12 
MR. J. W. CAPSTICK ON THE RATIO OF THE SPECIFIC HEATS 
AB is a glass tube 60 centims. long and 35 inillims. in diameter (see fig. 3), closed 
at A and sealed at B to the curved tube CB, the middle part of which is straight 
and horizontal. Before sealing this to the tube CD, the latter is calibrated, and the 
volume determined between the end D and a file mark at K, near the upper end. 
CD is then attached to AC, and the volume of the whole determined by filling with 
water and weighing. Subtracting from this the volume of DK we get the volume of 
AK, and as the tube CD has been already calibrated the volume to any other point 
is known if required 
Fig. 3. 
w 
A 
Next the side tube, EF, of the same bore as CD, is sealed on, and the three-way 
tap, G, making connection with an air pump or with the mercury reservoir, H. 
L is a thermometer graduated to fifths of a degree. 
A small quantity of the liquid whose vapour density is required is sealed up in a 
small tube with capillary ends and weighed. This is introduced at D, and made 
to rest at M, by inverting the apparatus for a moment. 
Next, D is closed with an india-rubber stopper, E being also closed, whilst the 
apparatus is exhausted through the three-way tap, G, after which operation G is 
turned so as to allow the mercury to flow in from the reservoir, and E is opened. 
The difference of the levels of the mercury in the two tubes is read by means of a 
cathetometer, and this difference subtracted from the height of the barometer gives 
the pressure of any air left in the apparatus. 
The small tube is then broken, by tilting the apparatus a little and allowing it to 
slide over into AB, where the capillary end breaks off and allows the liquid to 
evaporate. 
By reading the levels a second time we get the pressure of the vapour, and 
