226 "WELLS'S NATURAL PHILOSOPHY. 



A piece of brown paper submitted to the action of a burning-glass, ignites 

 much more quickly than a piece of white paper. The reason of this is, that 

 the white paper reflects the rays of the sun, and though but slightly heated 

 appears highly lummous ; while the brown paper, which absorbs the rays, 

 readily becomes heated to ignition. For the same reason a kettle whose bot- 

 tom and sides are covered with soot, heats water more readily than a kettle 

 whose sides are bright and clean. 



Light-colored fabrics are most suitable for dresses in summer, since they 

 reflect the direct heat of the sun, and do not absorb it ; black outside gar- 

 ments, on the contrary, are most suitable for winter, as they absOTb heat 

 readily, but do not reflect it. 



Hoar-frost, in the spring and autumn, may be observed to remain longer 

 in the presence of the morning sun, on liglit-colored substances than upon the 

 dark-colored soil, etc. ; the former do not absorb the heat, as the dark-colored 

 bodies do, but reflect it, and in consequence of this they remain too cold to 

 thaw the frost deposited upon their surfaces. 



How is the at- ^^"^ '^^ absorbs heat very slowly, and does not readily 

 xnosphere heat- part with it. Air rarely radiates heat, and is not heated to 

 ^^^ great extent by the direct rays of the sun. The spn, however, 



heats the surface of the earth, and the air resting upon it is heated by contact 

 with it, and ascends, its place being supplied by colder portions, which in turn 

 are heated also. 



This reluctance of air to part with its heat occasions some very curious dif- 

 ferences between its burning temperature and that of other bodies. Metals, 

 which are generally the best conductors, and therefore communicate heat 

 most readily, can not be handled with impunity when raised to a temperature 

 of more than 120° F. ; wa,ter becomes scalding hot at 150° F. ; but air ap- 

 phed to the skin occasions no very painful sensation when its heat is far be- 

 yond that of boiling water. 



Some curious experiments have been made in reference to the power of the 

 human body to withstand the influence of heated air. Sir Joseph Banks en- 

 tered an oven heated 52° above the boiling point, and remained there some 

 time without inconvenience. During the time, eggs, placed on a metal frame, 

 were roasted hard, and a beefsteak was overdone. But though he could 

 thus bear the contact of the heated air, he could not bear to touch any metal- 

 lic substance, as a watch-chain, money, etc. Workmen, also, enter ovens, 

 in the manufacture of molds of plaster of Paris, in which the thermometer 

 stands 100° above the temperature of boiling water, and sustain no mjury. 



In what man- 515. HGcat, ill passing through most sub- 

 tr'iv'nBmitted^''* stancGs, 01 media, is retained, or intercepted 

 Inrsub^t'ln^es"? ^^ ^^^ passagc in a greater or less degree. The 

 capacity of solids and liquids for transmitting 

 heat is not always in proportion to their transparency, or 

 capacity for transmitting light. 



