770 COEFFICIENT OF EXPANSION. 



than 0'5 or 1, and if the displacement affects both the freezing 

 and boiling point by nearly the same amount, the thermometer may 

 still be used, provided that the error is noted, and at each reading 

 applied as a correction. 



When a body expands we may consider solely the 

 increase of one of the linear dimensions of the body: 

 this is called the linear expansion of the substance of 

 which the body consists, and the elongation of the unit 

 of length of a body, when its temperature rises from 

 to 1, is called the coefficient of linear expansion of the 

 substance. Similarly we may consider the increase in 

 area of any portion of its surface ; this is called the 

 superficial expansion, and the increase of the unit of 

 surface in being heated from to 1 is called the co- 

 efficient of superficial expansion. Finally, the increase 

 in volume is called the cubical expansion, and the in- 

 crease of the unit of volume when the body is heated 

 from to lis the coefficient of cubical expansion. 



The determination of the coefficients of expansion 

 of substances requires complicated contrivances and 

 laborious experiments, for a description of which the 

 student must consult larger works. Table II., at the 

 end of this book, gives the coefficients of linear expansion 

 for a variety of the more important substances as they 

 have been determined by such experiments. 



If the coefficient of linear expansion of a body is 

 known its cubical expansion can be easily calculated. 

 Thus, according to Table II., the coefficient of linear 

 expansion of zinc is 0*0000294 ; a cube of zinc, of which 

 each edge is just l cm long at 0, so that its volume at 

 zero is l cc , will, when heated to 1 C., increase each of its 

 dimensions by O cm '0000294, and its volume will thus be- 



