28 
MR. J. W. CAPSTICK ON THE RATIO OF THE SPECIFIC HEATS 
Two other experiments were made at pressures below 100 millims., which gave 
results about 2 per cent, below these, but the dust figures obtained were poor, 
and the density and correction factor had to be obtained by extrapolation. As the 
'pv curve is getting a little irregular, so as to make the correction term uncertain, even 
at the lowest pressures actually observed, it is unsafe to go beyond the limits of 
direct experiment, hence they have been omitted. 
§ 10. Ethyl Bromide. 
In the case of this compound the experiments were conducted in a slightly different 
way from those already described. A vapour density determination was made in the 
usual way with a particular sample of the liquid, and when the pressure had been 
measured, the Kundt apparatus was filled to the same pressure with the vapour of an 
exactly similar specimen. Thus each line in the table below gives the result of a pair 
of parallel experiments, the vapour density determinations not being comparable with 
each other, as they were made on samples of liquid which had received different 
treatment. 
Ethyl bromide seems to be more subject to impurity than any of the other 
substances. The first sample used was given to me by a friend, but its vapour 
density was so abnormally low that I discarded it without attempt at purification, and 
procured a supply from Kahlbaum. This had a fairly steady boiling point, almost all 
coming over betv^een 38° and 39°, and was used after a simple fractionation in the first 
experiment. 
The vapour density determination gave a result ‘3 per cent, below the theoretical 
value. As the boiling jDoint was constant and at the right temperature, this raised a 
suspicion that there was some impurity present which had nearly the same boiling 
point as ethyl bromide, but a lower vapour density. If, as is not unlikely, the 
substance had been prepared from potassium bromide, alcohol, and sulphuric acid, 
the impurity might be ether, so the remainder was shaken with strong sulphuric acid 
and redistilled, which raised the density by nearly 1 per cent. This shows the 
insufficiency of the boiling point alone as a test of the purity of a liquid, and the value 
of a vapour density determination as a confirmatory test. 
The second and third experiments were made on the liquid after this treatment, and 
the fourth, after a repetition of the process. 
The densities obtained in this way are insufficient to give the correction factor, so a 
Boyle’s Law experiment was made in addition, with the following results, all at a 
temperature of 19° 
