PROPERTIES OF MATTER IN THE GASEOUS STATE. 
761 
nected with the chambers, one on each side of the porous plate, the object of these 
flasks being simply to enlarge the capacity of the chambers. 
The branch to the flask on the right was outside the tap P, so that by closing this 
tap the flask would be cut off' from the instrument, and the action of the pump would 
be confined to that one flask. 
In this condition the mercury pump had a definite capacity—about 6 fluid oz., the 
capacity of the flasks was definite—about 8 fluid oz. each, and besides these there were 
the tubes and chambers in the diffusiometer also of definite capacity—about 3 oz. on 
each side of the plate. 
The vacuum gauge was cut off during these experiments, so that the movement of 
the mercury in the siphon gauge constituted the only source of variation in capacity, 
and this was small. 
This constancy in the capacity of the several parts of the apparatus, if not absolutely 
essential for these experiments, was very important, as it did away with the necessity 
of any process of reduction in comparing the results of the experiments at different 
pressures. This may be seen as follows. 
Equal volumes. 
35. Starting with the pump full of mercury, and the taps open so that the pressure, 
whatever it might be, is the same throughout the instrument, both taps being then 
closed, one stroke of the pump draws a definite proportion of the entire air in the 
instrument out of the right-hand flask, lowering the pressure in this flask in a definite 
ratio. Or in other words, one stroke of the pump withdraws from the flask on the 
right a definite volume of gas as measured at the pressure in the instrument. 
This condition would be maintained until the tap P, between the right-hand flask 
and the instrument, was opened. Then the pressure on the right hand side of the 
porous plate would fall in a definite ratio. Transpiration would commence, and by the 
time the pressure on the two sides of the plate had again become equal, a definite 
volume of air, about half that withdrawn by the pump, must have passed through the 
porous plate. 
The time from the opening of the tap, before complete equalisation is effected, is 
then seen to be the time of transpiration of a definite volume of gas measured at 
either the initial or the final pressures in the instrument, under differences of pressure 
which, although varying, are ad corresponding stages proportional to the initial or 
final pressures in the instrument. 
This time, which is called by Graham the time of transpiration of equal volumes, is 
directly measured in these experiments. 
Measurement of the time. 
36. The time at which transpiration commenced was the time at which the tap was 
opened, the tap and the tubes being sufficiently large to allow almost instantaneous 
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