FREE AND PERFECTLY ELASTIC MOLECULES IN A STATE OF MOTION. 25 
and the absolute zero corresponds to — 461° of Fahr. scale (taking Budberg’s constant 
of expansion, see § 6). By employing' the equation e = v 2 A 3 and (-j = y 2 ; (XYJ), 
it is easy, by substituting the barometric height for e, to compute the result of any 
given experiment as if it had been performed on the hypothetical medium. This I 
have done in the case of MM. Clement and Desormes’ experiments. Beferring to 
the account given of them in the ‘ Mecanique Celeste,’ the minus interior pressure of 
the medium would have been 3'42 millims. ; the mean result is given as 3 - 61, the 
difference being only about ylryth part of an inch of mercury. If the experiment gave 
the nascent ratio, the minus pressure at the end would have been one-fourth of 
the minus pressure at the beginning, or 3'45 millims. In MM. Gay-Lussac and 
Welter’s experiments, the difference of pressure at the end was 4'44 millims. ; the 
nascent ratio in a medium would in a like experiment be 4'09 millims., the difference 
being about -yyth of an inch of mercury. 
The evidence afforded by these experiments may be summed up as follows :— 
The initial ratio of the increment of vis viva under a constant volume to the 
increment required to effect the same change of vis viva under a constant pressure in 
the medium is... 1^3 • 
In air the same ratio of the increments of heat under the same circumstances by 
MM. Clement and Desormes’ experiments, is. 
In air the same ratio, by MM. Gay-Lussac and Welter’s experiments, is 
A difference in the reading of the height of the mercury in the manometer of y^th 
and -yyth part of an inch would bring the respective experiments to coincide with the 
theory.* 
§ 25. In the more recent of Mr. Joule’s physical researches that gentleman has 
applied mechanical force to the compressing of air surrounded with water, to collect 
the heat evolved, and has found that about 800 lbs. descending through the height of 
one foot increases the temperature of a pound of water one degree. The same result 
nearly was obtained by forcing water through narrow tubes. Mechanical force was 
expended, and the same proportionate amount of heat was produced in the water. It 
is remarkable that the same mechanical value was found for the heat generated by the 
magneto-electric machine. Such accordance in the results, as Mr. Joule remarks, 
seems strongly to favour the vis viva or vibratory theory of heat. 
If air is similar in its constitution to a medium we may employ the deductions of 
this section to determine the mechanical value of any quantity of heat applied to it, 
1 
1 ■ 3 5 1 
1 
1 -3 T 
* [The fair agreement of the erroneously deduced value of 7 , viz., f, with observation, was doubtless 
the reason of the author not discovering his mistake of calculation. We know that upon his principles 
the calculated value should be ■§■, which accords much less well with the results observed for ordinary 
gases than does ■§■. It should be borne in mind that the observed value, 7 =l - 40-5, has not, even at the 
piesent time, been reconciled with theory, although reasons may be given for a departure from 7 = -§. 
-R.] 
MDOCCXCII. 
-A. 
