4 
MR. J. W. CAPSTICK ON THE RATIO OP THE SPECIFIC HEATS 
The method adopted for the determination of the ratio of the specific heats was 
Kundt’s velocity of sound method. It has the disadvantage of requiring the density 
of the gas to be known, and hence being very sensitive to impurities ; but this is 
probably counterbalanced by our knowing from Kijndt’s investigations all the con¬ 
ditions on which accuracy depends. 
Most of the gases used diverge considerably from agreement with Boyle’s law, and 
have not had their vapour densities determined except by the rough methods used in 
fixing molecular formulm; and even if they had, it would be unsafe to trust the 
results, for the usual test of the ^Jurity of organic liquids, constancy of boiling point, 
may easily lead to erroneous conclusions, as will be seen by the work on ethyl bromide 
described below. To avoid error from this source a direct experimental determi¬ 
nation has been made on the compounds as they were used in the velocity of sound 
experiment. 
The formula that has been used by most investigators for calculating the ratio 
of the specific heats from the velocity of sound is 
y = y> {ili'f 
where 
y = the ratio of the specific heats of air, 
p = the specific gravity of the gas referred to air at the same temperature 
and pressure, 
I — the wave-length in the gas, 
V = the wave-length in air. 
This formula is only true for a perfect gas, for the square of the velocity of sound 
is Ypv only if is a constant at any one temperature. 
In the present work I have used a formula obtained as follows — 
The equation = {dpjdp)^, where the symbols have their usual meanings, is true 
for any homogeneous substance. (Bayleigh’s Sound, § 244.) 
From this we have 
id — — 7 ?/ {d2jldv)i. 
But 
dpivldv = p -p vdpjdv, 
the differentiation being at constant temjjerature. 
Hence 
I 
and 
id — yv (2) — dpvldv) 
— yP^ ~ ' d2n'ldv) 
(1)- 
