24 
MR. J. J. WATERSTON OK THE PHYSICS OE MEDIA COMPOSED OF 
atmospheric pressure. Media also lose their vis viva if allowed to expand against 
pressure. 
Thus by XVI., if at 60° Fahr. a vessel containing air of double density is allowed 
to empty itself into the atmosphere, the decrement of temperature in the air that 
remains ought to be — 48° by the XVI. deduction. If on the contrary its density is 
suddenly increased from 1 to 2, its temperature ought to rise to 196°.'" 
Thus the analogy between media and gases and between vis viva and temperature is 
still maintained, and the phenomena of latent heat in gases appear also in media as 
the transference of force during a change of volume : out of the medium when it 
expands, and exerts, or gives out, mechanical force ; into the medium when it is 
compressed and acted upon, or receives mechanical force. 
The phenomena of latent heat thus appear to be the conversion of mechanical force 
into molecular vis viva; the visible into the invisible, as in condensation: and 
molecular vis viva into mechanical force; the invisible into the visible, as in 
expansion. 
It is a necessary consequence of the conservation of vis viva or indestructibity of 
force among perfectly elastic bodies. 
§ 24. Several experiments have been made on the ratio of the increment of 
temperature evolved by a small condensation of a volume of air to the diminution of 
temperature required to produce the same condensation under a constant pressure, and 
found it to be f. Mr. Ivory (‘ Phil. Mag.,’ 1827) has proved that this ratio is constant 
under every change of temperature or density so long as Dalton and Gay-Lussac’s 
law is maintained, or the air thermometer is an exact measure of heat. MM. Gay-¬ 
Lussac and Welter have also proved this experimentally for a considerable range. 
Mr. Ivory has also expressed his opinion that the nascent value of this ratio will be 
found to be b, and that the cause is probably connected with the proportion that 
subsists between the linear and the solid increments of expansion. 
The value of this ratio in all media, whatever may be their condition of density or 
vis viva, is j (§ 21); and the synthetical demonstration rests on the same fundamental 
principle that determines the proportion of linear to solid increments of expansion. 
This ratio ^ applies only to infinitesimal changes of volume, and it slowly increases 
with the amount of condensation. When the medium is compressed from 1 to 1 ‘20 
the ratio becomes f.t 
In the experiments of MM. Gay-Lussac and Welter referred to in the ‘Me- 
canique Celeste,’ the condensation did not exceed -^-th part of an atmosphere. 
The discrepancy may be exactly ascertained by performing the same experiments as 
it were on the medium by computation. The absolute temperature is denoted by v 2 
* Note C (temperature of condensed air). 
t Note D (to find the compression that corresponds with a given ratio between latent and sensible 
heat.) 
