48o THE POPULAR SCIENCE MONTHLY. 



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 api^lication when a molecule con- 

 tains more than one atom, and, especially as it did not in the case of 

 conduction of heat give results ratified by the experimental test. Max- 

 well abandoned it in favor of the idea that molecules repel each other 

 according to the inverse fifth power 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 tempera- 

 ture, but it also helped its author over a considerable mathematical 

 difficulty. Further experiments have shaken our faith in the first of 

 these two reasons, and the second is not sufficient to induce us to adopt 

 without further inquiry 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 modified by, the vibrations of the molecules themselves.' 

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

 repulsive forces, and vibrating molecules will affect each other in a simi- 

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

 tions play any important part in a molecular encounter, the spectroscope 

 will, I fancy, give us some information. If two molecules of the same 

 kind encounter, the periods of vibration are the same, and the forces 

 due to vibration will remain the same during, perhaps, the whole en- 

 counter. 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 encoun- 

 ter is observed when a molecule of a different kind is substituted for a 

 molecule having the same periods 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 ob- 

 served spectrum. I have endeavored by means of theoretical consid- 

 erations, 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 toithout 

 change of temjyerature produces the same effect as loould 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 spectrum. At 

 the high terhperature of the arc or spark the red line becomes weak, 



