CHANGE OF VOLUME DURING SOLIDIFICATION. 791 



water is then poured off, and the melted alloy allowed to flow into 

 a little cylindrical mould made of paper, 5 or 6 mm in diameter. 

 Wood's alloy is rather brittle when near its melting point ; tin at 

 about 200 shows the same behaviour. At ordinary temperatures 

 the alloy is pretty hard and somewhat elastic. 



The passage of a body from the solid into the liquid, 

 or from the liquid into the solid state, is generally ac- 

 companied by a change of volume. The behaviour of 

 different substances is very different in this respect. In 

 many cases the alteration of volume is very slight, in 

 others it is very considerable, and in some substances 

 the nature of the change is of an opposite kind to that 

 which is observable in other substances, in other words, 

 some bodies contract at the moment of liquefaction, 

 while in others the volume becomes increased while 

 they are passing into the liquid state. Whether one or 

 the other change is taking place may easily be decided 

 by observing whether any portion of the substance 

 which is still in the solid state, while liquefaction pro- 

 ceeds, floats in that portion which is already liquid, or 

 sinks in it. 



If a piece of stearine candle from which the wick 

 has been removed, about 20 grammes in weight and 

 broken up into several pieces, be heated in a capacious 

 test-tube until a portion of the stearine is melted, the 

 pieces which are still solid will be seen to remain at the 

 bottom of the tube. Solid stearine is therefore heavier 

 than liquid stearine stearine expands during liquefac- 

 tion. 



On the other hand, ice floats upon water, as is well 

 known. Ice is thus lighter than water ; ice contracts 

 during liquefaction. The specific gravity of ice, when 

 perfectly free from air bubbles, is nearly 1^, or more 



