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may 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- 
coming 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 80 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 76 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 
