442 
SIR B. C. BRODIE ON THE ACTION OF ELECTRICITY ON GASES. 
attached, also of glass, as shown in the figure, the points being finely ground. This 
glass rod is attached to the sides of the tube by a little resinous cement. The rod 
may be thus fixed after the cylinder a has been cemented into the lower cap f. The 
capacity of the cylinder a , between each point, is ascertained by calibration with mercury, 
which is effected before the reservoir is attached to the apparatus. In order to calibrate 
the cylinder a , the stopcock c is closed and k opened to the air ; the cylinder a is then 
filled with mercury (which has been carefully purified, and of which the specific gravity 
has been determined by experiment) up to the stopcock k. The mercury is then run 
out of the apparatus by means of the stopcock c to a level a little below the first 
point, 1 ; the stopcock c being again closed, a portion of the same mercury, exactly 
sufficient to bring the level of the mercury to the point 1, is poured back into the appa- 
ratus by means of the pressure-tube, the level of the mercury is read on the pressure- 
tube, and the mercury which has been run out of the apparatus is weighed. From these 
data the capacity of the cylinder «, taken together with the capacity of the pressure- 
tube g from the stopcock k to the first point 1, may be calculated. By perfectly similar 
operations the capacity of the cylinder and pressure-tube at the other points (2, 3, 4, 5, 
6, 7) may be ascertained, and the heights of those points upon the pressure-tube g 
determined. In order finally to determine the capacity of the cylinder a at the various 
points, we have to deduct from the capacity of the cylinder and pressure-tube at those 
points as thus ascertained the capacity of the pressure-tube. This capacity is readily 
determined by filling the apparatus as before with mercury, closing the stopcock k , and 
running the mercury out, by opening the stopcock c (the vessel b being detached), 
from successive portions of the pressure-tube alone, care being taken to run out so 
much as to pour back in each case a little mercury in order to secure the right reading 
of the meniscus ; this mercury is then weighed. The pressure-tube being graduated 
in millimetres, a simple calculation informs us of its capacity at any desired interval. 
Deducting the capacity of the pressure-tube from the capacity of the cylinder and pres 
sure-tube as previously determined, or, rather, deducting the weight of the mercury at a 
known temperature and of a known density corresponding to the capacity of the pres- 
sure-tube from the weight of mercury corresponding to the capacity of the cylinder and 
pressure-tube, we arrive at the capacity of the cylinder at any desired point. 
The cylinder and pressure-tube are enclosed in a second glass cylinder (n), which is 
fastened into an iron ring, by means of which it is attached to the frame of the appa- 
ratus ; this cylinder is filled with water, in which a thermometer is placed. 
The apparatus thus put together is to be carefully examined to ascertain that there is 
no leakage in the various joints and stopcocks; with this view the reservoir of mercury 
having been now attached to the apparatus, the stopcock k is opened and the cylinder a 
is partially filled with mercury ; the stopcock k is closed, the stopcock c opened, and a 
considerable pressure put upon the gas by means of the forcing-pump before referred 
to ; the stopcock c is then closed, and the height of the mercury in the pressure-tube 
read : the level of the mercury in the pressure-tube should not appreciably vary for 
several hours. 
