202 CHEMISTRY. 



gether. Lead, potassium, and sodium can be welded without 

 being heated, and mercury can be welded when it is frozen. 



278. Alloys and Amalgams. Metals unite together to 

 form alloys. Some of the most common of these we will 

 mention. Brass is an alloy composed of copper and zinc, 

 the copper making from two thirds to three fourths of the 

 whole. The color of the mixture is intermediate between 

 the deep color of the copper and the light color of the zinc. 

 What is called pinchbeck is a kind of brass, with a larger 

 proportion of zinc than ordinary brass. What is called 

 German silver is a sort of brass with the addition of another 

 metal, nickel, the whiteness of which gives this alloy its re- 

 semblance to silver. Bronze is an alloy of copper and tin, 

 the latter being commonly one tenth of the whole. Bell- 

 metal is the same, with a larger proportion of tin. Common 

 type-metal is an alloy of lead with different proportions of 

 zinc, tin, bismuth, and antimony. Solders are commonly al- 

 loys of lead and tin. Pewter is tin alloyed with lead or 

 antimony. What is called Britannia ware is a kind of pew- 

 ter. The alloys which various metals form with mercury 

 are called amalgams. A familiar example you have in the 

 silvering of mirrors. The amalgam is formed in this case 

 by pouring mercury upon tin-foil laid over the glass. 



279. Nature of Alloys. An alloy is generally considered 

 as a mixture, and not a compound, for two reasons: 1. In 

 making alloys there are no fixed proportions in which the 

 metals must be combined. 2, The qualities of alloys are in- 

 termediate, for the most part, to those of their constituents. 

 Thus the color of brass is intermediate to the colors of the 

 copper and the zinc, and the hardness of type-metal to that 

 of the copper and that of the lead. But there are some 

 marked exceptions to this second characteristic of mixtures, 

 which seem to indicate the existence of some degree of 

 chemical affinity, sufficient to produce decidedly new quali- 



