36 SCIENTIFIC AMUSEMENTS. 



crystals of snow become coherent together, and the mass tends to 

 become somewhat translucent. With the aid of a blacksmith's vice 

 or other screw press enabling powerful pressure to be developed, it 

 is possible to squeeze a quantity of loose snow in a mould or a pair 

 of dies, so as to convert it into a shaped solid block of ice, which 

 will be almost perfectly clear if the snow were quite clean and the 

 pressure powerful enough, as for example when a hydraulic press 

 is used. 



It is remarkable that whilst water is affected by pressure in 

 such a way that the freezing-point is lowered by increase of 

 temperature, the opposite is the case with many other substances. 

 The following simple rule connects the two kinds of action. If 

 the solid substance is lighter than the same body when melted (as 

 is the case with ice), increased pressure lowers the melting-point, 

 and vice versa. This is due to the circumstance that in the case 

 of ice and such like bodies, the tendency of pressure being to 

 squeeze the mass into a smaller bulk, an increase of pressure will 

 tend to produce the same result as fusion (i.e., shrinkage in bulk) ; 

 so that the greater the pressure the less heat is required to produce 

 melting ; whilst in the case of bodies that are heavier in the solid 

 state than when melted, increase of pressure tends to produce the 

 opposite effect to that produced by fusion (i.e., in this case, increase 

 in bulk) ; so that now the greater the pressure the more heat will 

 be requisite to overcome the effect of the pressure and produce 

 melting. 



Welding. 



Many solids when heated become soft and plastic before they 

 actually melt : in this condition two lumps of hot substance may 

 be squeezed together and will cohere to a solid mass just as two 

 lumps of soft clay or putty may be squeezed together into one. 

 Metals exhibiting this property are said to weld together, wrought 

 iron and platinum furnishing two of the best examples. In the 

 manufacture of iron articles two pieces of metal are frequently 

 fastened together by this process, on which a large part of the art 

 of the " blacksmith," or worker of wrought iron, depends. The 

 rods or other pieces of iron to be welded are heated nearly white 

 hot in a "forge" or cinder-fire urged by bellows; and if applied 

 together and skilfully hammered whilst sufficiently hot (at a 

 " welding heat ") they cohere perfectly. Sometimes a little sand 

 or other silicious matter is sprinkled over the surface, the object 

 of which is to form a coating of fusible matter over the surface of 

 the metal ; this is forced out by the pressure, leaving clean surfaces 

 of metal close together, which thus adhere firmly. 



