00 Mr. J. Gill on the Dynamical Theory of Heat. 



obviate a serious source of error, though still it should be borne 

 in mind that glass has an attractive power for moisture which 

 prevents its free evaporation at temperatures considerably higher 

 than the regular boiling-point under any given pressure. 



I must be allowed to express my opinion that the experiments 

 of M. Hirn are of great practical value, and I submit that the 

 importance of the facts which they furnish is not depreciated by 

 the circumstance that the experimenter, apparently borne down 

 by scientific authority, recently presented to the Academy of 

 Sciences an interpretation of his results in accordance with the 

 dynamical theory. It should also be borne in mind that the 

 results of Hirer's experiments coincide in the main with those of 

 Seguin made more than twenty-five years ago, when his reason- 

 ing on the mode of action of the steam-engine pointed out the 

 necessity of supposing a conversion of heat into work, of which, 

 nevertheless, he could find no appreciable indication in the results 

 of the numerous experiments made by him with this express 

 purpose, and conducted with the attention and accuracy sug- 

 gested by his great scientific knowledge and superior practical 

 skill in thermo-dynamics as then known. 



In my experiments above alluded to on the specific heat of 

 air at different densities, I found that certain proportions of sur- 

 face in the apparatus relative to the masses of air, their condi- 

 tion, and the times occupied by their passage through the calo- 

 rimeter, were essential to correct results ; and I venture to think 

 that some concealed source of error must have existed in Reg- 

 nault's experiments on this subject, undetected even by his extra- 

 ordinary ability in similar researches. 



I would also repeat that my long practical acquaintance with 

 the general phenomena of steam, as well as the results of direct 

 experiment, have tended to convince me that the total heat of 

 steam is a constant quantity. I believe that this important point 

 is again engaging the attention of physicists ; and it may be well 

 to remark that in the condensation of high-pressure steam there 

 is a tendency to produce heat from the effects of tumultuous 

 expansion, which may seriously affect the results, showing, as in 

 some of Hirn's experiments, more heat than is due to the con- 

 stitution of the steam. In the cooling of gases under ordinary 

 circumstances there is no change of state, and the contraction of 

 the volume should result from a gradual shrinking of the sphere 

 of action of each particle in the mass, a process which suggests 

 no idea of tumultuous motion or percussive action of the parti- 

 cles. In saturated vapours each particle must possess the pre- 

 cise specific amount of heat requisite for its existence in the 

 vaporous state : if heat be added, the vapour becomes a gas ; if 

 heat be withdrawn, some of the particles must collapse into 



