6o Conjiderathns on Dr. Hut Ion's Theory of Rain. 



The capacity of water (which Crawford terms the com- 

 parative heat) being taken at i,ooo, that of air proves by 

 his experiments to be as 1,790, and that of aqueous vapour 

 as 1,550, at equal weights and temperatures. 



Then if to dry air we add fuccelhve portions of water at 

 increafing temperatures, we are forming a compound (not 

 merely heating a homogeneous niafs), the nature of which, 

 as in all other refpefts, fo in that of the capacity for caloric, 

 niuft (till continue approaching to that of aqueous vapour. 



Now in any two portions of this compound, formed at 

 different temperatures, the capacities will differ. It will 

 hi lefs in that whicli was faturated at the higher tempera- 

 ture ; and vvhatfoever may have been the precife rate at which 

 the folution advanced at increafing temperatures, the mixture 

 of the two portions will not produce a temperature in the 

 arithmetical mean, but below it, as being moll influenced 

 by the drier portion. 



Thus is the principal condition invalidated, viz. that por- 

 tions of faturated air at diffVering temperatures will poff'efs, 

 after mixtuie, a temperature in the arithmetical mean. This 

 argument mav appear at ilrH; fight ftill to favour thqiconclu- 

 Ijon rcfpcfting the mixture in the curve mdr, fincc, if pre- 

 cipitation take place at the mean temperature, much more 

 ihould it at a lower than the mean : but perhaps it may be 

 eafy to demonftrate, on the fame principles as before, that 

 the temperature of fnch a compound, as it refults from this 

 circumflance of mixture only, will in no cafe aff'ett the 

 Hate of folution, and that neither fuper-nor under-faturaiion 

 will enfue therefrom. 



We will take for this purpofe the curve mdr, and exa- 

 mine the real circumftances of a folution advancing with the 

 temperature but in an accelerated ratio; premifing that the 

 quellion regards the effects of heat (caloric) only as the caufe 

 both of increafed temperature and increafed folution, and 

 that, according to Crawford, equal additions of this (ihq 

 capacity of the receiving body continuing unchanged) pro- 

 duce equal expanfions in the common thermometer. 



That the folution of water in air goes on increafing with 

 the temperature of the compound is true, and it is equally 

 fo that it increafc? at the cxpenfe of that poijuer luhich gives 

 the teviperalure ; ior water held by air is itfelf in the flate of 

 gas, and gas of whatfoever kind requires a large p.roportiou 

 of caloric for its conflitution, which is not at all meafurable 

 by the thermometer. 



Suppofing therefore a unit of dry air at a certain tcmpe-. 

 rature to confill of 0,005 meafures of liquid bale and 0,99,5 



caloric. 



