420 
LORD RAYLEIGH ON THE LA\Y OF THE PRESSURE OF GASES 
open through AB to the pump. When the mercury is raised to the middle mark H, 
the volume is halved, and the pressure to be dealt Avith is doubled. Gas sufficient 
to exert the single pressure (75 millims.) must be supplied to the intermediate 
chamber, which is now isolated from the pump by the mercury .standing uj) in AB. 
Both manometers can now be set, and the doubling of the pressure verified. 
The communication through F Avith the jDump is free from obstruction, but on a 
side tube a three-way tap is provided communicating on the one hand Avith the gas 
supply and on the other with a A^ertical tube, more than a barometer-height long 
and terminating below under mercury, by means of Avhich a wash-out of the gene¬ 
rating vessels can be eftected Avhen it is not desired to evacuate them. The fiA’e 
tubes leading downwards from A, E, C, I, K are all over a barometer-height in 
length and are terminated by suitable hoses and reservoirs for the supply of 
mercury. When settings are actually in progress, the mercury in the- hoses is 
isolated from the reservoirs l^y pinch-cocks and the adjustment of the supply is 
effected by squeezing the hoses. As explained in my former paper, the final adjust¬ 
ment must be made by squeezers AA'hich operate upon parts of the hoses Avhich lie 
flat upon the large Avooden mercury tray underlying the Avhole. The adjustment 
being somewhat complicated, a convenient arrangement is almost a necessity. 
The Side Apparatus^ 
By the aid of these manometers the determination of pressure is far more accurate 
than Avith the ordinary mercury column and cathetometer, but since the pressures are 
defined Ijeforehand, the adjustment is throAvn upon the volume. The A'ariable 
volume is introduced in the side-tube JK. This was graduated to cub. centim., in 
the first instance by mercury from a burette. Subsequently the narroAv parts aboA^e 
and l^eloAv the bulb (Avhich as Avill presently be seen are alone of importance) Avere 
calibrated Avith a Aveighed column of mercury of volume equal to ^ cub. centim. 
and occupying about 80 millims. of the length of the tube. The AAdiole capacity 
of the tube betAveen the loAvest and highest marks Avas 20|- cub. centims. The object 
of this addition is to meet a difficulty Avhich ineAutably presents itself in apparatus 
of this sort. The volume occupied by the gas cannot be limited to the capacities 
susceptible of being accurately gauged. BetAveen the upper mark G and the 
mercury surface in C AA’hen set, a volume is necessarily included Avhich cannot be 
gauged Avith the same accuracy as the volumes betAveen G and H and between H 
and I. The simplest vieAv of the side apparatus is that it is designed to measure 
this volume. In the notation suhsetpiently used Vg is the Amlume included Avhen the 
mercury stands at C, at G, and at the top mark J. Let us suppose that Avith a 
certain quantity of gas imprisoned it is necessary in order to set the manometer CD, 
the \ipper chamber being Amcuous, to add to Vg a further Amlume Y-, amounting to 
the greater part of the capacity of the side-tube, so that the Avhole Amlume is Vg + V-. 
