September 13, 1912] 



SCIENCE 



333 



heats, would appear to lead naturally to a 

 molecular theory of calorie. For instance, 

 it has often been noticed that the molecular 

 latent heats of vaporization of similar com- 

 pounds at their boiling points are propor- 

 tional to the absolute temperature. It fol- 

 lows that the molecular latent caloric of 

 vaporization is the same for all such com- 

 pounds, or that they require the same num- 

 ber of molecules of caloric to effect the same 

 change of state, irrespective of the absolute 

 temperatures of their boiling points. From 

 this point of view one may naturally regard 

 the liquid and gaseous states as conjugate 

 solutions of calorie in matter and matter in 

 caloric respectively. The proportion of 

 caloric to matter varies regularly with pres- 

 sure and temperature, and there is a defi- 

 nite saturation limit of solubility at each 

 temperature. 



One of the most difSeult cases of the gen- 

 eration of caloric to follow in detail is that 

 which occurs whenever there is exchange of 

 heat by radiation between bodies at differ- 

 ent temperatures. If radiation is an elec- 

 tro-magnetic wave-motion, we must suppose 

 that there is some kind of electric oscillator 

 or resonator in the constitution of a ma- 

 terial molecule which is capable of re- 

 sponding to the electric oscillations. If the 

 natural periods of the resonators correspond 

 sufficiently closely with those of the inci- 

 dent radiation the amplitude of the vibra- 

 tion excited may be sufficient to cause the 

 ejection of a corpuscle of caloric. It is gen- 

 erally admitted that the ejection of an elec- 

 tron may be brought about in this manner, 

 but it would evidently require far less en- 

 ergy to produce the emission of a neutral 

 corpuscle, which ought therefore to be a 

 much more common effect. On this view, 

 the conversion of energy of radiation into 

 energy of caloric is a discontinuous process 

 taking place by definite molecular incre- 



ments, but the absorption or emission of 

 radiation itself is a continuous process. 

 Professor Planck, by a most ingenious ar- 

 gument based on the probability of the 

 distribution of energy among a large num- 

 ber of similar electric oscillators (in which 

 the entropy is taken as the logarithm of the 

 probability, and the temperature as the 

 rate of increase of energy per unit of en- 

 tropy) , has succeeded in deducing his well- 

 known formula for the distribution of en- 

 ergy in full radiation at any temperature ; 

 and has recently, by a further extension of 

 the same line of argument, arrived at the 

 remarkable conclusion that, while the ab- 

 sorption of radiation is continuous, th& 

 emission of radiation is discontinuous, oc- 

 curring in discrete elements or quanta. 

 Where an argument depends on so many 

 intricate hypotheses and analogies the pos- 

 sible interpretations of the mathematical 

 formula are to some extent uncertain ; but 

 it would appear that Professor Planck's 

 equations are not necessarily inconsistent 

 with the view above expressed that both 

 emission and absorption of radiation are 

 continuous, and that his elementa quanta, 

 the energy of which varies with their fre- 

 quency, should rather be identified with the 

 molecules of caloric, representing the con- 

 version of the electro-magnetic energy of 

 radiation into the form of heat, and pos- 

 sessing energy in proportion to their tem- 

 perature. 



Among the difficulties felt rather than 

 explicitly stated, in regarding entropy or 

 caloric as the measure of heat quantity, is 

 its awkward habit of becoming infinite, ac- 

 cording to the usual approximate formulae, 

 at extremes of pressure or temperature. If 

 caloric is to be regarded as the measure of 

 heat quantity, the quantity existing in a 

 finite body must be finite, and must vanish 

 at the absolute zero of temperature. In 



