January 24, 1913] 



SCIENCE 



127 



sought to do,-^ or by seeking for causes of 

 these specific heat relations which have 

 nothing to do with quantum theory. The 

 one conclusion which this experimental 

 work in atomic heats drives home is that 

 the principle of equi-partition of energy, 

 while valid when applied to atomic vibra- 

 tors for certain ranges of temperature, has 

 no general validity. This is, however, 

 nothing new. If we can not get rid of it 

 without a quantum theory, as Planck and 

 Poincare and Jeans all imply, then some 

 form of quantum theory has been demon- 

 strated to be a necessity. If these atomic 

 heat experiments stood alone, however, I 

 fancy that other and more easily visual- 

 izable explanations would be sought. For 

 example, so far, they seem to be qualita- 

 tively consistent with an assumption like 

 this, namely, that as the absolute zero is 

 approached, the atoms begin to freeze to- 

 gether, and thus the number of effective 

 carriers of energy is diminished. The 

 higher the atomic frequency the higher the 

 temperature at which this freezing-up 

 process begins. The atoms of solids would 

 then be imagined to freeze into rigid sys- 

 tems of continually increasing size, each 

 system being endowed, however, with the 

 kinetic energy of agitation appropriate to 

 its temperature. It might then become 

 possible, before absolute zero was reached, 

 for the total kinetic energy content of the 

 whole mass to become that of a single mole- 

 cule of the surrounding gas. Such an hy- 

 pothesis would seem to account well for the 

 exceedingly high thermal conductivity of 

 non-metals at low temperatures,^^ since the 

 transfer of energy from point to point 

 would be effected by a diminishing number 

 of intermediaries as the temperature fell. 

 If, however, a quantum theory must be 



-'^ Nernst and Lindemann, Zeit. fiir Elektro- 

 chemie, 17, p. 867, 1911. 



^^Euken, Ann. der Phys., 34, p. 185. 



called in to account for phenomena in 

 other fields, it is of course in the interest of 

 simplicity to make it do service in the field 

 of atomic heats as well. All that can now 

 be said is that the attempts thus far made 

 to apply it quantitatively in this field have 

 not been particularly successful, though 

 they have been sufficiently suggestive to 

 stimulate to further experimenting. New 

 data are sure to pour in rapidly in the near 

 future. 



3. We now come to the forrhs of atom- 

 istic theory which make radical assump- 

 tions regarding the distribution of radiant 

 energy in space, rather than in time. The 

 least radical of these, because the least gen- 

 eral, is that of which Professor Bragg"^ is 

 the most active exponent. It is frankly 

 corpuscular. It was developed, however, 

 with a view of explaining the properties 

 of one type of radiation only, namely, 

 X- and y-rays, and at a time when there 

 was some justification for regarding these 

 as isolated phenomena. Recent develop- 

 ments have strongly emphasized the sim- 

 ilarities, rather than the differences, be- 

 tween these and other types of so-called 

 ethereal radiations. But this in no way 

 weakens the positive arguments for a coi'- 

 puscular form of X-ray. The most com- 

 pelling of these arguments is as follows : 



X-rays unquestionably pass over, or 

 pass all but an exceedingly minute frac- 

 tion of the atoms contained in the space 

 traversed, without spending any energy 

 upon them or influencing them in any 

 observable way. But here and there 

 they find an atom from which they hurl 

 an electron with enormous speed. This 

 is the most interesting and most signifi- 

 cant characteristic of the X-ray, and 

 one which distinguishes it from a- and 

 ^-rays just as sharply as does the property 



^ Bragg, "Studies in Radioactivity," 1912. 



