RADIANT HEAT AND ITS RELATIONS. 223 



Placing: a concave silvered mirror behind the electric 

 light I converge its rajs to a focus of dazzling brilliancy. 

 I place in the path of the rays, between the light and the 

 focus, a vessel of water, and now introduce at the focus a 

 piece of ice. The ice is not melted by the concentrated 

 beam which has passed through the water, though matches 

 are ignited at the focus and wood is set on fire. The pow- 

 erful heat, then, of this luminous beam is incompetent to 

 melt the ice. I withdraw the cell of water ; the ice imme- 

 diately liquefies, and you see the water trickling from it 

 in drops. I reintroduce the cell of water; the fusion is 

 arrested and the drops cease to fall. The 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 no 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 inverted 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 ad- 

 duced in illustration of the action of these lightless rays. 

 Here, for example, are two powders, both white, and undis- 

 tinguishable from each other by the eye. The luminous 

 rays of the lamp are unabsorbed by both powders — from 



