56 EXPERIMENTAL GENERAL SCIENCE 



56. Casting. A large number of metal objects are ham- 

 mered into shape but the majority are "cast" by being melted 

 and poured into moulds. Sharp castings cannot be made of 

 most metals for the reason that they contract on solidifying. 

 Brass and cast iron are two common substances that expand 

 slightly in turning from the liquid to the solid state and so 

 are prime favorites with the foundry man. Type metal is a 

 mixture of various metals that expands on solidifying, though 

 the metals alone contract on cooling. The expansion of type 

 metal is highly desirable; in fact, the several metals composing 

 it are selected with this end in view, for if it did not expand on 

 solidifying, it would not fill the moulds and form the sharp 

 outlines so necessary for type. Gold and silver, on the con- 

 trary, contract on solidifying, and coins cannot be cast of 



FIG. 19. Ring and ball set. 



these metals. When coins are made, therefore, pieces of the 

 metal have to be forced into dies under great pressure to give 

 them the proper sharpness. Of all the common metals, zinc 

 contracts most when heat is withdrawn. 



56. Some Practical Applications. The expansion and con- 

 traction of all substances with changes of temperature make 

 it necessary to take this into account daily in many operations. 

 Sometimes we take advantage of it, as when steel girders are 

 fastened together with red-hot rivets which contract and tighten 

 as they cool; sometimes considerable ingenuity must be devel- 

 oped to avoid its effects. Large steel bridges expand so much 

 in the summer sunshine that the sections are mounted on rollers 

 or have telescoping joints to permit them to change in length. 

 Long bridges sometimes vary more than a foot in length in 



