48 
MR. J. J. WATERSTON ON THE PHYSICS OE MEDIA COMPOSED OF 
The air engines of the atmospheric railway may perhaps afford the means of testing 
this equation. 
Note I ).— To find the Compression that Corresponds to a Given Ratio of Latent to 
Sensible Heat. 
This may be computed by means of IY. and XVI. 
Suppose the medium is compressed so that the mean molecular distance changes 
CC X | - 3. • 
from 1 to ——, th vis viva increases from 1 to "—-— (XVI.), the increment being 
X i i- X 
1 jx — L. 
By withdrawing molecular vis viva under a constant pressure, let the mean 
X . 
distance change from 1 to-- ; the molecular vis viva must be diminished from 1 to 
& x + 1 
(IV.), the decrement being 
(x + l) 3 — x 3 
= K. The ratio K/L is given to find 
as+l/ ' ' n “““ " & (x + l ) 3 
x + 1\ 3 
— I , which is the amount of compression from unity that makes the ratio 
X J 
between the sensible and latent, or evolved vis viva, equal to K/L. 
If we put x/(x + 1) = y, the equation resolves itself into y ?J +if + y = K/L = f 
in the case given. This equation may easily he solved by inspection of a table of square 
and cube numbers. 1 /y s is the compression from unity to give the ratio K/L of the 
sensible to the latent heat. 
Note E.—Specific Heat of Air. 
It is probable that the specific heat of mercury and water are better determined 
than that of any other bodies. Assuming that the specific heat of liquid mercury 
(0'033) is the same as that of its vapour, and that all gases and vapours have for the 
same volume the same specific heat when in equilibrio of pressure and temperature, 
the specific heat of air in terms of that of water is (P238. The mean experimental 
value is, according to the French chemists, (P2G7. Mercury is thus the only liquid, 
so far as is known, whose specific heat is the same as what it ought to be in the state 
of vapour, if it conforms to the general law. Water is nearly three times greater 
than steam ought to he. Alcohol five times that of its vapour. Ether seven times, 
&c. ; see Note B. 
This accordance of the specific heat of mercury with its vapour seems to prove that 
there is little or no part of the heat required to raise the temperature of the liquid 
absorbed in a latent form. This is an important point in the vis viva theory. 
It is remarkable that there is no instance, so far as yet known, of mercury 
combining in fractional parts of a volume. Arsenic, sulphur, phosphorus, and some 
others combine in fractional volumes, and their specific heat in the form of vapour by 
