ON RADIANT HEAT. 63 



transparent water of the cell exerts no sensible absorption 

 on the luminous rays, still it withdraws something from 

 the beam, which, when permitted to act, is competent to 

 melt the ice. This something is the dark radiation of the 

 electric light. Again, I place a slab of pure ice in front 

 of the electric lamp; send a luminous beam first through 

 our cell of water and then through the ice. By means of 

 a lens an image of the slab is cast upon a white screen. 

 The beam, sifted by the water, has little power upon the 

 ice. But observe what occurs when the water is removed; 

 we have here a star and there a star, each star resembling 

 a flower of six petals, and growing visibly larger before our 

 eyes. As the leaves enlarge, their edges become serrated, 

 but there is no deviation from the six-rayed type. We 

 have here, in fact, the crystallization of the ice reversed 

 by the invisible rays of the electric beam. They take the 

 molecules down in this wonderful way, and reveal to us 

 the exquisite atomic structure of the substance with which 

 Nature every winter roofs our ponds and lakes. 



Numberless effects, apparently anomalous, might be 

 adduced in illustration of the action of these lightless rays. 

 These two powders, for example, are both white, and un- 

 distinguishable from each other by the eye. The luminous 

 rays of the sun are unabsorbed by both from such rays 

 these powders acquire no heat; still one of them, sugar /is 

 heated so highly by the concentrated beam of the electric 

 lamp, that it first smokes and then violently inflames, while 

 the other substance, salt, is barely warmed at the focus. 

 Placing two perfectly transparent liquids in test-tubes at 

 the focus, one of them boils in a couple of seconds, while 

 the other, in a similar position, is hardly warmed. The 

 boiling-point of the first liquid is 78 degrees C., which is 

 speedily reached; that of the second liquid is only 48 

 degrees 0., which is never reached at all. These anomalies 

 are entirely due to the unseen element which mingles with 

 the luminous rays of the electric beam, and indeed consti- 

 tutes 90 per cent, of its calorific power. 



A substance, as many of you know, has been discovered, 

 by which these dark rays may be detached from the total 

 emission of the electric lamp. This ray-filter is a liquid, 

 black as pitch to the luminous, but bright as a diamond to 

 the non-luminous, radiation. It mercilessly cuts off the 

 former, but allows the latter free transmission. When 



