214 Dr. A. Naumann on the Specific Heat of Gases 



conditions in which their specific heats were determined. A 

 further confirmation of this is afforded by the fact that the the- 

 oretically calculated specific heats are almost always smaller, 

 sometimes considerably smaller, but never notably greater than 

 the observed values ; for the further a gas is from the perfect 

 gaseous condition — the more molecular attractions have still to 

 be overcome when it is heated, — the greater must be the excess 

 of the specific heat as found by experiment over that calculated 

 theoretically. In the case of chlorine, which in one of its com- 

 pounds (hydrochloric acid) conforms to the above law in regard 

 to its specific heat, we are certainly justified in concluding that 

 in the free state it is comparatively far from following the laws 

 of Mariotte and Gay-Lussac. Thus starting from the simpler 

 relations, which in reality hold only approximately and in a 

 minority of cases, we have a method of investigating the more 

 complex relations presented by the majority of substances. Reg- 

 nault himself takes as the basis of his reasoning the theore- 

 tical behaviour of ideal gases, in order to judge of the molecular 

 state of the gases in accordance with their observed specific 

 heats, in cases where a knowledge of the compressibility and 

 expansion is defective or altogether wanting, as, for instance, in 

 relation to carbonic acid* and the gases f enumerated in the 

 second half of the above Table. 



For chemically allied bodies containing equal numbers of atoms 

 in a molecule, regularities are almost always observable. In 

 such cases the difference between the observed specific heat and 

 that calculated for the perfectly gaseous state is greater, and in 

 ^general also the boiling-point is higher, in proportion as the 

 molecular weight is greater, as is shown by the following- 

 Table (p. 215). 



Since bodies of the same number of atoms and of correspond- 

 ing chemical composition thus depart in general so much the 

 more from the laws which apply to perfect gases as their mole- 

 cular weight is greater, we may conclude that the mutual attrac- 

 tions of similar atoms which still subsist in the ordinary gaseous 

 condition become stronger under otherwise identical conditions 

 as the mass of the molecules increases — a result which is in per- 

 fect accordance with one of the fundamental laws of attraction. 



* Op. cit. p. 299. f Ibid. pp. 319, 320. 



