OF THE PARAFFINS AND THEIR MONOHALOGEN DERIVATIVES. 
13 
knowing- its weight, volume, and temperature, we have all the materials required for 
calculating its specific gravity. 
It is not advisable to exhaust the apparatus very completely in the first instance, 
for then the evaporation of the liquid is so violent that fragments of glass, and 
sometimes even the whole tube, get blown over the bend at the top on to the surface 
of the mercury, making it difficult to read the position of the surface. If 15 or 
20 milluns. of air is left in this seldom happens, but time must of course be given to 
allow the gases to diffuse into each other. 
To avoid draughts and inequalities of temperature, the whole apparatus, with the 
exception of the reservoir, H, is enclosed in a box, with vertical openings at the front 
and back, through which the levels can be read with the cathetometer. 
The calculations are simplified if the reservoir is always adjusted so that the level 
of the mercury in CD stands at the same point. This makes the pressure of the 
residual air the same in the two measurements, provided the temperature is constant. 
In my experiments I always brought the level to the file mark, K. 
To empty the apparatus the reservoir is lowered till the mercury in CD sinks to 
the level of G—E being of course closed—and air is allowed to enter through the 
three-way tap. 
Fig. 4. 
The vapour densities were generally required in the neighbourhood of certain 
determinate pressures. To secure this the liquid was always sealed in a tube of the 
same diameter, and a preliminary filling and weighing being made with water, a 
simple calculation gave the length required in any case. It was then easy to draw 
off a piece of such a length as would hold within 5 or 10 per cent, of the required 
amount. 
When the liquid has a very low boiling point special arrangements are needed for 
filling and sealing the tubes. The followiug method has been found to be quite 
satisfactory, but requires careful manipulation to avoid breaking the capillaries. 
A piece of glass tube is drawn out into the form ABCDE (fig. 4), with a capillary 
part at B, and a capillary end, DE, and weighed. 
The end. A, is then connected with a water pump, by means of a piece of india- 
rubber tube closed with a spring clamp. The part C, which is that which is to 
be filled, rests in a lead tray, slightly inclined, and filled with a suitable freezing 
