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IX. Bakerian Lecture.— On the Law of the Pressure of Gases between 
75 and 150 h\[ilHmetres of Mercury. 
By Lord Rayleigh, F.R.S. 
Received January 15,—Read February 27, 1902. 
In a recently published paper* I have examined, with the aid of a ne’w manometer, 
the behaviour of gases at very low jiressures, rising to 1'5 millims. of mercury, with 
the result that Boyle’s law was verified to a high degree of precision. There is, how¬ 
ever, a great gap between the highest pressure there dealt Avith and that of the 
atmosphere—a gap which it appeared desirable in some Avay to liridge over. The 
sloping manometer, described in tlie paper referred to, does not lend itself well to tlie 
use of much greater pressures, at least if Ave desire to secure the higher proportional 
accuracy that should accompany the rise of jn-essure. The present communication 
gives the results of observations, by another method, of the laAv of pressure in 
gases between 75 millims. and 150 millims. of mercury. It Avill be seen that for air 
and hydrogen Boyle’s laAv is verified to the utmost. In the case of oxygen, the 
agreement is rather less satisfactory, and the accordance of separate observations is 
less close. But even here the departure from Boate’s laAv amounts only to one part 
in 4000, and may jierhaps be referred to some reaction betAveen the gas and the 
mercury. In the case of argon too the deviation, though very small, seems to he 
beyond the limits of experimental errors. Whether it is due to a real minute 
departure from Boyle’s laAv, or to some complication arising out of the conditions of 
experiment, must remain an open question. 
In the case of pressures not greatly beloAV atmosphere, the determination Avith the 
usual column of mercury read by a cathetometer (after Regnault) is sufficiently 
accurate. But when the pressure falls to say one-tenth of an atmosphere, the diffi¬ 
culties of this method begin to increase. The guiding idea in the present iiiAmstiga- 
tion has been the avoidance of such difficulties by the use of manometric gauges 
combined in a special manner. The object is to test Avhether Avhen the volume of a 
gas is halved its pressure is doubled, and its attainment requires tAvo gauges indi¬ 
cating pressures Avliich ai'e in the ratio of 2 : 1. To this end Ave may employ a pair 
of independent gauges as nearly as possible similar to one another, the similarity 
being tested by combination in jiarallel, to borroAv an electrical term. When con- 
* ‘Phil. Trans.,’ A, a''o 1. 196, p. 205, Feb., 1901. 
VOL. CXCVIII.—-A 308. 3 H 
12.4.1902. 
