ELASTIC FORCES OF AQUEOUS VAPOUR. 577 
exhausted the globe for the last time and closed the tube with 
the lamp. The globe being surrounded with ice, I observed the 
difference in the level of the two mercurial columns. Admitting 
that the tension of mercurial vapour is nothing at 0°, the ob- 
served difference of level is the simple measurement of the 
elastic force of the air remaining in the apparatus. The globe 
is now placed in water, the temperature of which is gradually 
raised to the boiling-point, and the precautions mentioned above 
are attended to for obtaining a stationary temperature at the 
time of observation. The differences of height in the two co- 
lumns of mercury are then caused by the elastic force of the air 
remaining in the apparatus and by the tension of the mercurial 
vapour; this latter may be obtained alone, by deducting from 
the difference of height observed, the elastic force of the air for 
the temperature in question, calculated from its elastic force ac- 
tually observed at zero. 
In a first experiment I left purposely a rather notable quan- 
tity of air in the apparatus; the elastic force of this air at 0° was 
found to be 4:22 millimetres. The tensions of the mercurial 
vapour were found to be— 
0°00 C. 0-000 millimetre. 
23°-57 0068 sons 
38°01 0°098 owe 
100°60 0°555 cet 
In another experiment an almost perfect vacuum was pro- 
duced by heating to ebullition the mercury in the globe whilst 
the air was exhausted. The elastic force of the air remaining in 
the globe was found to be 0:10 millimetre at 0°; the following 
values were then obtained for the tensions of the mercurial va- 
pour :— 0°00 C. 0:000 millimetres. 
25°39 0°034 eee 
49°15 0087 ase 
79°74 tes | ors. 
100°11 0°407 ees 
* I made some determinations of the tension of mercurial vapour at more 
elevated temperatures, by placing the globe in an oil-bath. Towards 200° the 
experiments became evidently erroneous from distillation of the mercury. Al- 
though this was the case, I subjoin the numbers that I obtained; they must 
only be regarded as approximations :—— 
1007-6 C. 0:56 millimetre. 
146°°3 3°46 one 
177°-9 10:72 
200°°5 22:01 
