HEAT AND EXPANSION 107 



injury to the active ingredient they contain. By working 

 under a low pressure, clarified sugar juices, food extracts, 

 glycerin, dyewood, gelatin, and other liquors can be con- 

 centrated to any desired extent without injury. If such 

 liquors were heated to a temperature of boiling water for any 

 prolonged period, as would be necessary were they evaporated 

 in an open pan, their nature or constitution would to a greater 

 or lesser extent undergo a change and they would be spoiled. 

 For vacuum evaporation, a pump is necessary, first for 

 exhausting the air and the steam from the vacuum pan and 

 then for sending both to a vessel called a condenser where 

 the vapors are condensed. One of the most practical devices 

 is called the multiple effect system. This device consists of 

 four simple vacuum pans so connected that the steam from 

 the boiling liquid of the first is made to pass through the 

 others. In this way the heat of the steam of the first pan 

 is sufficient to heat the liquid of the second to the boiling 

 point, the heat of the steam of the second raises the tempera- 

 ture of the third, and so on. 



109. Expansion of Metals. Heat causes metals to ex- 

 pand. The expansion of unit of length for one degree is called 

 the linear coefficient of expansion. The increase per degree for 

 unit of surf ace is called surface expansion; for unit of volume 

 it is called cubic expansion. A steel joist 3 ft. long is, for 

 example, about J/g in. longer in summer than in winter; 

 hence long steel structures must not be rigidly fixed at 

 both ends. Steel car-rails are laid about y% in. apart to 

 allow for expansion. The amount of expansion of various 

 substances in length, area, and cubic contents or capacity 

 is given in the following table. For each degree of heat the 

 metal expands the fraction of an inch indicated. 



