216 
LORD RAYLEIOH OX A XEW MANOMETER, 
coiTespoiidiiig to volumes Y + 0, V + 40 the pressures were 344-9, IBS’S 'We have 
now only to calculate V from the eepiation 
A' + 40 _ 344 9 184-9 A’' + 170 
' A^^ "" 3454 ’ 
or ' Y- + 110 + 2800 = 1-0072 {\~ + 170 Y) ; 
wlience A’ = 45-5 cub. centims. 
The adopted value, derived from observations upon nitrogen and hydi'ogen, is 
A" = 45-6 cul). centims. 
lu charging the apparatus, tlie first stej) is to make a good vacuum throughout, the 
cross-tap Ijeiug o})en. The gas su]j})ly being started, the first portions are allowed to 
IjIow off from under mercury, and then, by use of the tliree-way tap, a sufficiency is 
introduced into the apparatus to an absolute pressure of, perhaps, 10 centims. of 
mercury. The gas-leading tube would then l)e sealed off. Ultimately the remainder 
of the supply tube and the ljlow-(jff tube were exhausted to diminish the risk of 
leakage. 
Tlie “nitrogen” was jaepared from air by passage over red-hot copper and desicca¬ 
tion with })hosphoric anhydride. Accordingly it contained argon to the amount of 
about 1 2 )er cent. 
In taking a set of ol)servations the procedure would he as follows. Assurance 
having l>een ol^tained that the vacuum was good, tire next step would l)e to set the 
mercury in tlie volume chamber so tliat v =■ 190 cub. centims., then after a few minutes 
to adjust the sloping manometer and to read the telescope scale. It was of course 
necessary to ensure that sufficient time was allowed for uniformity of jDressure to 
establish itself, and oliservations were frequently renewed after a quarter of an hour 
or longer. In the case of oxygen, to lie considered later, several hours were sometimes 
allowed. If operations wei’e leisurely conducted, with first a rough setting of the 
volume and tlien a rough setting of the manometer followed liv accurate settings in 
the same order, little or no cliange could afterwards be detected. Indeed I Avas 
rather surprised to find hoAV rapidly ecpiilibrium seemed to lie established. The next 
smaller A'olume, r.r/., v = 150, aa-ouII then lie observed, and so on until v — 40. In 
observations to he used for the examination of Boyle’s laAv v Avas not further reduced, 
as too much stress might thereby he throAvn upon the accuracy of Ah The same 
observations Avere then repeated in reA’erse order and the mean taken. The numbers 
recorded are thus the mean of tAvo settings only of the manometer. 
The next ste|i Avas to alloAv aliout half the gas to esca}ie. The mercury at the 
pump AA-as alloAA^ed to rise so as to cut off fhe pump-head and A" -fi v Avas so adjusted 
as to he equal to tlie A’olume remaining iqiou the other side, about ISO cub. centims. 
The cross-taj) aavis then opened, and after a sufficient interA'al of time the zero, 
corresponding to no pressure, Avas read. In the course of the obseiwations upon 
