Royal Society. 321 



•* On the Thermal Effects of Fluids in Motion."— No. II. By 

 J. P. Joule, Esq., F.R.S., and Professor W. Thomson, F.R.S. 



The first experiments described in this paper show that the ano- 

 malies exhibited in the last table of experiments, in the paper pre- 

 ceding it*, are due to fluctuations of temperature in the issuing stream 

 consequent on a change of the pressure with which the entering air is 

 forced into the plug. It appears from these experiments, that when 

 a considerable alteration is suddenly made in the pressure of the en- 

 tering stream, the issuing stream experiences remarkable successions 

 of augmentations and diminutions of temperature, which are some- 

 times perceptible for half an hour after the pressure of the entering 

 stream has ceased to vary. 



Several series of experiments are next described in which air is 

 forced (by means of the large pump and other apparatus described 

 in the first paper) through a plug of cotton wool, or unspun silk 

 pressed together, at pressures varying in their excess above the 

 atmospheric pressure, from five or six uj) to fifty or sixty pounds on 

 the square inch. By these it appears that the cooling effect which 

 the air, as found in the authors' previous experiments, always ex- 

 periences in passing through the porous plug, varies proportionally 

 to the excess of the pressure of the air on entering the plug above 

 that with which it is allowed to escape. Seven series of experi- 

 ments, in each of which the air entered the plug at a temperature of 

 about 16° Cent., gave a mean cooling effect of about '0175° Cent., 

 per pound on the square inch, or '27° Cent, per atmosphere, of dif- 

 ference of pressure. Experiments made at lower and at higher tem- 

 peratures showed that the cooling effect is very sensibly less for 

 high than for low temperatures, but have not yet led to sufficiently 

 exact results at other temperatures than that stated (16° Cent.) to 

 indicate the law according to which it varies with the temperature. 



Experiments on carbonic acid at different temperatures are also 

 described, which show that at about 16° Cent., this gas experiences 

 4\ times as great a cooling effect as air. They agree well at all the 

 different temperatures with a theoretical result derived according to 

 the general dynamical theory from an empirical formula for the 

 pressure of carbonic acid in terras of its temperature and density, 

 which was kindly communicated by Mr. Rankine to the authors, 

 having been investigated by him upon no other experimental data 

 than those of Regnault on the expansion of the gas by heat and its 

 compressibility. 



Experiments were also made on hydrogen gas, which, although 

 not such as to lead to accurate determinations, appeared to indicate 

 very decidedly a cooling effect amounting to a small fraction, per- 

 haps about y^g, of that which air would experience in the same cir- 

 cumstances. 



The following theoretical deductions from these experiments are 

 made : — 



I. The relations between the heat generated and the work spent 

 in compressing carbonic acid, air and hydrogen, are investigated 

 from the experimental results. In each case the relation is nearly 

 * Philosophical Magazine for September 1853, p. 230. 



Phil. Mag. S. 4. Vol. 8. No. 52. Oct. 1854. \ Y 



