EXPANSION OF SOLIDS. 5 



selves in their sockets. When cast iron pipes are employed to 

 conduct hot air or steam through a factory, they are never 

 allowed to abut against a wall or obstacle which they might in 

 expanding overturn. 



A compound bar, made by soldering together two thin plates 

 of copper and platinum, affords a good illustration of unequal 

 expansion by heat. The copper plate, being much more expan- 

 sible than platinum, the bar is bent upon the application of 

 heat to it ; and in such a manner, that the copper is on the 

 outside of the curve. It may easily be conceived, that by a 

 proper attention to the expansions of the metals of which it is 

 composed, a bar of this kind might be so constructed, that 

 although it was heated and expanded, its extreme points should 

 always remain at the same distance from each other, the length- 

 ening being compensated for by the bending. The balance 

 wheels of chronometers are preserved invariable in their dia- 

 meters, at all temperatures, by a contrivance of this kind. It 

 has also been applied to the construction of a thermometer of 

 solid materials that of Breguet. 



When hot water is suddenly poured upon a thick plate of 

 glass, the upper surface is heated and expanded before the heat 

 penetrates to the lower surface of the plate. There is here 

 unequal expansion, as in the case of the slip of copper and 

 platinum. The glass tends to bend, with the hot and expanded 

 surface on the outside of the curve, but is broken from its want 

 of flexibility. The occurrence of such fractures is best avoided 

 by applying heat to glass vessels in a gradual manner, so as to 

 occasion no great inequality of expansion ; or by using very 

 thin vessels, through the substance of which heat is rapidly 

 transmitted. 



This effect of heat on glass may by a little address be turned 

 to advantage. Watch glasses are cut out of a thin globe of 

 glass, by conducting a crack in a proper direction, by means of 

 an iron rod, or piece of tobacco pipe, heated to redness. Glass 

 vessels damaged in the laboratory may often be divided in the 

 same manner, and still made available for many useful pur- 

 poses. 



Both cast iron and glass are peculiarly liable to accidents 

 from unequal expansion, when in the state of flat plates. Plate 

 glass indeed can never be heated without risk of its breaking. 

 The flat iron plates placed across chimneys as dampers, are also 



