397 
temperature of the bath is kept uniform throughout by constant 
stirring, and made to rise very slowly. When within a few degrees 
of the temperature at which the determination is to be made, the 
current of hydrogen is almost stopped, so that the bulbs C and C 
may contain less vapour than will fully saturate the gas at the tem- 
perature of sealing. The water of the bath is then made to sub- 
side, by opening a large tap placed near the bottom. The bends a 
and a are thus exposed, the bulbs CC remaining covered. Imme- 
diately the current of hydrogen having been stopped, the flame is 
applied at aa y so as to seal the apparatus hermetically. The tem- 
perature of the bath, as well as the height of the barometer, must 
now be observed. After being cleaned, the apparatus (which now 
consists of three portions, viz., the portion CC hermetically sealed 
and the two ends b and D) must be weighed. 
The capacity of the apparatus is found by filling it completely 
with water and weighing ; but previously to this operation the volume 
of hydrogen enclosed at the time of sealing must be found. On 
breaking one extremity under water, the water will rise in the bulbs, 
and, after a while, will have absorbed all of the vapour, but will leave 
the hydrogen. The bulbs must then be lifted out of the water, 
without altering their temperature, and, with the water that has 
entered, weighed. The difference between the latter weighing and 
the weight of the bulbs quite full of water gives the weight in 
grammes, which expresses in cubic centimeters the volume of hydro- 
gen enclosed ; the pressure is the height of the barometer minus 
the column of water which had entered the bulbs ; the temperature 
is that of the water. 
An example of a determination of the vapour density of alcohol at 
30° C. below its boiling point is subjoined : — 
Height of the barometer (at 0° C.), 
Temperature of the balance case, . 
Weight of apparatus in dry air, 
Temperature at time of sealing, 
W eight of apparatus -J- hydrogen + vapour, 
763*09 m.m 
7 - 5 ° C. 
69-959 grm. 
48° C. 
69 5275 grm. 
Weight of apparatus + water (at 5.2° C.) 191*76 grm. 
Weight of apparatus filled with water, . 545"36 grm. 
Height of water column, . . . 122 m.m. 
From which is deduced : — 
