492 



SCIENCE 



[N. S. Vol. XXX. No. 771 



jP = normal radiation-pressure corresponding 

 to the radiant energy per unit of volume of 

 the ether = ethereal moment of momentum 

 transferred per unit of time to a normal sur- 

 face; m = the mass of a body, an atom for 

 example, E, its energy, either actual or poten- 

 tial, V, its velocity ; V = velocity of light ; and 

 i = time. Then according to the Maxwell- 

 Bartoli formula, 



V 



X-iU 



where p is the reflecting power of the surface 

 which receives the radiation. Under the 

 action of this radiant pressure, there is a 

 transference of energy from the ether to mat- 

 ter which may be regarded as the transferring 

 of a definite amount of angular momentum 

 from the ether to matter, according to the 

 equation 



M'/t = dE = dmXr\ (1) 



Whether the energy received by an absorb- 

 ent particle from radiation shall be manifested 

 in the body as kinetic energy of mechanical 

 motion, or as thermal energy, is a detail de- 

 pending on whether the particle is free to 

 move, or is constrained by its linkage to other 

 particles. 



We may distinguish between the free ether 

 and the ether which is bound in material vor- 

 tical motion; but in a slightly different sense 

 the ether can never be said to be free, for it is 

 everywhere in contact with itself. The angu- 

 lar momentum of a free material body is 

 invariable. It can not be altered from within, 

 and can only be transferred by contact. But 

 the angular momentum of the ether which is 

 everywhere in contact is always transferred. 

 Mass, as a property of ether, is purely tem- 

 porary. The ethereal mass is strictly propor- 

 tional to the radiant energy and disappears 

 with it. Equation (1) which expresses a rela- 

 tion between ether and matter, may, however, 

 be applied to the ether alone, if m and E 

 denote its transient mass and energy during 

 radiant activity. 



The earth, by virtue of its orbital motion, 

 possesses a great store of potential energy for 

 the production of heat. An atom, by virtue 

 of the internal motions of its electrons, pos- 



sesses a relatively enormous potential energy. 

 Neither of these stores of energy can be con- 

 verted into heat except by some sort of col- 

 lision which disturbs and disarranges the har- 

 monious motion. The distinction between 

 actual and potential energy depends upon the 

 point of view. The kinetic energy of terres- 

 trial orbital motion is actual mechanical en- 

 ergy, but in relation to thermal energy the 

 kinetic energy is potential. Upon this energy 

 of a planet's orbital revolution there are super- 

 imposed the kinetic energy of its axial rota- 

 tion, that of the motion of the entire solar 

 system along the sun's way, relatively to the 

 gravitational center of motion in the galactic 

 agglomeration of which our sun forms a part, 

 and the unknovsm motion of the galaxy as a 

 whole, relatively to some point in absolute 

 space. The total energy of a body is an un- 

 known quantity, save as it becomes theoretic- 

 ally possible to predict it in the way that Pro- 

 fessor Lewis has pointed out. 



Eecognizing that only a small part of the 

 total energy-content of a body comes within 

 our cognizance, and that comparatively only a 

 brief interval in its history can come under 

 observation, I think that we may, with all 

 humility in acknowledgment of our limita- 

 tions, nevertheless find in the relations between 

 light and matter an epitome of the history of 

 the physical universe. If mass can be trans- 

 ferred through light, it follows that mass may 

 be said to originate from light, that is, from 

 ethereal rotation so circumstanced that the 

 motion becomes circumscribed and the energy 

 prevented from dissipation by ordinary 

 agencies. 



The mechanical action of light observed by 

 Crookes, although not certainly separated by 

 him from mechanical action due to the residual 

 gas of his vacuum tubes, has now been com- 

 pletely demonstrated by Lebedew, and by 

 Nichols and Hull, and has been shown by the 

 latter to be independent of the wave-length, 

 and in agreement with the prediction from 

 electromagnetic theory. 



Although the rapid motion of finely divided 

 matter in comets' tails may be explained on 

 the hypothesis of electric repulsion, the de- 

 mand for a large solar electric field has been 



