336 
MR. J. P. JOULE AND PROFESSOR THOMSON ON THE 
Although hot air had been passed through the plug for half an hour before the 
readings in the preceding Table were obtained, it is probable that the numbers T444 
and T399, representing the cooling effect of atmospheric air, are not so accurate as 
the value l°-050. Taking this latter figure for the effect of an excess of pressure of 
89-618— 14*68 = 74*938 lbs., we find a considerable decrease of cooling effect owing 
to elevation of temperature, for that pressure, at the low temperatures previously 
employed, is able to produce a cooling effect of l o, 309. 
In order to obtain the effect of carbonic acid unmixed with atmospheric air, we 
shall, in accordance with the principle already adhered to, consider the thermal 
capacities of the gases to be equal for equal volumes. Then the cooling effect of the 
3-472 x100- 1-052x4-49 o0 ro/ . 
pure gas= =3 *586. 
Collecting these results, we have, — 
Temperature 
of bath. 
Excess of 
pressure. 
Cooling 
effect. 
Cooling effect 
reduced to 100 lbs. 
pressure. 
Theoretical cooling 
effect for 100 lbs. 
pressure. 
12-844 
60-601 
5-049 
8-33 
8-27 
19-077 
37-248 
2-938 
7-89 
8-07 
91-516 
74-938 
3-586 
4-78 
4-96 
Note . — The numbers shown in the last column of the Table are calculated by the 
general expression given in our former paper* for the cooling effect, from an empirical 
formula for the pressure of carbonic acid, recently communicated by Mr. Rankine in 
a letter, from which the following is extracted. 
“ Glasgow, May 9, 1854. 
“ Annexed I send you formulae for carbonic acid, in which the coefficient a has been 
determined solely from Regnault’s experiments on the increase of pressure at constant 
volume between 0° and 100° Cent. It gives most satisfactory results for expansion 
at constant pressure, compression at constant temperature, and also (I think) for 
cooling by free expansion” [*. e. the cooling effect in our experiments]. 
“ Carbonic Acid Gas. 
P pressure in pounds per square foot. 
V volume of one pound in cubic feet. 
P 0 one atmosphere. 
V 0 theoretical volume, in the state of perfect gas , of one lb. at the pressure P 0 and 
the temperature of melting ice. 
P 0 V 0 for carbonic acid 17116 feet, log P 0 V 0 = 4*2334023. 
(P 0 V 0 actually , at 0°, 17145.) 
Kp dynam. spec, heat at constant pressure 300-7 feet; log K p =2-4781334. 
C absolute temperature of melting ice, 274° Cent. 
* Philosophical Transactions, June 1853. 
