THE TEACHINGS OF MODERN SPECTROSCOPY. 479 



to proceed would consist in carefully observing the spectra in different 

 layers of the sun. Supposing we observe a change at one point, we 

 may investigate at what temperature that change takes place, and we 

 miay then ascribe the same temperature to that particular place at the 

 solar surface, if no other cause has interfered which may have affected 

 our result. This last conditional limitation leads us to the discussion 

 of the important but difficult question, whether we can determine any 

 such interfering cause, which, not being temperature, 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 vary- 

 ing 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 ulti- 

 mately be obtained, a remarkable complication of facts has delayed 

 the realization 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 recognized 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 mo- 

 tion, is surrounded by other molecules. These are, indeed, very closely 

 packed, and continually moving about with enormous velocities. Gen- 

 erally 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 mole- 

 cule 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 



