from the Cold induced by the Evaporation of Water. 103 



the formation of vapour. It might indeed be supposed, that 

 the stem of the thermometer should convey a little heat to the 

 bulb ; but this being of glass, is a very bad conductor, and any 

 heat which it supplies must be quite inappreciable, as it makes 

 no sensible difference whether we apply a wet covering to the 

 bulb alone, or continue it along a part of the stem. To me, at 

 least, it is as difficult to comprehend how the same piece of coal 

 could continue to burn for an indefinite period, as that the wet 

 bulb could continue to furnish any heat which it does not receive 

 from other bodies. But Dr Anderson certainly carries this idea 

 to perfection, in supposing the wet bulb alone to furnish, that 

 is, to create all the heat expended on the formation of vapour. 



Without pretending to give a complete theory of the rate at 

 which thp depression varies in air of different densities, it does 

 not seem so very difficult as the other gentleman supposes, to 

 explain why the depression should be greater as the air is more 

 attenuated. When air is dilated, its capacity for heat, or the 

 absolute heat which a given mass of it can contain at a particu- 

 lar temperature, is no doubt increased ; but I am rather at a 

 loss to see what concern this has with the question. It is sure- 

 ly not the total or absolute heat of a given mass, but rather the 

 specific heat of a given volume, or the heat which that volume 

 of air can give out in being cooled through a small range that 

 is to be imparted to the bulb, and spent in saturating the same, 

 or an equal volume, with vapour*. Now, although the rarer 

 the air is, the fewer of its particles will touch the cold surface, 



• Nothing is known, or likely ever to be known, of the total or absolute 

 quantity of heat contained in any body, or of the ratio which the absolute 

 heat of one body bears to that of another ; yet many chemical writers take it 

 for granted that both are perfectly ascertained, as also that the specific heat is 

 exactly proportional to the absolute heat, and that the former is the same at 

 every temperature. Of the fallacy of such assumptions we have abundant 

 evidence in the many examples which Mr Dalton has collected of the ridi- 

 culously different positions which computations founded on them give to the 

 absolute zero. In one case, however, a notable exception is made with re- 

 spect to the ratio between the absolute and specific heats : the absolute heat 

 of steam is obviously greater than that of water, and yet the specific heat of 

 the former, notwithstanding its vastly greater volume, is only reckoned about 

 a fourth of that of the latter ; which affords a strong presumption that en- 

 largement of volume does not increase the specific heat, however much it may 

 enlarge the capacity for the absolute. 



