IV. On a New Manometer^ and on the Law of the Presmre oj Gases between 
1'5 and O’Ol MiUinietres of Mercury. 
By Lord Hayleigii, F.R.S. 
Received January 15,—Read February 21, 1901. 
Introduction. 
The behaviour of air and other gases at low densities is a sidyect wiiich presents 
peculiar difficulties to the experimenter, and liighly discrepant results have been 
arrived at as to the relations between density and pressure. While Mendeleee and 
SiLJERSTRoM have announced considera,ble deviations from Boyle’s law, Amagat* 
finds that law verified in the case of air to the full degree of accuracy that the 
observations admit of. In principle Amagat’s method is very simple. Tlie reservoir 
consists mainly of ffivo nearly equal 1ju11)s, situated one aljove the other and con¬ 
nected by a comparatively narrow passage. By the rise of mercury from a mark 
l^elow the lower bulb to another on the connecting passage, the volume is altered in 
a known ratio which is nearly that of 2:1. The corresponding pressures are read 
with a specially constructed differential manometer. Of tiiis the lower part whicli 
penetrates the mercury of the cistern is single. Near the top it divides into a U, 
widening at the level of the surface of the mercury into tuljes of 2 centims. diameter. 
Higher up again tliese tid^jes re-unite and l)y means of a three-way ta}) can l)e con¬ 
nected either with an air-pump or with the upper bull). Suitable taps are provided 
by which tlie two branches can be isolated from one another. During tlie ol)serva- 
tions one branch is vacuous and tlie other communicates wltli the enclosed gas, so 
that the difference of levels represents the pressure. This difference is measured liy 
a cathetometer. 
It is evident that when the pressure is very low the principal difficulty relates 
to the measurement of tliis quantity, and that the errors to be feared in respect to 
volume and tenqierature are of little Importance. Amagat, fully alive to this 
aspect of the matter, took extraordinary pains with the manometer and witli the 
cathetometer by which it was read. An insidious error may enter from the refraction 
of the walls of the tubes through which the mercury surfaces are seen. But after 
all his precautions Amagat found that he could not count upon anything less than 
* ‘ Ann. de Chimie,’ vok 28, p. 480, 1883. 
16.3.1901 
