-mentioned law is that which actu- 

 ally prevails *. 



It is obvious that this principle affords an explanation of the 

 formation of clouds in the atmofphere, and that currents of air, 



Vol. V.— P. III. I or 



* It has been fuppofed that the chemical folution of humidity in air is necef- 

 farily implied in this theory of rain. The truth is, that the air is here con- 

 sidered only as the vehicle of the vapour, and that the tranfparent flate of the 

 latter is fuppofed to depend on the temperature, or the quantity of heat; but 

 whether that heat act on the vapour folely and directly, or indirectly; by increa- 

 ling the power of the air to retain it in folution, is, with refpect to this theory, al- 

 together immaterial. 



Dr. Hutton has indeed ufed the common language concerning the folution of 

 humidity in air ; but the fuppofition of fuch folution is not eflential to his theory. 

 He feemed, indeed, to entertain doubts about the reality of that operation, found- 

 ed on the circumftance of evaporation taking place in vacuo. Experiments made 

 by M. Dalton fince the death of Dr Hutton, Ihew that there is great reafon for 

 fuppoling that the air has no chemical action whatever on the aqueous vapour 

 contained in it. Manchejler Memoirs, vol. v. p. 538. 



