;3S 



NA TURE 



[February 7, 1895 



ratio of the specific heats is one and two-thirds, and 

 that therefore a similar relation must hold for argon, is 

 to understate the case. Taking the very outside values, 

 no diatomic gas has a ratio greater than about \\i ; 

 and to place among these a substance for which the 

 ratio is i"66, would be entirely opposed to all the 

 other indications of a theory which, though ad- 

 mittedly only approximate, nevertheless in all other 

 cases accords fairly wiih the conceptions of the 

 chemist. The behaviour of mercury vapour suggests 

 that there are two classes of phenomena which occur 

 within the molecule — a coarser group with which the 

 ordinary mechanical theory of gases is concerned, and 

 a more refined type detected by the spectroscope. It is 

 the former, and not the latter, which are of the same 

 order as the chemical facts which lead to conclusions as 

 to the number of atoms in the molecule. 



Thus, in the case of mercury vapour, the mechanical 

 theory of gases ignores the vibrations, whether 

 mechanical or electrical, which produce the spectrum, 

 and gives the precise value of the ratio of the 

 specific heats which corresponds to three degrees of 

 freedom. Whe;her we frame a mental picture of a 

 Boscovitch point, or are content with the more usual 

 contradictory image of a smooth sphere, perfectly elastic, 

 and yet incapable of internal vibrations, is for the present 

 purpose comparatively unimportant. It is impossible to 

 connect the conception of three degrees of freedom with 

 anything that does not behave as one single thing, in- 

 capable of being set in rotation, and incapable of internal 

 vibration. That such a thing may have structure is not 

 denied. It is only affirmed that as far as the phenomena 

 under investigation are concerned, structure is not 

 recognisable. It is not denied that it may be subject to 

 internal changes. It is only affirmed that these changes 

 are of different order from the causes which affect the 

 behaviour of the molecule in what may be called 

 the pressure-producing machinery of a gas. Where 

 the ordinary dynamical theory stops, there, and there 

 precisely, chemical analysis stops also. The chemical 

 facts which prove that mercury is monatomic have 

 nothing to do with its spectrum. The arguments used 

 would be valid if it had no spectrum at all. Analysis 

 recognises no structure in the indivisible molecule of 

 mercury. In chemistry, and in the approximate theory 

 of gases, "monatomic" means — in this case, at all 

 events— the same thing. 



.\cxt take the case of the diatomic gases. 

 The results of the dynamical theory can be most easily 

 represented if we suppose the atoms to be smooth spheres, 

 which may, if it is desired, be regarded as mere gco- 

 incincal surfaces surrounding Boscovitch centres. 



The theory shows that unless the two atoms when 

 united can be fairly represented by a single point or a 

 single smooth sphere, the ratio of the specific heats ought 

 not to be greater, but may be less, than 1-4. This value 

 would correspond to the case of two smooth spheres the 

 surfaces of which were maintained in contact, or to two 

 points maintained at a certain fixed distance apart, but 

 ■ itherwisc free. Smaller values would indicate greater 

 internal freedom. 



The gases may be divided into two classes, viz. 

 firstly, 0.,Nj, H„ CO, NO and HCI, and secondly, CI.,, 

 NO. 1319. VOL. 51] 



Br, and r,. The mean of the ratios of the specific heats 

 for the first six is, according to Masson,' i'399. The 

 corresponding figure deduced from Kegnauli's experi- 

 ments is I '410. The highest value obtained from 

 Regnault is 142 in the case of HCI, but Masson's value 

 for the same gas is only vy)Z, the mean being i 406. In 

 the cases of CL, Br^, and L the values are lower, lying 

 between i'293 and I'S^s. The larger values thus ir» 

 some cases slightly exceed, but are in all cases very 

 close to, the limit fixed by the theory on the assump- 

 tion that the dual character of the molecule can be 

 recognised by it. The smaller values fall well within the 

 limit. 



The ratio of the specific heats has not been directly 

 determined for many substances, but it can be calcu- 

 lated by well-known formuUc for all gases of which 

 the specific heat at constant pressure is known, 

 and though the values thus obtained are only ap- 

 proximate, they are sufficient to prove that the mole- 

 cules of the more complex gases have always more 

 than the minimum number of degrees of freedom which 

 are consistent with the idea of their being built of 

 smooth spheres constrained to remain in cont.ict, and 

 equ.il in number to the number of atoms within the 

 molecule. Whereas, if argon is a diatomic substance, 

 the molecule has f^wer degrees of freedom than the 

 theory indicates, and is thus the single exception to a 

 universal rule. In that event it is not too much to 

 say that the result will indicate a connection between its 

 atoms of a kind absolutely different from that in any other 

 known substance. In all other cases the approximate 

 dynamical theory is in close agreement with tlie view that 

 molecular structures made up of the union of two or more 

 like or unhke things, have a more or less irregular form, 

 and are capable of rotation. If argon is not monatomic, 

 this rule will for the first time be broken. Quite apart 

 from the absolute validity of the theoretical grounds on 

 which it is based, and regarded only as suggested and 

 not as completely justified by theory, it is nevertheless 

 an empirical generalisation which up to the present has 

 stood every test. 



It is not suflicient to explain the difficulty by saying 

 that the bonds between the constituents of the argon mole- 

 cules must be very s:rong, for we have already assumed 

 that the tie which unites the centres of the hydrogen 

 or oxygen atoms is proof against the collisions which 

 occur in the gas — i.e. is or the purposes of the approxi- 

 mate theory infinitely strong. It is further necessary 

 that the molecule must be incapable of rotation, that 

 the two points must coincide, or the two spheres be 

 crushed out of shape, so that the surface is spherical. 

 Such violent assumptions would be quite unjustifiable 

 unless we are driven to them by facts which cannot be 

 disputed. . Until it is directly proved that argon is 

 diatomic, we must agree with the discoverers that the 

 weight of evidence is in favour of the molecule being 

 indivisible. Whether in the future other and more 

 convincing evidence will be adduced on the other 

 side, the future alone can show. The courts of science 

 are always open, and every litigant has an unrestricted 

 right of moving for a writ of error. 



Sec K. Strcckcr. Wieii. Ann, 1 (, 1881, p. 41. 



