400 Dynamic Theory. 



a plate of mica be interposed between the nicols, the heat instantly gets 

 through and the needle of the pole is deflected. If, instead of the mica, 

 a plate of quartz crystal is interposed, the heat rays get through this 

 also, showing the rotary polarization. The double refraction of heat 

 in passing through Iceland spar is also shown by placing the pile in the 

 path of either of the refracted rays. ( Compare these effects with those 

 of light, mentioned above. ) 



As many bodies are transparent to the waves of both light and heat, 

 so there are some quite transparent to one and not to the other. A 

 solution of alum in a glass cell, or cell with glass sides, allows all the 

 light to pass, but effectually stops the heat rays. On the other hand, 

 those chemical elements that are gases, also the elementaiy liquids, like 

 bromine, and the liquid solutions of sulphur, phosphorus and iodine, are 

 very transparent to heat. * When either the heat rays or the light rays 

 are wanted exclusively, they can be sifted out by the use of a filter made 

 of the appropriate material. 



The resistance which different bodies offer to the passage of radiant 

 heat through them is not the same for all sources of heat. Melloni experi- 

 mented with heat from ( 1 ) a lamp, ( 2 ) platinum heated to incandes- 

 cence, ( 3 ) copper heated to a temperature of 400 C and ( 4 ) copper at 

 100 C. The percentage of the heat from a ray of each of these sources 

 which passed through different bodies as compared with air is shown in 

 the following table. Each substance was made to be ^ of an inch thick. 

 1234 1234 



Rock salt allows nearly all the heat to pass through it, while alum, 

 sugar-candy and ice allow but little to pass. Observe too that very dif- 

 ferent amounts of the different sorts of heat get through the same body. 

 Thus 39 per ct. of the heat of the lamp gets through Iceland spar, while 

 only 28 per ct. of the incandescent platinum heat, 6 per ct. of the 400 

 copper heat, and none at all of the 100 copper heat make the passage. 

 The difference in the heat consists of difference in the wave lengths and 

 the consequent periods of vibration, the hotter the radiating body the 

 shorter the waves and the higher the pitch. So the above examples ap- 



1 Prof. Tyndall's Six Lectures. His light filter consisted of a cell of transparent bi-sul' 

 phide of carbon, in which some pieces of iodine were dissolved; the more iodine the 

 greater the imperviousness to light, but the diathermancy or perviousness to heat is not 

 impaired. 



