KADIANT HEAT AND ITS RELATIONS. 85 



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 re- 

 moved ; 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 crystal- 

 lisation 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 undistinguishable 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 C., which is speedily reached; 

 that of the second liquid is only 48 C., 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 constitutes 90 per cent, 

 of its calorific power. 



A substance, as many of you know, has been dis- 

 covered, by which these dark rays may be detached from 



