FREE AND PERFECTLY ELASTIC MOLECULES IN A STATE OF MOTION. 45 
If it is more than about one-third that of water, if for equal volumes it is not the 
same as that of air and the other gases, then is the theory incomplete and altogether 
defective. 
It is stated to be about 0‘84, but this requires confirmation. The specific heat of 
the vapour of alcohol ought to be only -g-th that of the liquid. The vapour of ether 
only yth that of the liquid. Vapour of etherine jfh. Vapour of sulphuret of carbon 
Tfh. Vapour of oil of turpentine xotli. Vapour of bromine ytli, &c. 
These are from MM. de la Rive and Marcel's experiments. 
Is there any direct evidence in favour of or against this view ? So far as I can 
discover, little, if any, on either side. It is quite undetermined as yet by experiment, 
and is so surrounded by practical difficulties that it will probably long remain so. 
But analogy favours it in so many ways as to make it highly probable. Liquid 
etherine has four times the specific heat that its vapour ought to have, judging from 
its specific gravity. Now, olefiant gas is isomeric with etherine, and its specific 
gravity is one-half that of etherine vapour. It can hardly be doubted that the 
specific heat of liquefied olefiant gas for equal weights is the same as that of etherine, 
which corresponds very nearly with naphtha, turpentine, and the other hydrocarburets. 
If it is so it will be double what it ought to be, if for equal volumes it is the same as 
that of air and the other gases. Now, the eminent French chemists who have at 
different times made experiments on the specific heat of this gas agree that it is 
nearly 1*5 times that of air, while Mr. Haycraft (‘ Edin. Trans.’), on the other hand, 
with his simple and apparently most efficient apparatus, found it to be the same as 
that of air, and accounts for the higher number of the French chemists by the great 
difficulty there is in freeing it from ethereal vapours. 
It is the same with carbonic acid gas (the only other exception to the law of equal 
specific heat for equal volumes). The French chemists agree that its specific heat lies 
between 1T75 and 1 ‘258 ; Mr. Haycraet, by many experiments, that if carefully 
dried, it offers no exception to the general law. Judging from the analogy of other 
similar binary compounds, there cannot be a doubt that the specific heat of liquefied 
carbonic acid is double, if not three times, that of the gas. 
It would be a most valuable addition to our knowledge if this great change in the 
specific heat of a body when it becomes vapour could be thoroughly established, and it 
is in vain to proceed with the subject of vapours until it is so. 
Sulphuric ether is probably better adapted for the experiment than any other body. 
Its boiling temperature is very low, and there is a vast disproportion between the 
specific heat of the liquid and what may be expected in the vapour. Suppose a volume 
of it and of air are maintained in equilibrium of pressure and temperature by means 
of a bent tube with mercury. If a sudden small and equal dilatation is made in both 
at the same instant, the difference of pressure that will then become apparent will 
indicate the value of the latent heat of the vapour in terms of its specific heat, and, 
as we know the value of the same in terms of the specific heat of water, we shall have 
