138 - METALLIC ALLOYS. 



pure water, kept in leaden cisterns, or passing 

 through leaden pipes, often dissolves so much lead 

 as to become unwholesome, or even poisonous ; and 

 for the same reason leaden covers to cisterns are 

 equally objectionable, because the water which con- 

 denses on the cover, being of course pure (73), cor- 

 rodes and dissolves the lead, and dripping down again 

 into the cistern, contaminates the water, which other- 

 wise might have remained pure and wholesome : a 

 great deal of disease is probably caused by want of 

 attention to these facts. 



314. Many of the metals, when melted together, 

 combine to form what are called alloys, or mixed 

 metals ; some of these appear to be regular definite 

 compounds, though others are obviously mere mix- 

 tures. The most important of the alloys are gold 

 and copper, and silver and copper, which are harder 

 than gold or silver alone ; these alloys are used for 

 plate, coin, &c. Zinc and copper, or brass ; tin and 

 copper, or bell-metal; tin and iron, or common tin 

 plate, which is often supposed to be merely tin, 

 though it really consists of thin plates of iron, alloyed 

 on the surface with tin, so as to have the strength 

 and stiffness of the iron, together with the free- 

 dom from rusting of the tin. Zinc and iron, or "gal- 

 vanized iron," as it is frequently called, is iron 

 alloyed or covered on the surface with a film of zinc, 

 which greatly protects it from corrosion; and, lastly, 

 lead and tin, or pewter, and common solder. 



