Prof. Norton on Molecular Physics. 285 



subsequent cooling*." According to what has just been stated, 

 all such bodies must evolve heat because of the electric conden- 

 sation that attends the formation of new compound molecules 

 after the primitive molecules have been decomposed. Now, if 

 these new molecules be cooled, the tendency would be to increase 

 the molecular attraction by which their constituents are held 

 together, rather than to favour their decomposition. The in- 

 stances cited, viz. protoxide of nitrogen, peroxide of hydrogen, 

 chlorous and chloric acids, the chloride, iodide, and sulphide of 

 nitrogen, seem to sustain this explanation. 



It is frequently found by experiment that if heat is absorbed 

 in the act of decomposition, the separated components will re- 

 unite on cooling. It is probable that in such instances there 

 is no formation of new molecules following the first decomposi- 

 tion, and that the heat which may be absorbed in addition to 

 that consumed in the act of separation, is due solely to an expan- 

 sion taking place after the receding molecules have been urged 

 beyond the limit of molecular attraction and become subject to 

 molecular repulsion. In that event a subsequent cooling, by 

 contracting the atmospheres of the molecules and so increasing 

 their attractive force, might favour a reunion. Hydrate of lime 

 may be cited as a case admitting of this explanation. 



We have seen that heat, by expanding and augmenting the 

 repulsive energy of molecular atmospheres, tends virtually to 

 polarize a surface positively, and that in this fact we have the 

 probable origin of thermo-electricity (/. c.p. 276), and the explana- 

 tion of the effect of heat in the development of electricity by 

 friction (/. c. p. 108) . The same polarizing action of heat should 

 favour the oxidation of metallic surfaces. The condensing action 

 of platinum and certain other metals upon gases, and of sur- 

 faces generally upon aqueous vapour with evolution of heat (see 

 Silliman's American Journal, vol. xxxviii. p. 109), may be referred 

 to a polarization developed by the mutual action of the surface 

 and the gas or vapour. The attraction thus resulting, we must 

 suppose, is not in general sufficient to bring the gases within the 

 range of the natural molecular attraction, and so into complete 

 union with the surface. 



Combination by Volume. — The assumption is now generally 

 made, that the molecules of all substances in the gaseous state 

 occupy the same volume. This conclusion, to which chemists 

 have been conducted, implies that there are the same number of 

 molecules in equal volumes of different substances, and therefore 

 that the elastic tension of the individual molecules is the same 

 for all gases. This result corresponds with the determination of 

 the repulsive energy of molecules at the greater distances, as given 

 in Table I. (vol. xxviii. p. 200), if we regard the coefficient m as con- 



