Chap. xxxv.] ABSORPTION OF HEAT. 461 



can be foeussed, so they can be reflected. A source 

 of heat placed at a distance from a concave mirror 

 has a conjugate focus in front of a mirror, and if an 

 explosive substance be placed in the conjugate focus, 

 the concentration of heat at that point is speedily 

 evident. 



Heat rays are also subject to interference like 

 light rays. 



Absorption and emission of heat. In 

 other ways heat rays present remarkable analogies to 

 light rays. It has been observed how certain substances 

 transmit the undulations of light, and how others 

 intercept them, the former being called transparent, 

 the latter opaque bodies. The explanation of their 

 action is that certain bodies permit the vibrations to 

 pass through them unaffected, while others pick out 

 the vibrations and thus absorb them. We have seen 

 also how this explains the production of colour. 

 Various substances act selectively on light rays, 

 which consist of vibrations of various lengths, one 

 substance selecting vibrations of particular lengths, 

 which it intercepts, while it permits others to pass. 

 Thus, a piece of red glass permits the vibrations of 

 red to pass, but absorbs the others. In the same 

 way different bodies act differently on heat rays. 

 One body will permit them to pass unchallenged, 

 another body will intercept them wholly or partially. 

 Thus glass refuses to pass on heat vibrations, while 

 it permits light rays to pass. It is transparent to 

 light, but not transparent (that is, opaque) to heat. 

 Rock salt, on the other hand, permits heat rays to 

 traverse its substance very readily. Water inter- 

 cepts heat to a large extent, so also does a solution 

 of alum, while bisulphide of carbon gives them a free 

 way. So that if the beam from an electric lamp be 

 passed through a cell of alum solution, its heat rays 

 will, to a very large extent, be sifted out, while the 



