272 



SCIENCE 



[N. S. Vol. XXX. No. 765 



per year is only one part in 20 billionths 

 (1 in 2 X 10'^) of the mass of the sun, and 

 as this diminution in mass is not neces- 

 sarily accompanied by any decrease in its 

 gravitational attraction, we can not expect 

 to be able to get any evidence of this 

 effect. 



As our knowledge of the properties of 

 light has progressed, we have been driven 

 to recognize that the ether, when transmit- 

 ting light, possesses properties which, be- 

 fore the introduction of the electro-mag- 

 netic theory, would have been thought to 

 be peculiar to an emission theory of light 

 and to be fatal to the theory that light con- 

 sists of undulations. 



Take, for example, the pressure exerted 

 by light. This would follow as a matter 

 of course if we supposed light to be small 

 particles moving with great velocities, for 

 these, if they struck against a body, would 

 manifestly tend to push it forward, while 

 on the undulatory theory there seemed no 

 reason why any effect of this kind should 

 take place. 



Indeed, in 1792, this very point was re- 

 garded as a test between the theories, and 

 Bennet made experiments to see whether or 

 not he could find any traces of this pres- 

 sure. We now know that the pressure is 

 there, and if Bonnet's instrument had been 

 more sensitive he must have observed it. 

 It is perhaps fortunate that Bennet had 

 not at his command more delicate appa- 

 ratus. Had he discovered the pressure of 

 light, it would have shaken confidence in 

 the undulatory theory and checked that 

 magnificent work at the beginning of the 

 last century which so greatly increased our 

 knowledge of optics. 



As another example, take the question of 

 the distribution of energy in a wave of 

 light. On the emission theory the energy 

 in the light is the kinetic energy of the 

 light particles. Thus the energy of light is 



made up of distinct units, the unit being 

 the energy of one of the particles. 



The idea that the energy has a structure 

 of this kind has lately received a good deal 

 of support. Planck, in a very remarkable 

 series of investigations on the thermody- 

 namics of radiation, pointed out that the 

 expressions for the energy and entropy of 

 radiant energy were of such a form as to 

 suggest that the energy of radiation, like 

 that of a gas on the molecular theory, was 

 made up of distinct units, the magnitude 

 of the unit depending on the color of the 

 light; and on this assumption he was able 

 to calculate the value of the unit, and from 

 this deduce incidentally the value of Avo- 

 gadro's constant— the number of molecules 

 in a cubic centimeter of gas at standard 

 temperature and pressure. 



This result is most interesting and im- 

 portant because if it were a legitimate de- 

 duction from the second law of thermody- 

 namics, it would appear that only a par- 

 ticular type of mechanism for the vibrators 

 which give out light and the absorbers 

 which absorb it could be in accordance with 

 that law. 



If this were so, then, regarding the uni- 

 verse as a collection of machines all obey- 

 ing the laws of dynamics, the second law of 

 thermodynamics would only be true for a 

 particular kind of machine. 



There seems, however, grave objection to 

 this view, which I may illustrate by the 

 case of the first law of thermodynamics, 

 the principle of the conservation of energy. 

 This must be true whatever be the nature 

 of the machines which make up the uni- 

 verse, provided they obey the laws of dy- 

 namics, any application of the principle 

 of the conservation of energy could not 

 discriminate between one type of machine 

 and another. 



Now, the second law of thermodynamics, 

 though not a dynamical principle in as 



