THE EEASON WHY. 57 



* The fear of tho Lord is the beginning of wisdom : a good understanding have 

 all they that do his commandments." PSALM cxi. 



object in a line with it, we shall find that the plate will reflect the 

 rays it has received by radiation, on to the object so placed ; but if 

 we place an object between the fire and the plate, we shall find 

 that the rays of heat will be intercepted, and that the latter can no 

 longer reflect heat. 



188. Does the agitation of the air interfere witli the direc- 

 tion of rays of heat ? 



It has been found that the agitation of the air does not affect the 

 direction of rays of heat. 



189. Why, then, if a current of air passes through a space 

 across which heat is radiating, does the air become warmer ? 



Because it takes up some portion of the heat, but it does not 

 alter the direction of the rays. 



[This is clearly illustrated by reference to rays of light which are seen under 

 many circumstances. But they are never bent, moved, nor in any way affected 

 by the wind.] 



190. Why will not a current of air disturb the rays of 

 heat, just as if would a spider's web, or threads of silk ? 



Because heat is an imponderable agent, that is, something which 

 cannot Be acted upon by the ordinary physical agencies. It has 

 no weight, presents no substantial body, and is, in these latter 

 respects, similar to light and electricity. 



191. What other sources of radiation of heat are there 

 besides the sun and the fire ? 



The earth, and all minor bodies, are, in some degree, radiatort 

 of heat. 



192. What substances are the best radiators ? 



All rough and dark coloured substances and surfaces are the 

 best radiators of heat, 



193. What substances are the worst radiators of heat ? 



All smooth, bright, and light coloured surfaces are bad radiatort 

 if heat. 



[Dr. Stark, of Edinburgh, has proved, by a series of experiments, the influence 

 which the colours of bodies have upon the velocity of radiation. He surrounded 



