1881.] on the Teachings of Modern Spectroscopy. 605 



inatioal difficulty. Further cxi)crimcnts Lave sliakcn our faith in the 

 iirst of these two rea8t)iis, aud the second is not sufficient to induce us 

 to ad()i)t without further incpiry the new law of action between two 

 molecules. 



It is exceedingly likely that the forces acting between two mole- 

 cules when they are in close proximity to each other are partly due to, 

 or at least moditicd by the vibrations of the molecules themselves. 

 Such vibrations must, as in the case of sound, produce attractive and 

 repulsive forces, aud vibrating molecules will affiict each other in a 

 similar way as two tuning-forks would. Now, if the forces due to 

 vibrations play any important part in a molecular encounter, the 

 spectroscope will, I fancy, give us some information. If two mole- 

 cules of the same kind encounter, the i)eriods of vibration are the same, 

 and the forces due to vibration will remain the same during, perhaps, 

 the whole encounter. If two dissimilar molecules encounter, the 

 relative phase of the vibrations, and hence the forces, will constantly 

 change. Attraction will rapidly follow repulsion, and the whole 

 average effect will be much smaller than in the case of two atoms of 

 the same kind. We have no clear notion how such differences may 

 act, and we must have recourse to experiment to decide whether any 

 change in the effect of an encounter is observed when a molecule of 

 a different kind is substituted for a molecule having the same jieriods 

 of vibration. 



When a body loses energy by radiation, that energy is restored 

 during an encounter ; the way in which this energy is restored will 

 profoundly affect the vibrations of the molecule, and hence the 

 observed S2)ectrum. I have endeavoured by means of theoretical 

 considerations, or speculations, as you may perhaps feel inclined to call 

 them, to lead you on to an experimental law which I believe to be of 

 very great importance. The spectrum of a molecule is in fact 

 variable at any given temperature, and changes if the molecule is 

 surrounded by others of different nature. 



Placing a molecule in an atmosphere of different nature ivithout 

 change of temperature produces the same effect as would he observed on 

 lowering of temperature. 



Let me give you one example. Lithium at the temperature of the 

 Bunsen flame has almost exclusively one red line in its S2)ectrum. 

 At the high temperature of the arc or spark the red line becomes 

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

 orange line, which is already slightly visible, though weak, at low 

 temperatui'es, and by additional green and blue lines. 



But even at the high temperature of the spark wo 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 tem2)erature spec- 

 trum of lithium when taken from a dilute solution of a lithium salt, 

 and the high temperature spectrum when that solution is concen- 

 trated. 



