THE TEACHINGS OF MODERN SPECTROSCOPY. 481 



and almost entirely disappears. It is replaced by a strong orange line, 

 which is already slightly visible, though weak, at low temperatures, 

 and by additional green and blue lines. 



But even at the high temperature of the spark we may obtain again 

 a spectrum containing the red line only if we mix a small quantity of 

 lithium with a large quantity of other material. The same spark, for 

 instance, will give us the low-temperature spectrum of lithium when 

 taken from a dilute solution of a lithium salt, and the high-temperature 

 spectrum when that solution is concentrated. 



The spectra of zinc and tin furnish us other examples in the same 

 direction, but the spectra of nearly all bodies show the same law in a 

 more or less striking way. 



If this law which I have given you is a true one,* and I believe it 

 will stand any test to which no doubt it will be subjected, we shall be 

 able to draw some important conclusions from it. In the first place, it 

 will be proved that the forces between atoms do depend on their vibra- 

 tions. If this is true, any change in the vibrations of the spectrum, 

 however small, will entail a corresponding change in all the other prop- 

 erties of the body. On the other hand, any change in the affinities of 

 the element observed by other means will be represented by a change 

 in the spectrum. 



It is also possible that the introduction of forces due to vibratory 

 motion will help us over a considerable difficulty in the molecular 

 theory of gases. Some of the conclusions of that theory are at pres- 

 ent absolutely contrary to fact. A spectroscopist, for instance, who is 

 acquainted with the mercury spectrum and all the changes in that 

 spectrum which can take place, feels more than skeptical when he is 

 told that the molecule of mercury contains only one atom, which 

 neither rotates nor vibrates. 



Nor can it be of advantage to science to pass silently over this 

 difficulty, or to neglect it as unessential, as is often done by modern 

 writers. The late Professor Maxwell, at least, was well aware of 

 its importance, and has often expressed in private conversation how 

 serious a check he considered the molecular theory of gases to have 

 received. This is not the place to enter more fully into this point, and 

 to consider how the vibratory forces may affect some of the suppo- 

 sitions on which the theoretical consequences are founded. 



However important the effects of concentration or dilution on the 



* Lockyer, " Studies in Spectrum Analysis," p. 140, draws attention to the fact that 

 an admixture of a second element dims the spectrum of the first, and he expresses this 

 fact by saying, " In encounters of dissimilar molecules the vibrations of each are 

 damped." Later he has shown that the lines of oxygen and nitrogen, which are wide 

 at atmospheric pressure, thin out when the gases' are only present in small quantities. 

 Lecoq de Boisbaudran in his "Atlas" gives several examples of the differences in the 

 relative brilliancy of lines produced by concentrating or diluting the solution from which 

 the spark is taken. The complete parallelism of this change to the changes produced by 

 increased temperature has, however, never received sufficient attention. 



VOT. TTY. 81 



