790 ALLOYS. 



About 100 or 200 grammes of sodium hyposulphite should be 

 melted in a small glass flask in a hot- water bath ; if heated over a 

 flame the flask is very apt to break. It is best to place the flask 

 in a small pot of water, so narrow that the flask cannot be upset, 

 and to support the pot suitably over a spirit-flame, until the last 

 grain of the solid salt is melted. If even the smallest quantity of 

 solid salt remains in the liquid, it would prevent the retardation of 

 solidification taking place. It will take some hours for the melted 

 mass to cool to the temperature of the atmosphere. 



Alloys, that is mixtures of metals formed by fusing 

 and mixing together several metals, have mostly a lower 

 melting point than would be expected by taking the aver- 

 age of the melting points of the metals of which they are 

 composed ; it even happens in many cases that the melt- 

 ing point of the alloy is lower than that of any of the 

 component metals taken separately. Thus soft solder, 

 an alloy of tin and lead, melts at 170, or 60 below the 

 melting point of tin. Most striking is the low melting 

 point of 4 Wood's fusible metal/ which is an alloy of 

 7 parts, by weight, bismuth, 4 parts lead, 2 parts tin, 

 and 1 part cadmium. This alloy melts between 66 

 and 70. It becomes liquid when placed in pretty hot 

 water ; or the end of a small bar of it may be melted 

 over the flame and the liquid metal allowed to fall upon 

 the fingers without burning them. 



Bismuth is a very brittle metal of a reddish- white colour, and can 

 be reduced to powder by pounding it in a mortar. Cadmium is a 

 white soft metal, which has a great resemblance to zinc. For pre- 

 paring Wood's metal the bismuth should be fused first, in an iron 

 ladle, then the lead, tin, and cadmium should be added, stirring the 

 whole with a splinter of wood. No more heat should be applied 

 than is sufficient to fuse the whole, as otherwise too much of the 

 metals combines chemically with oxygen, that is, it is burnt away. 

 It is advisable to take not less than 14 grammes bismuth, 8 grammes 

 lead, 4 grammes tin, and 2 grammes cadmium. For preparing a 

 small bar, the alloy is melted in a test-tube of boiling water, the 



