SIR B. C. BRODIE ON THE ACTION OF ELECTRICITY ON OASES. 
443 
In order to determine the volume of a gas enclosed in the cylinder a , the gas must 
he first expanded, if necessary, below the point at which the reading is to be made ; 
this is effected by opening the stopcock c , the communications being open between 
the reservoir b and the external air ; the stopcock c is then closed, and the pressure 
put upon the mercury in the reservoir by means of the forcing-pump ; the stopcock c 
is then partially and carefully opened so as to allow the mercury very slowly to rise 
until it is brought to the level of the point ; the stopcock is instantaneously closed, 
and the level of the mercury on the pressure-tube read : the temperature of the water 
in the external cylinder is given by means of the thermometer placed in it ; the baro- 
meter is also read, and from these data, the capacity of the cylinder at the point at which 
the observation is made being known, the volume of the gas at 0° C. and 760 millims. 
pressure may be calculated. 
With a view of testing the accuracy of this method of measurement, the ratio of the 
pressure under which the same volume of air exists at two adjacent points was deter- 
mined throughout the apparatus, and these ratios were compared with the ratios of the 
capacities of the same two adjacent points as determined by mercurial calibration; the 
numbers should be the same in both cases ; thus, putting^, y; 3 , .... as the pressures 
under which any volume of air exists at the points 1, 2, 3, .... , and v x , v 2 , v 3 , .... as 
the capacities of the apparatus at the same points, as determined by mercurial calibration, 
the following results were obtained. The ratios of the pressures given below are in each 
case the mean of not less than three experiments; the several observations at two adja- 
cent points were made with slightly varying volumes of air, so as to get different readings 
—=2-0576, 
v i 
—=1-6541, 
?'- 2 
—=1-4462, 
*3 
^—1-3990, 
V 4 
— = 1-4696. 
V 5 
Now the capacity of the aspirator at the fifth point may also be determined by 
drawing over into the aspirator a pipette of air, and ascertaining the pressure at 
which that volume of air exists at that point ; assuming this to be the true capacity of 
the aspirator at that point, and applying the ratios of pressures to the numbers thus 
obtained for that capacity, the following numbers are arrived at : — 
on the pressure tube in each case. 
P 
P 
Jh 
Pz 
—=2-0597, 
P, 
= 1-6534, 
; - 3 =l-4464, 
V 4 
—=1-3989, 
Pi 
Pi 
Pf, 
=1-4685, 
o P 
J1DCCCLXXII. 
