AXD ON THE LAAV OF PRESSURE OF OASES, ETC. 
215 
have been overcome l;)y the use of a suitaljle cathetometer, but such was not to hand. 
The most direct method by actually gauging with mercury the spaces concerned 
being scarcely feasible, I devised another method which has the advantage of easy 
execution and is practically independent of the assumption of Boyle’s law. The 
opportunity was taken to increase the range over which the volume could be varied. 
The new chamber, composed mainly of tubing of 18 millims. diameter, is graduated 
at intervals of 10 cub. centims. over a total range of 200 cub. centims. It is 
prolonged above and Ijelow by narrow tubing in order to connect it with the sloping 
manometer bulb and with the hose and mercury reservoir as before. The zero mai’k 
is situated on the upper tube a few centimetres above its junction with the wider 
one. It is scarcely necessary to say that no rubber -Nvas employed except for the 
hoses, and that these were always occupied by mercury under a pressure above 
atmosphere. The mercury reservoirs themselves were protected against damp by 
chloride of calcium. 
If we call the ungauged volume (from the zero mark to tlie bulb of the sloping 
manometer with “ point ” set) V, and the gauged volume v, the total volume occupied 
by the gas is V + f ; and the problem is how to determine V. If we may assume 
the correctness of Boyle’s law for rare gases and may rely upon tlie sloping 
manometer, the process is simple enough. We have only to find the pressures 
exerted by the included gas at volumes Y and V + r, whence by Boyle's law the 
ratio of these volumes is kncwn and thus Y determined in terms of v. In order to 
avoid the use of Boyle’s law, further observations are necessary. 
The requisite data can Ije obtained Ijy changing tlie quantity of gas. Suppose that 
with the original (plantity of gas certain pressures, P, P', correspond to total volumes, 
V + V 4- tq, and tliat with a reduced amount of gas the same jiressures are 
recorded with vedmnes Y -f v-^, Y -|- W Since the pre.ssure is a function of the 
density, whether Boyle’s lav' be applicable or not. it must follow that 
A + ''i _ Tq 
A + To ^ "t ('[ 
whence Y is determined in terms of tlie known volumes I'j, Co. iq, iq,. It may lie 
remarked that this argument does not a.ssume even the correctness of the scale of 
pressures. 
In carrying out the method practically it was necessary to work to tlie fixed marks 
of the volume chamber, and thus the same pressures could not be recovered cxaetly. 
But the use of Boyle’s law in order to make what is equivalent to small corrections is 
unobjectionable. 
AVitli this explanation it may suffice to give the details of an actual determination 
executed with nitrogen. With the original quantity of gas, volumes Y + 70, Y + 170 
gave pressures proportional to 345’4, 184'0. Sufficient gas was now removed to allow 
the remainder to give nearly the .same higher pressure as before with v — 0. Thus, 
