OF THE PARAFFIN'S AND THEIR MONOHALOGEN DERIVATIVES. 
7 
A point requiring some consideration was the question at what temperature the 
experiments should be made. According to WIillner carbonic acid, carbon monoxide, 
nitrous oxide, and ammonia have values of the ratio of the specific heats which change 
in some cases by as much as 4 per cent, between 0° and 100°, and if this were so in 
all cases, it might well be asked at what temperature the results would be comparable. 
There are many gases, however, for which y is constant; oxygen and nitrogen are 
such, and Strecker showed that over a long range of temperature the change, if it 
existed at all, was very small for the halogens and halogen acids. Muller, too, 
found no indications of change in the gases investigated by him. His assumption 
that the compressions and rarefactions in his apparatus are adiabatic is so improbable 
that we are bound to suppose his results are too low, but the method should be capable 
of showing relative changes. 
In fact, no observer but Wullner has ever found any appreciable change of y with 
the temperature, and it is possible that the aiTangement of his apparatus at least 
exaggerated the change he found. 
Hence, so far as previous observations go, there is a presumption in favour of the 
constancy of y. 
Independently of this, there is something to be said in favour of choosing some 
constant temperature, for as the chief interest of y arises from its relation to the 
internal energy, it seems desirable to secure that either the internal, the translational, 
or the total energy should be constant, and we can make the translational energy 
constant by working at a constant temperature. Consequently it was decided to 
work at the temperature of the room. 
^ 2. The Kundt A 2 :)paratus. 
The apparatus used for the determination of the velocity of sound in the gases was 
in all essential features the same as that described by Kundt in ‘ Poggendorff’s 
Annalen,’ vol. 135. The double ajqDaratus was used, as it makes accurate temperature 
observations unnecessary, the tubes containing air and the gas under investigation 
lying side by side. It also ensures the figures in air and in the other gas corre¬ 
sponding to exactly the same note, so that change of pitch in the vibrating tube from 
change of temperature or any other cause has no effect. 
It will be sufiicient to describe one end, as the two are almost identical in arrange¬ 
ment. 
The vibrator, AB (see fig. 1), is a closed glass tube 150 centims. long and 35 millims. 
in diameter, and was chosen from a considerable number tried as giving the best 
figures. It is not desirable that it should give a very loud tone, for this scatters the 
dust too much, but it should speak readily, so that the intensity can be adjusted. 
An important point is to choose one that gives a note as free from overtones as 
