ACTION OF DARK RADIATIONS. 205 



heat-waves, and allow the light-waves alone to pass. These may be 

 concentrated by suitable lenses and sent into watei without sensibly 

 warming it. Let the light-waves now be withdrawn, and the larger 

 heat-waves concentrated in the same manner ; they may be caused to 

 to boil the water almost instantaneously. 



This is the point to which I wished to lead you, and which without 

 due preparation could not be understood. You now perceive the im- 

 portant part played by these large clarkness-waves, if I may use the 

 term, in the work of evaporation. AVhen they plunge into seas, lakes, 

 and rivers, they are intercepted close to the surface, and they heat the 

 water at the surface, thus causing it to evaporate ; the light-waves at 

 the same time entering to great depths without sensibly heating the 

 water through which they pass. Not only, therefore, is it the sun's fire 

 which produces evaporation, but a particular constituent of that fire, 

 the existence of which you probably were not aware of. 



Further, it is these self-same lightless waves which, falling upon the 

 glaciers of the Alps, melt the ice and produce all the rivers flowing 

 from the glaciers; for I shall prove to you presently that the light- 

 waves, even when concentrated to the uttermost, are unable to melt 

 the most delicate hoar-frost ; much less would they be able to produce 

 the copious liquefaction observed upon the glaciers. 



These large lightless waves of the sun, as well as the heat-waves 

 issuing from non-luminous hot bodies, are frequently called obscure or 

 invisible heat. 



We have here an example of the manner in which phenomena, ap- 

 parently remote, are connected together in this wonderful system of 

 things that we call Nature. You cannot study a snow-flake profoundly 

 without being led back by it step by step to the constitution of the 

 sun. It is thus throughout Nature. All its parts are interdependent, 

 and the study of any one part completely would really involve the 

 study of all. 



Heat issuing from any source not visibly red cannot be concentrated 

 so as to produce the intense effects just referred to. To produce these 

 it is necessary to employ the obscure heat of a body raised to the high 

 est possible state of incandescence. The sun is such a body, and its 

 dark heat is therefore suitable for experiments of this nature. But in 

 the atmosphere of London, and for experiments such as ours, the heat- 

 waves emitted by coke, raised to intense whiteness by a current of elec- 

 tricity, are much more manageable than the sun's waves. The electric 

 light has also the advantage that its dark radiation embraces a larger 

 proportion of the total radiation than the dark heat of the sun. In fact, 

 the force or energy, if I may use the term, of the dark waves of the 

 electric light is fully seven times that of its light-waves. The electric 

 light, therefore, shall be employed in our experimental demonstrations. 



From this source a powerful beam is sent through the room, re- 

 vealing its track by the motes floating in the air of the room ; for, were 



