346 On the Modijicaticfis of Clouds, and 



is on the nature of this process, the state in which the va- 



Eour subsists for a time, and the means by which the watc? 

 ecomes aeain visible, that the greatest diversity of opinion 

 has prevailed. 



The chemical philosopher, seduced by analogy, and ac- 

 customed more to the action of liquids on solids, naturally 

 regards evaporation as a solution of water in the atmosphere, 

 and the appearance of cloud as the first indication of its pre- 

 cipitation ; which becoming afterwards (under favourable 

 circumstances) more abundant, produces rain. The. theory 

 of Dr. IJutton goes a step furtlier, assumes a certain rate 

 of solution difiermg from that of the advance of tempera- 

 iure bv which it is effected, and deduces a general explana- 

 tion of clouds and rain from the precipitation which, ac- 

 cording to his rule, should result from every mixture of 

 ditferent portions of saturated air. The fundamental prin- 

 ciple of this theory has been disproved in an essay hereto- 

 fore presented to the society*, and which was written under 

 the opinion, at present generally adopted by chemists, that 

 evaporation depends on a solvent power in the atmosphere, 

 and follows the general rules of chemical solution. 



The author has since espoused a theory of evaporation 

 which altogether exclud-:s the above-mentioned opinion (and 

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

 a cousSderable degree indebted to it for the origin of the 

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

 to state the fundamental propositions of this theory, with 

 sxich other parts as appear inuncdiatcly necessary, referring 

 tor mathematical demonstrations and detail of experiments 

 to the work itself, wliieh is entitled " Expcrimenla! Essays 

 en the Constitution of mixed Gases ; on the Force of Steam 

 or \ apour from Water and other Liquids in different Tem- 

 peratures, both in a Torricellian \^acuum and in Air; on 

 iuvaporation ; and on the Expansion of Elastic Fluids by 

 ileat. Cy John Dalton,"^ — See Memoirs of the Literary and 

 Philosophical Society of Manchester, vol. v. part 2. — The 

 propasiiions arc as follow : 



" I. Wlien two elastic fluids, denoted by A and B, are 

 mixed together, there is no mutual repulsion amongst their 

 particles j t'lat is, the particles of A do not repel those of i?, 

 ii-> tiiey do one another. Conscijuetilly, the pressure or 

 whole weight upon any ovie particle arises solely froiu 

 those of i;s own kind. 



" 2. The force of steam from all liquidsis the same at equal 

 tlistances above or beltiw the several temperatures at which 

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



• Slc Fail. ^^ai.'. vol xiv- p. ;;. 



anv 



