Septembee 19, 1913] 



SCIENCE 



389 



pair represent kinetic energy, the second 

 potential energy; and all the activities of 

 the material universe are represented by 

 alternations from one of these forms to the 

 other. 



Whenever this transference and trans- 

 formation of energy occur, work is done, 

 and some effect is produced, but the energy 

 is never diminished in quantity: it is 

 merely passed on from one body to another, 

 always from ether to matter or vice versa 

 — except in the case of radiation, which 

 simulates matter — and from one form to 

 another. 



The forms of energy can be classified as 

 either a translation, a rotation or a vibra- 

 tion of pieces of matter of different sizes, 

 from stars and planets down to atoms and 

 electrons; or else an etherial strain which 

 in various different ways is manifested by 

 the behavior of such masses of matter as 

 appeal to our senses.' 



Some of the facts responsible for the 

 suggestion that energy is atomic seem to 

 me to depend on the discontinuous nature 

 of the structure of a material atom, and on 

 the high velocity of its constituent par- 

 ticles. The apparently discontinuous emis- 

 sion of radiation is, I believe, due to fea- 

 tures in the real discontinuity of matter. 

 Disturbances inside an atom appear to be 

 essentially catastrophic; a portion is liable 

 to be ejected with violence. There appears 

 to be a critical velocity below which ejec- 

 tion does not take place ; and, when it does, 

 there also occurs a sudden rearrangement 

 of parts which is presumably responsible 

 for some perceptible etherial radiation. 

 Hence it is, I suppose, that radiation 

 comes off in gushes or bursts; and hence it 

 appears to consist of indivisible units. 

 The occasional phenomenon of new stars, 



'See, in the Philosophical Magazine for 1879, 

 my article on "A Classification of the Forms of 

 Energy. ' ' 



as compared with the steady orbital mo- 

 tion of the millions of recognized bodies, 

 may be suggested as an astronomical 

 analogue. 



The hypothesis of quanta was devised to 

 reconcile the law that the energy of a 

 group of colliding molecules must in the 

 long run be equally shared among all their 

 degrees of freedom, with the observed fact 

 that the energy is really shared into only a 

 small number of equal parts. For if vi- 

 bration-possibilities have to be taken into 

 account, the number of degrees of molecu- 

 lar freedom must be very large, and energy 

 shared among them ought soon to be all 

 frittered away; whereas it is not. Hence 

 the idea is suggested that minor degrees of 

 freedom are initially excluded from sharing 

 the energy, because they can not be sup- 

 plied with less than one atom of it. 



I should prefer to express the fact by 

 saying that the ordinary encountex's of 

 molecules, are not of a kind able to excite 

 atomic vibrations, or in any way to disturb 

 the ether. Spectroscopic or luminous vi- 

 brations of an atom are excited only by an 

 exceptionally violent kind of collision, 

 which may be spoken of as chemical clash ; 

 the ordinary molecular orbital encounters, 

 always going on at the rate of millions a 

 second, are ineffective in that respect, ex- 

 cept in the case of phosphorescent or 

 luminescent substances. That common 

 molecular deflexions are ineffective is cer- 

 tain, else all the energy would be dissi- 

 pated or transferred from matter into the 

 ether; and the reasonableness of their 

 radiative inefficiency is not far to seek, 

 when we consider the comparatively 

 leisurely character of molecular move- 

 ments, at speeds comparable with the ve- 

 locity of sound. Admittedly, however, the 

 effective rigidity of molecules must be com- 

 plete, otherwise the sharing of energy must 

 ultimately occur. They do not seem able 



