Cbnjiderathns on Dr. Hullon's Theory of "Ram. 57 



40 and at 10 be mixed ; then op\s the quantity of water 

 contained in this mixture at the mean heat 25, while e xs 

 the ordinate of fohition : confcquently ep is the quantity of 

 water that cannol be retained in loiution in the mean tem- 

 perature produced l)v the mixture. 



" In the curve m g r let equal portions at 10 and 40 be 

 mixed; and the ordinate being drawn in the mean 25, sk 

 will be the whole power of Tolution, and the quantity of 

 water that air is capable of dilfolving in this degree of heat; 

 but op being the quantity of water aftually in the mixture, 

 the air is iinderfaturated by the quantity fk, 



" Thus the aftual curve of evaporation being known, the 

 effecl of any mixture of two portions at different tempera- 

 ments may be afcertained. If the folution of water in air 

 increafcs equally with the heat, there will be neither fuper- 

 nor uuder-faturation in a mixture of portions at different 

 temperatures. 



" If the folution incrcafes with the heat but in a dc- 

 creafing rate, there will be under-faturation ; if it increafes 

 with the heat but in an increaling rate, there will be iupcr- 

 faturation. 



" The lafl: cafe applies to the phaenomena of breath and 

 fleam* rendered vilible in mixing with air colder than them- 

 felves, and to various appearances that mav occur in mixing 

 together feveral portions of air differentlv falurated with hu- 

 midity and at different temperatures : for it is not every 

 mixture of air at diflerent temperatures that forms a vifible 

 condcnfation, this efl'ciit depending on the degree of faiura- 

 tion with humidity. But if two portions of the atmor|)here, 

 both faturated with humidiiv, fliould be mixed, let there be 

 but a difierence in their temperatures, a condcnfation pro- 

 portionate to this difference will take place. 



** At prefcnt, from the influence of the afcending fun, 



* Aqueous jras at iii* (or dcam) produces vifible clouds in mixing 

 with air at a lower tcmperiuiire ; but fteam is not a folutio 1 of water in 

 air, neither is thi^ appearance peculiar to it, but attends the cooling (in 

 contact with air) ot the tranfparcnt vapours of cairphor and all volatile 

 oils, as well a. of benzriic acid and other fubft inces quite infjiublt in air. 

 The fublim-ittoii of the latter very aptly extmplitits to a fpt6lator of it that 

 jromentarv produ(^tion of fnow which has btcn witntfled when the ir.» 

 tenfciv cold and denl'e air of northern climates hns been (uddcniv admitted 

 into the nioiil and \>'aim atmoCpheic of the apartments in ufc ihirc. 



Viliblc ttc:am atifes frmn the diRcrinfi; c.ip.iciries of air and of vapour for 

 caloric. The former, in acquiiinj; temiieiiture from the latter, lobs it of 

 a portion of its ro.r/!iliieni c.iioric; fance a momentary rcprodudiou of 

 water, which is afterwards dtpofucd or diilolved according to circum-* 

 ft«ncts. iVc/.V' iy tbc Author of the pirfnt t.jftj, 



two 



