RADIANT HEAT AND ITS RELATIONS 91 



focus a piece of ice, the ice is not melted by the concen- 

 trated beam. Matches, at the same place, are ignited, and 

 wood is set on fire. The powerful heat, then, of this lu- 

 minous beam is incompetent to melt the ice. On with- 

 drawing the cell of water, the ice immediately liquefies, 

 and the water trickles from it in drops. Eeintroducing the 

 cell of water, the fusion is arrested, and the drops cease 

 to fall. The transparent water of the cell exerts no sen- 

 sible absorption on the luminous fays, 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 ad- 

 duced in illustration of the action of these lightless rays. 

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

 distinguishable from each other by the eye. The luminous 

 rays of the sun are unabsorbed by both — ^from such rays 



