the Rarefaction and Condensation of Air. 381 



condensation in the experiments of Table I. : also a certain 

 quantity of cold would have been produced in the experiments 

 given in Table III. The results are, however, such as might 

 have been deduced a priori from any theory in which heat is 

 regarded as a state of motion among the constituent particles 

 of bodies. It is easy to understand how the mechanical force 

 expended in the condensation of air may be communicated to 

 these particles so as to increase the rapidity of their motion, 

 and thus may produce the phaenomenon of increase of tempe- 

 rature. In the experiments of Table III. no cold was pro- 

 duced, because the momentum of these particles was not per- 

 manently converted into mechanical power; but had the mo- 

 tion of the air from one vessel to the other been opposed in 

 such a manner as to develope power at the outside of the jar, 

 which might have been accomplished by means of a cylinder 

 and piston, then loss of heat would have occurred, just as in 

 Tables IV., V. and VI., where the force was applied in lifting 

 the atmosphere of the earth. 



It is quite evident that the reason why the cold in the ex- 

 periments of Table IV. was so much inferior in quantity to 

 the heat evolved in those of Table I., is that all the force of 

 the air, over and above that employed in lifting the atmo- 

 sphere, was applied in overcoming the resistance of the stop- 

 cock, and was there converted back again into its equivalent 

 of heat. 



The discovery of Dulong*, that equal volumes of all elastic 

 fluids, taken at the same temperature and under the same 

 pressure, when suddenly compressed or dilated to the same 

 i'raction of their volume, disengage or absorb the same 

 absolute quantity of heat, accords perfectly with these prin- 

 ciples. 



The mechanical equivalents of heat determined by the va- 

 rious series of experiments given in this paper are 823, 795, 

 820, 814, and 760. The mean of the last three, which I take 

 as least liable to error, is 798 lbs., a result so near 838 lbs., 

 the equivalent which I deduced from my magnetical experi- 

 ments, as to confirm, in a remarkable manner, the above ex- 

 planation of the phacnomena described in this paper ; and to 

 afford a new, and to my mind, powerful argument in favour 

 of the dynamical theory of heat which originated with Bacon, 

 Newton, and Boyle, and has been at a later period so well 

 supported by the experiments of Rumford, D^avy and Forbes. 

 With regard to the detail of the theory, much uncertainty at 

 present exists. The beautiful idea of Davy, that the heat of 

 elastic fluids depends partly upon a motion of particles round 

 * Annalei de Chimie, vol. xli. p. 156. 



