TAKING THE DENSITY OF VAPOUR OF VOLATILE LIQUIDS. 451 
The object of this is to avoid complete saturation of the gas with vapour. By 
this device, the greater part of the gas which is present in C and C, at the end of 
the operation, has entered at a lower temperature than that at which the appa- 
ratus is sealed, and, consequently, is only partially saturated atthe moment of 
sealing the apparatus. 
The desired temperature having been attained, sufficient water to expose the 
bends a a, but not the large bulbs C C, is suddenly let out of the bath, by opening 
a large tap situated close to the bottom of the bath. The current of hydrogen 
having been entirely stopped, the flame is applied to @ and a, until the apparatus 
is sealed up in both places. The temperature of the bath having been again 
noted, and also the height of the barometer, the bulbs C and C which contain 
the vapour and hydrogen, are removed from the bath, carefully cleaned, and sub- 
sequently, along with the other two fragments (viz. the portion A to a and the 
portion a to D, also carefully cleaned), transferred to the balance-case and 
weighed. 
The difference between this latter weighing and that at the beginning of the 
operation gives, when the weight of the displaced air is added to it, the weight of 
C Cfilled with a mixture of hydrogen and vapour at the barometric pressure and 
temperature of the bath at the time of sealing. 
The remainder of the operation having for its object to find how much hydro- 
gen is enclosed in C C, and also the capacity of C C, is conducted thus :—The her- 
metically sealed C Cis placed in cold water, and one of its sealed extremities nipped 
off whilst still under water, which enters and occupies the space not filled by 
hydrogen. After remaining in water for a number of hours, until the vapour has 
been completely absorbed, the apparatus is lifted vertically out of the water, 
care being taken not to heat the hydrogen contained in the bulbs, for which 
purpose a holder is employed. The temperature of the water, also the height 
of the barometer, is observed. 
The apparatus, together with the water which has thus entered, is then 
weighed. It is afterwards completely filled with water, and again weighed. 
The difference between the weight of the bulbs empty, and the weight of them 
filled with water, gives the total capacity of the apparatus. 
The difference between the weight after the first portion of water has entered, 
and the weight of the apparatus quite full of water, gives the volume of the hy- 
drogen at temperature of the water and at the barometric pressure, minus the 
column of water contained in the bulbs when they are held over the water out 
of which they had been lifted. This column is measured after the first weighing 
of bulbs and water had been made. 
Such is the process which we employ to take vapour densities of bodies in- 
capable of being heated to their boiling points without decomposition. 
Exception may perhaps be taken to the mode of measuring the hydrogen 
