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ttiay have given, of the nature of Dr. Apjohn's method, may be at- 

 tained by a short study of that first experiment described by him, in 

 which it was found that in a stream of dry hydrogen gas, in which a 

 thermometer with a dry bulb stood at 68°, the one that had the mois- 

 tened bulb was cooled to 48° of Fahrenheit ; the barometer indi- 

 cating at the same time an atmospheric pressure of 30.114 inches. 

 From the stationary state to which the second of these two ther- 

 mometers had been reduced, it is clear that the continual supply of 

 heat, required for the continuing evaporation of moisture from the 

 bulb, was supplied neither from the water with which that bulb was 

 moistened, nor from the mercury which it contained, but only from 

 the stream of warmer gas which continued to pass along it ; the small 

 effect of radiation from surrounding bodies being neglected in com- 

 parison herewith. Each new portion of the current of hydrogen, in 

 cooling from 68° to 48°, must therefore have given out very nearly 

 the precise amount of heat absorbed by that new portion of moisture, 

 which passed at the same time from the state of water to the state of 

 vapour, at the temperature of 48°. It is also assumed, apparently 

 upon good grounds, that after the moist bulb attains its stationary 

 temperature, the whole (or almost the whole) of the new gas, in be- 

 commg fully cooled, becomes at the same time fully moistened^ or 

 saturated with the new vapour ; this vapour being intimately mixed 

 with the gas which had assisted to form it ; and every cubic inch of 

 this mixture containing exactly (or almost exactly) as much moisture 

 as a cubic inch could contain, in the form of vapour, at its own tem- 

 perature : a quantity which is known from the results of Dalton, re- 

 specting the elastic force of vapour. From those results it follows, 

 that in the present case, the temperature of the vapour being 48°, its 

 elastic force must have been such that it could by itself have sup- 

 ported the pressure of a column of mercury, 35 hundredths of an 

 inch in height ; but the pressure upon the mixture was equivalent to 

 a column 30 inches and 11 hundredths high; therefore the pressure 

 which could have been supported by the hydrogen alone, at the same 

 temperature of 48°, was equivalent to 29 inches and 7Q hundredths : 

 so that, by the known proportionality between density and pressure, 

 the weight of the gas which was contained in the whole or in any part 

 of this mixture would have exceeded the weight of the vapour in the 



