ELASTIC FORCES OF AQUEOUS VAPOUR. 5973 
metric tubes, the whole is left until the following day; the globe 
is then surrounded with ice, and the elastic force of the air re- 
maining in the apparatus is determined at 0°. Without re- 
moving the ice which surrounds the globe, the bulb enclosed in 
the tube mn is broken by expansion, and the water distils into 
the globe; the tube mn is detached by fusing the capillary 
portion at k with the lamp. The apparatus is now arranged, the 
experiments are made in the same manner as before; in this 
manner the results I, J, K of Table III. were obtained, which 
show the elastic force of aqueous vapour at 0° to be somewhat 
less than was obtained by the first mode of procedure. 
It is easy, lastly, to make use of the apparatus at figs. 1 and 2 
for determining the tension of aqueous vapour in an absolute 
vacuum. For this purpose the water is not enclosed in a bulb, 
but a certain quantity is placed at once in the globe. When the 
apparatus is arranged in the ordinary manner, a vacuum is pro- 
_ duced by the air-pump, and the globe A is heated with some 
charcoal, in order to distil over the small quantity of water which 
has condensed in the barometric tube af. On continuing the 
action of the air-pump, a constant distillation of the water in the 
globe and of that in the barometric tube is produced; this water 
condenses in the tube M N containing the pumice-stone moist- 
ened with sulphuric acid. Having distilled in this manner 
_ a few grammes of water under very low pressure, it may be 
presumed that the air has been completely expelled from the 
apparatus, the tube / is then sealed by the lamp. The determi- 
nation of the elastic force of the vapour is then made in the usual 
manner. 
The series L, M of Table III. were obtained in this manner. 
III. The modes of procedure which I have just described are 
alike applicable to the determination of the tension of vapours 
produced from other liquids besides water, and require but a 
very small quantity of material. When these liquids are of a 
nature to attack the cement, the tubes must be cemented on to 
the exterior of the connecting piece ed/; it is also convenient 
to use tubes ground with emery to fit the copper connecting 
piece as nearly as possible. The apparatus represented at figs. 
1 and 2 is however only applicable to the determination of ela- 
stic forces below 200 millimetres. When it is required to deter- 
mine more considerable tensions, I employ the apparatus re- 
presented at fig. 5. This apparatus I never used for the vapour 
