504 Br. Arthur Schuster [Jan. 28, 



yet produces the same change in a spectrum which we have hitherto 

 only ascribed to changes of temperature. 



I must here remark that a change in type is not the only spectro- 

 scopic change in the spectrum which is observed to take place on 

 varying the temperature. Line spectra especially are subject to 

 curious variations in the relative intensities of their lines. These 

 variations follow no general rule, and must be investigated separately 

 for each element. The cause of this variation is a subject on which 

 there exists a great difference of opinion ; but, whatever this cause may 

 be, if the changes always take place at one fixed temperature, we can 

 turn them into account in measuring that temperature. However strong 

 our wish that such a spectroscopic measurement of temperature may 

 ultimately be obtained, a remarkable complication of facts has delayed 

 the realisation of this hope for at least a considerable period of time. 



We have to enter partly into a theoretical question, and I must 

 necessarily allude to some of the facts recognised by all who believe 

 in the molecular theory of gases. Each molecule, which, as we have 

 seen, sends out rays of light and heat on account of its internal 

 motion, is surrounded by other molecules. These are, indeed, very 

 closely packed, and continually moving about with enormous velocities. 

 Generally they move in straight lines, but it must necessarily happen 

 that often they come very near, and then affect and deflect each other. 

 Perhaps they come into actual contact, perhaps they repel each other 

 so strongly when near, that contact never takes place. The time 

 elapsing between two such collisions is very small. If you can 

 imagine one second of time to be magnified to the length of a 

 hundred years, it would only take about a second, on the average, 

 from the time a molecule has encountered one other molecule until it 

 encounters the second. During the greatest part of this very short 

 time, it moves in a straight line, for the forces between molecules are 

 so small that they do not affect each other, unless their distance is 

 exceedingly small. It is, therefore, only during a very small fraction 

 of time that one molecule is under the influence of another, and it is 

 one of the greatest problems of molecular physics to find out what 

 happens during that short element of time. I should like to explain 

 to you how I believe the spectroscope may contribute its share to the 

 settlement of that question. In his first great paper on the molecular 

 theory of gases, the late Professor Clerk Maxwell assumed that two 

 molecules may actually come into contact, that they may strike each 

 other, as two billiard balls do, and then separate, according to the 

 laws of elastic bodies. This theory is difficult of application when a 

 molecule contains more than one atom, and especially as it did not in 

 the case of conduction of heat give results ratified by the experimental 

 test. Maxwell abandoned it in favour of the idea that molecules repel 

 each other according to the inverse fifth j)ower of the distance. This 

 second theory not only gave what at the time was believed to be the 

 correct law for the dependence of the coefficient of conduction on 

 temperature, but it also helped its author over a considerable mathe- 



