526 Prof. TyndalFs Contributions to Molecular Physics. 



and still we find the whole of the heat intercepted*. A layer 

 of water, O07 of an inch in thickness, is sensibly opake to the 

 radiation from a hydrogen-flame. Hence we may infer the coin- 

 cidence in period between cold water and aqueous vapour heated 

 to a temperature of 3259° C. ; and inasmuch as the period of 

 the water-molecules has been proved to be extra-red, the period 

 of the. vapour-molecules in the hydrogen-flame must be extra- 

 red also. 



Another point of considerable interest may here be adverted 

 to. Professor Stokes has demonstrated that a change of period 

 is possible to those rays which belong to the violet and extra- 

 violet end of the spectrum, the change showing itself by a 

 degradation of the refrangibility. That is to say, vibrations of a 

 rapid period are absorbed, and the absorbing substance has 

 become the source of vibrations of a longer period. Efforts, I 

 believe, have been made to obtain an analogous result at the red 

 end of the spectrum, but hitherto without result ; and it has been 

 considered improbable that a change of period can occur which 

 should raise the refrangibility of the light or heat. Such a 

 change, I believe, occurs when we plunge a platinum wire into a 

 hydrogen-flame. The platinum is rendered white by the colli- 

 sion of molecules whose periods of oscillation are incompetent to 

 excite vision. There is in this common experiment an actual 

 breaking up of the long periods into short ones — a true render- 

 ing of un visual periods visual. The change of refrangibility 

 differs from that of Professor Stokes, firstly, in its being in the 

 opposite direction — that is, from low to high ; and secondly, in 

 the circumstance that the platinum is heated by the collision of 

 the molecules of aqueous vapour, and before their heat has 

 assumed the radiant form. But it cannot be doubted that the 

 same effect would be produced by radiant heat of the same period, 

 provided the motion of the ether could be raised to a sufficient 

 intensity. The effect in principle is the same, whether we con- 

 sider the platinum wire to be struck by a particle of aqueous 

 vapour oscillating at a certain rate, or by a particle of ether 

 oscillating at the same rate. And thus, I imagine, by a chain 

 of rigid reasoning, we arrive at the conclusion that a degree of 

 incandescence, equal to that of the sun itself, might be produced 

 by the impact of waves, of themselves incompetent to excite 

 vision f. 



* From the opacity of water to the radiation from aqueous vapour, we 

 may infer the opacity of aqueous vapour to the radiation from water, and 

 hence conclude that the very act of nocturnal refrigeration which causes 

 the condensation of water on the earth's surface gives to terrestrial radia- 

 tion that particular character which renders it most liable to be intercepted 

 by the aqueous vapour of the air. 



t Some time after this was written I learned that Dr. Akin had previ- 



