10 Mr. Ivory on the Laws of the Condensation and Dilatation 



that it has to the changes of volume some other unknown re- 

 lation nearly equivalent. 



We may likewise infer (at least with great probability) that 

 the same equations which express the laws of the condensation 

 and dilatation of air will apply generally to all the permanent 

 gases. Every gas absorbs heat when it enlarges its dimen- 

 sions, and evolves heat when it contracts. The volume, or 

 the density, is therefore in every case a function of heat the of 

 combination. But it is proved, by the experiments of Dalton 

 and Gay-Lussac, that, when the pressure is the same, the 

 volumes of air and all the gases vary in the same proportion 

 by equal changes of temperature. The second of the equa- 

 tions (E) is therefore true of all the gases for every given 

 pressure. But if two volumes of air and a gas, subjected to 

 the same pressure, vary in the same manner when exposed to 

 equal degrees of heat, and if there be no other cause of the 

 alteration of the volumes than the heat of combination, we 

 must conclude that the common cause operates by the same 

 rule in both the cases; that is, we must suppose that the first 

 of the equations (E) is true of the gas as well as of the air. At 

 least, the equations, if we suppose them genei-ally true, will 

 agree with all the facts, as lar as is known ; and they may 

 therefore be considered as containing a physical account of the 

 condensation and rarefaction of air and the gases. 



What has been said of the gases will apply to the vapours 

 exhaled from fluids, so long as they follow the same laws of con- 

 densation and dilatation with the gases. But in the vapours 

 there is a maximum density when the quantity of latent heat 

 is the least possible compatible with the gaseous form. In 

 this case, when a greater degree of cold, or a greater com- 

 pressive force is applied, as there is no source whence the heat 

 necessary to a condensation can be supplied, a portion of the 

 vapour returns to a fluid state. 



The equations (E) show that a thermometer of air, or a gas, 

 under a constant pressure, will exactly measure the variations 

 of temperature by the changes of its bulk. But in reading the 

 indications of such an instrument, the air must be allowed to 

 resume a permanent volume, in order that the heat of com- 

 bination i may be dissipated. Judging by analogy, we may 

 infer that a thermometer of mercury or any other fluid, or a 

 solid thermometer, such as a bar of metal, will be an exact 

 measurer of temperature only so long as the cohesive force of 

 the particles remains nearly the same. For in these cases 

 the cohesion is the antagonist force to the expansion of heat, 

 as the pressure is in the case of the gases. 



4. One important apj)]ication of the foregoing theory is to 



determine 



