HEAT. 157 



what we have said upon radiation, it will be seen, that 

 dark colors radiate more heat than light ones : therefore, 

 dark clothing carries off more of the heat of the body, 

 than light colored would. 



The heat, produced by the direct rays of the sun upon 

 a body, seldom exceeds 120, but by a peculiar contri- 

 vance to prevent the heat from being carried off by the 

 surrounding bodies 220 or 230 may be produced. 



When the rays of the sun are concentrated, they pro- 

 duce a much greater effect, as with a burning glass 

 bodies may be set on fire at a considerable distance ; they 

 must be diret-ted, however, upon a body that will absorb 

 or retain them, if they are directed upon apiece of glass, 

 it will not be heated, nor will any transparent body, such 

 as air, or water. In these cases, the heating power of the 

 sun's rays, is not augmented by concentrating them ; the 

 effect is owing entirely to the great number of rays which 

 are brought upon one point. 



Combustion. Among all the natural phenomena which 

 daily take place around us, there is none more wonderful 

 than combustion, and there is none less understood ; and 

 yet its vast utility seems to demand that all should know 

 its cause, and the relative powers of the various combusti- 

 ble bodies which are used in the arts and trades, as well 

 as for domestic purposes. 



If a piece of iron, and one of wood, of the same 

 size be exposed to the same heat until the iron is red hot, 

 quite different effects will be produced upon them. The 

 iron continues to acquire heat, up to a certain point, 

 where it will remain, while the wood will heat to a certain 

 point, and then suddenly become much hotter of itself; 

 affording at the same time abundance of heat and light. 

 After a short time this heat and light will diminish, and 

 finally cease, although still exposed to the same heat as 

 at the beginning. If the two bodies be now withdrawn 

 from the heat and permitted to cool, the iron will be 

 found to have undergone no change, while what was 

 wood is quite another thing, having lost its shape, weight 

 and color, and is no longer capable of being set on fire as 

 before. Again, if charcoal be heated to about 800, it 

 takes fire and becomes intensely hot, which after a time 

 diminishes, and finally ceases, when it will be found 



