THE EFFECTS OF HEAT. 



22T 



516. The heat of the bud passes through transparent 

 ujdies without loss ; but heat from terrestrial sources is 

 in great part arrested by many substances which allow 

 light to pass freely, — such as water, alum, glass, etc. 



Thus, a plate of glass held between one's face and the sun mil not protect 

 it, but held between the face and a fire, it will intercept a large proportion of 

 heat 



517. Those substances which allow heat to pass freely 

 through them, are called d'latliermanous, and those which 

 retain nearly all the heat they receive, are called ather- 



manous. 



Rock-salt allows heat to pass through it more readily than any other known 

 substance ; while a thin plate of alum, which is nearly transparent, almost 

 entirely intercepts terrestrial heat Heat, indeed, will pass more readily 

 through a black glass, so dark that the sun at noon is scarcely discernible 

 through it, than through a thin plate of clear alum. Water is one of the least 

 diathermanous substances, although its transparency is nearly perfect li, 

 therefore, it is desired to transmit light without heat, or with greatly dimin- 

 ished heat, it is only necessary to let the rays pass through water, by which 

 they will be stramed of a great part of their heat. 



It has been found that the power of heat to penetrate a 

 dense, transparent substance, is increased in proportion as the 

 temperature of the body from which it is radiated is increased. 

 Heat, also, accompanied by light, is transmitted more readily 

 than heat without light 



518. Heat and light come to us conjointly from the sun. 

 "When a ray of light is caused to pass through a prism it is 

 analyzed or separated into seven brilliant colors, or element- 

 ary parts. If the heat ray which accompanies the light ia 

 treated in a similar manner, our organs of sight are so constituted that we 

 ao not discover any separation to have taken place in it. It is, however, es- 

 tablished beyond a doubt that in the same manner as a ray of white light 

 can be modified and divided, so a ray of radiant heat can be separated into 

 parts possessing qualities corresponding to the various colors. 



How flops the 

 tcini)iT;itureof 

 a body raduit- 

 iiv,' hiat aff.ict 

 its truusmis- 

 Bioii ? 



Is a ray of solar 

 heat simple or 

 compound in 

 its nature ? 



What effect 

 does heat pro- 

 duce upon all 

 bodies '/ 



SECTION III. 



THE EFFECTS OF HEAT. 



519. The general and most ob\aous effect (-^ 

 heat upon material substances, is to expand 

 them, or increase their dimensions. 



