28 HEAT. 



temperature do not recover at once from them, and do not give con- 

 sistent indications. 



There is one case in which the nearly equal expansion of two dif- 

 ferent substances is of great importance in the construction of scientific 

 apparatus that of glass and platinum. Referring to the table of 

 Lavoisier and Laplace, it will be seen that they found as the co-efficient 

 for glass '0000087, while Borda obtained for platinum a value nearly 

 0000086. If then a platinum wire be inserted in glass, and the glass be 

 fused round it, in cooling the two contract nearly equally. There is 

 therefore little strain, and the glass does not break away from the 

 platinum, as it would from other metals. Platinum wires can thus be 

 fused through glass, and through these wires electric currents can be 

 led into closed glass vessels, such as vacuum tubes and eudiometers. 



Volume Expansion Of Solids. We do not very often require 

 to know the volume expansion, or, as it is frequently termed, the 

 " cubical dilatation " of solids, except in so far as it is necessary in the 

 measurement of the expansion of liquids and gases, when we may require 

 to know the volume expansion of the containing vessel. We shall 

 describe how this expansion is determined in connection with the expan- 

 sion of liquids, and shall here only state that, if the solid expands 

 equally in all directions, its volume expansion may be found from its 

 linear as follows : If a solid of cubical form has a co-efficient of linear 

 expansion k, the length I of the edge at becomes I (1 +M) at t. The 

 volume, therefore, increases from Z 3 to Z 3 (l + Jct) & = Z 3 (l + 3kt + 3/^ 2 + k s t s ). 

 Now Jet is, for moderate temperatures, so small that we may with more 

 or less exactness neglect Jc 2 t 2 and & 3 ^ 3 , and the volume is very nearly 

 P(l + 3kt). The co-efficient of volume expansion the increase of the 

 volume which is 1 at for each degree rise of temperature is, there- 

 fore, 3/c or three times the linear coefficient of expansion. 



