66 



SCIENCE. 



[N. S. Vol. XXI. No. 524. 



not know definitely whether ozone is formed 

 by the action of ultra-violet light in the ab- 

 sence of electrical waves, though this is a 

 matter easily settled by experiment. We 

 know that ozone gives out light on decom- 

 posing, but we do not know anything about 

 the spectrum of this light. It is quite 

 probable also that we must formulate the 

 theorem of Le Chatelier more exactly than 

 we have hitherto done before we can apply 

 it successfully to the phenomena of light. 

 An instance based on electrical phenomena 

 will show what I mean. If a voltaic cell 

 be short-circuited the chemical change will 

 be such as to decrease the electromotive 

 force of the cell. If we do not keep the 

 cell at constant temperature the Joule heat 

 will cause the temperature to rise and this 

 may either raise or lower the electromotive 

 force of the cell. "We are then really con- 

 sidering two phenomena, the electrical and 

 the heat effects. One may mask the other 

 completely. 



In one case, at any rate, we know that 

 we can apply the theorem of Le Chatelier 

 to light phenomena. Suppose we have a 

 gas enclosed in a transparent adiabatic 

 vessel and concentrate upon it light of a 

 wave-length that is absorbed by the gas. 

 The temperature of the gas will rise and 

 equilibrium will be reached when the gas 

 has changed so that it no longer absorbs 

 light of that particular wave-length or 

 when the gas emits light of the same wave- 

 length and intensity as that which is acting 

 upon it. This emission by a gas at some 

 temperature of the light which it absorbs 

 at the same temperature is Kirchhoff 's law, 

 which thus appears as a special case of 

 what the chemists call the theorem of Le 

 Chatelier. To be frank, I do not now see 

 how we are to apply this theorem to the 

 phenomenon of phosphorescence, and yet 

 we are dealing with an absorption and an 

 emission of light. I venture to suggest 

 that it is to the application of the theorem 



of Le Chatelier that we must look for a 

 rational treatment of phosphorescence, 

 fluorescence, chemiluminescence, etc., rath- 

 er than to a theory of vibrating molecules. 

 It will be time enough to discuss the appli- 

 cation to radiations when we have solved 

 the simpler problem of the theory of cold 

 light. 



A discussion of equilibrium relations 

 would not be complete without some refer- 

 ence to the future of thermodynamics in 

 chemistry. There are two radically dis- 

 tinct ways of considering the relation of 

 thermodynamics to chemistry. One is to 

 look upon thermodynamics as a mathemat- 

 ical shorthand. The aim of thermody- 

 namics is then to present a consistent and 

 formal treatment of the known energy re- 

 lations. In this case thermodynamics deals 

 with the past and not with the future ; with 

 the classification of knowledge and not with 

 the discovery of new laws. This is the 

 point of view of most mathematical chem- 

 ists and it is because of this that Ave do not 

 turn to the mathematical chemist for new 

 ideas. There is another way of consider- 

 ing thermodynamics, namely, as an instru- 

 ment of research. It is not too much to 

 say that the mathematical chemist can 

 work out in a few hours or days results 

 which would take his less fortunate col- 

 league months or even years to obtain. At 

 present the race is to the tortoise and not 

 to the hare ; but I can not believe that this 

 will always be so. Other things being 

 equal, the man who can handle his thermo- 

 dynamics will beat the man who can not; 

 but in order to have that take place thermo- 

 dynamics must be considered as an instru- 

 ment of research and not as a branch of 

 metaphysics. We must confess that the 

 mathematical chemistry of the past decade 

 has not done what it should have done and 

 that there is no immediate prospect of any 

 improvement. In the meantime we do not 

 despair. There are great possibilities in 



