CHAP. 1. 10. OF CLOUDS. 31 



It is said again, that nimbi have been observed 

 to take place without the precurrence of other 



" The author has since espoused a theory of evaporation 

 which altogether excludes the abovementioned opinion (and 

 consequently Dr. Hutton's also), and considers himself in a 

 considerable degree indebted to it for the origin of the ex- 

 planation he is about to offer. It will be proper, therefore, to 

 state the fundamental propositions of this theory, with such 

 other parts as appear immediately necessary, referring for 

 mathematical demonstrations and detail of experiments to the 

 work itself, which is entitled " Experimental Essays on the 

 Constitution of mixed Gases; on the Force of Steam or Va- 

 pour from Water and other Liquids, in different Temperatures, 

 both in a Torricellian Vacuum and in Air; on Evaporation; 

 and on the expansion of Elastic Fluids by Heat. By John 

 Dalton." See Memoirs of the Literary and Philosophical 

 Society of Manchester, vol. v. part 2. The propositions are 

 as follow: 



' 1. When two elastic fluids, denoted by A and B, are 

 mixed together, there is no mutual repulsion amongst their 

 particles; that is, the particles of A do not repel those of B, as 

 they do one another. Consequently, the pressure or whole 

 weight upon any one particle solely arises from those of its 

 own kind. 



' 2. The force of stream from all liquids is the same at equal 

 distances above or below the several temperatures at which 

 they boil in the open air; and that force is the same under 

 any other pressure of another elastic fluid as it is in vacua. 

 Thus the force of aqueous vapour of 212 is equal to 30 inches 

 of mercury; at 30 below, or 182, it is of half that force; 

 and at 40 above, or 252, it is of double the force: so like- 



