18 



CHEMISTRY. 



Fig. 3. 



Fig. 4. 



shown by placing a red-hot 

 iron ball in the focus of a 

 parabolic mirror; the calo- 

 rific rays impinging on the 

 mirror, will be reflected, in 

 straight lines ; and if another 

 similar mirror be properly 

 adjusted, many feet distant, 

 these rays may be made to 

 converge again in its focus, 

 so as to affect a thermome- 

 ter placed there, or even to 

 set fire to phosphorus. 

 It is a law, that the best radiators are the worst reflectors, and vice 

 versa ; but the power of absorbing heat is in direct proportion to the 

 radiating power. The power of absorbing heat has been thought to 

 depend upon the colour of the substance; but this has been questioned. 

 The laws of radiation explain the formation of deiv ; the surface of 

 the ground being cooled down after sunset by radiation, the aqueous 

 vapour existing in the air is deposited in the form of dew. If the 

 night be cloudy, however, the rays of heat are intercepted by the 

 clouds which reflect them back, and hence little or no dew is depo- 

 sited. It is also well known, that the most perfect radiators, such as 

 grass, wood, leaves, &c., are the most covered with dew, while a 

 piece of polished metal, &c., would be scarcely moistened. In the 

 same manner also, the production of the land and sea breezes of 

 tropical islands is accounted for. 



Transmission of heat. — By this is meant the passage of the calorific 

 rays through certain media. Substances which transmit such rays are 

 called translvxent, or diatherinanous ; it may easily be shown by in- 

 terposing a piece of plate glass between a red-hot body and a mirror; 

 while all the rays of light will pass through the glass, many of the 

 rays of heat will be arrested by it, so that but a very feeble effect is 

 produced at the focus of the mirror. The only substance perfectly 

 diathermanous is rock salt ; other media, though entirely transparent, 

 intercept the rays of heat to a greater or less extent. It has also 

 been shown that radiant heat, like light, is capable of being po- 

 larized. 



Vaporization signifies the conversion of a solid or liquid into a 

 vapour or gas, by heat. Vapours diflfer from gases only in being 

 easily compressed into a liquid. Vaporization includes both ebulli- 

 lion and evaporation. In ebullition, the formation of the vapour is 

 so rapid, that it rises to the surface in the form of bubbles, which 

 then burst ; the point at which this takes place is called the boiling 

 pointy and is always constant under the same circumstances; it 

 varies very nfiuch for different liquids ; thus, for water, it is 212° F. ; 



