CLOUDS AND RIVERS, ICE AND GLACIERS. 15 



ivaves. The electric light, therefore, shall be employed 

 in our experimental demonstrations. 



40. From this source a powerful beam is sent through 

 the room, revealing its track by the motes floating in 

 the air of the room ; for were the motes entirely absent 

 the beam would be unseen. It falls upon a concave 

 mirror (a glass one silvered behind will answer) and 

 is gathered up by the mirror into a cone of reflected 

 rays ; the luminous apex of the cone, which is the focus 

 of the mirror, being about fifteen inches distant from its 

 reflecting surface. Let us mark the focus accurately by 

 a pointer. 



41. And now let us place in the path of the beam a 

 substance perfectly opaque to light. This substance is 

 iodine dissolved in a liquid called bisulphide of carbon. 

 The light at the focus instantly vanishes when the dark 

 solution is introduced. But the solution is intensely 

 transparent to the dark waves, and a focus of such 

 waves remains in the air of the room after the light 

 has been abolished. You may feel the heat of these 

 waves with your hand; you may let them fall upon a 

 thermometer, and thus prove their presence ; or, best 

 of all, you may cause them to produce a current of 

 electricity, which deflects a large magnetic needle. The 

 magnitude of the deflection is a measure of the heat. 



42. Our object now is, by the use of a more powerful 

 lamp, and a better mirror (one silvered in front 

 and with a shorter focal distance), to intensify the 



