PROPERTIES OF MATTER IN THE GASEOUS STATE. 
739 
which the differential gauge could be read. A special means of reading this gauge 
was devised. This consisted of a species of cathetometer almost close to the gauge, 
in which, instead of a telescope, a microscope with an inch object glass and a semi-disc 
in the focus of the eye-piece was used, the screw which moved the microscope had 
50 threads to an inch, and the head had 200 divisions, so that one division corresponded 
to the TooTToth part of an inch. Owing to the high magnifying powers, the effect ol 
a motion of one division was visible, and several readings taken from the same position 
of the mercury agreed to within one division. 
Testing the apparatus. 
16. The complicated character of the apparatus and the number of joints rendered 
it extremely difficult to make it perfectly tight. When working at the pressure of 
the atmosphere this was of no great moment, but when working with rarefied gas it 
was necessary that it should be so tight that the leak might cause no appreciable 
disturbance. 
At first india-rubber varnish was used to make the joints tight; but this did not 
answer, as the vapour from the varnish produced very considerable disturbance. After 
this the surfaces of the india-rubber were carefully washed, and then considerable 
pressure applied by wrapping wire on the tubes and screwing up the press. In this 
way, after a few days, the apparatus became what may be called perfectly tight. 
There was a slight leak or probably slight diffuson through the india-rubber, for after 
the experiments were concluded the apparatus was left full of hydrogen at the pressure 
of the atmosphere, and the tap communicating with the pump closed. It was then 
found that the pressure within the chambers steadily fell until it reached 9 inches 
of mercury. This point was reached after about one month. The pressure then began 
to rise, and in another month the gauge showed 12 inches. The entire volume of the 
chambers and tubes is only about 6 fluid ounces, so that it might well be imagined 
that the hydrogen had been absorbed by, or condensed on the india-rubber or the 
porous plate, but the fact that the pressure again rose seemed to imply that the 
hydrogen had escaped ; but whether through the india-rubber or not it is impossible 
to say. 
Such a leak, however, was entirely without effect on the results. In fact, a leak 
which admitted air at the rate of 1 inch of mercury in an hour into one chamber did 
not cause any appreciable alteration in the differential guage. 
Drying the gas. 
17. The presence of vapour in the gas was at first a source of great trouble. The 
tendency of porous plates to absorb moisture is so great, and the presence of vapour in 
the gas produces such a great disturbance even when the pressure of vapour is a long 
way below that at which it would condense on the cold surface, that for some time 
