THE EFFECTS OF HEAT. 233 



In what man- ^^2. GasGs and aeriform substances expand 

 e^an^dby^hSTtr l-490th of the bulk which they possess at 32° 

 lor every degree of heat which they receive 

 above that point, and contract in the same proportion for 

 every degree of heat withdrawn from them. 



Thus, 490 cubic inches of air at 32° would so expand as to occupy an inch 

 more space at 33°, and by the addition of another degree of heat, raising its 

 temperature to 34°, it would occupy an additional inch, and so on. In a like 

 manner, by the ■wnthdrawal of heat, 490 cubic inches of air would occupy an 

 inch less space at 31° than at 32° ; two inches less at 30° and so on. The 

 same law holds good for all other gases, and for vapors and steam. 



Illustrations of the expansion of air by heat are most familiar. If a bladder 

 partially fiUed with confined air be laid before the fire, the air contained in it 

 may be expanded to a degree sufficient to burst the bladder. Chestnuts laid 

 upon a heated surface, burst with a loud report on account of the expansion 

 of the air within their shells. The process of warming and ventilating build- 

 ings depends entirely upon the application of this principle of the expansion 

 and contraction of air by the increase and diminution of heat. 



How may the 533. As the magnitude of every body changes 

 c^tSn^Jff with the heat to which it is exposed, and as 

 l\^T to^ fhe ^^^ same body, when subjected to calorific in- 

 Shl^t?"^"* fluences under the same circumstances has al- 

 ways the same magnitude, the expansions and 

 contractions which are the constant effects of heat, may be 

 taken as the measure of the cause which produced them. 

 -What are the ^34. Thc instrumcuts for measuring heat 

 meLTuring'h^t ^^c Thermomcters and PjTometers. The for- 

 •^^•^^ mer are used for measuring moderate tempera- 



tures ; the latter for determining the more elevated de- 

 grees of heat. 



Liquids are better adapted than either solids or gases for measuring the 

 effects of heat by expansion and contraction ; since in solids the direct ex- 

 pansion by heat is so small as to be seen and recognized with difficulty, and 

 in air or gases it is too extensive, and too liable to be affected by variations 

 in the atmospheric pressure. From both of these disadvantages liquids are 

 free. 



The liquid generally used in the construction of thermometers is mercury, 

 or quicksilver. 



Mercury possesses greater advantages for this purpose than 

 l^e^e^lW any other liquid. It is, in the first place, eminently dis- 

 ada'pted for the tinguished for its fluidity at all ordinary temperatures ; it 

 thenncmeters r is, in addition, the only body in a liquid state whose va- 



