LESSONS IN PHYSICS. 33 



sudden blow, as glass or the diamond. The relative hardness of 

 two bodies is ascertained by trying which will scratch the other. 

 This test shows the diamond to be hardest of all substances and 

 the mineral, talc, to possess the property of hardness in the least 

 degree, or the property of softness in the highest degree. Many 

 substances after having been raised to a high temperature may 

 be tempered to almost any degree of hardness, and other sub- 

 stances, as glass, may be greatly toughened by the process of 

 annealing. 



In many cases when substances pass from the liquid or gase- 

 ous state into the solid form, their molecules arrange themselves 

 in regular geometrical forms called crystals, as ice, common salt, 

 sugar and quartz. The greater portion of the older rocks are of 

 a crystalline structure, and most of the metallic ores and miner- 

 als have distinctive crystalline forms. Metals, by jarring, often 

 become crystalline, losing their ductility and tenacity. From this 

 cause, bells long rung change their tone; cannon, after long use, 

 lose their strength ; and the perpetual jar of bridges, shafts of ma- 

 chinery, car axles, etc., gradually changes the tough, fibrous iron 

 into the weaker crystalline form, which explains many accidents. 



Most substances expand as they are heated, and contract as 

 they cool. In many cases the contraction and expansion is 

 somewhat uniform ; but those substances which crystallize when 

 cooling usually expand slightly as their temperature approaches 

 the point of solidification. Most crystalline structures occupy 

 more space than the same matter in a liquid form. Water con- 

 tracts as its temperature falls from the boiling point to 39 F., 

 and then it expands, so that the volume at 32 F. is the same 

 as it was at 48 F. If water continued to contract to the freez- 

 ing point, it would be denser and heavier than the warmer water 

 below, and would sink. Lakes, rivers and other bodies of water 

 would begin to freeze at the bottom, and would soon become 

 solid ice. But as the surface stratum of water approaches the 

 freezing point, it expands, and, becoming lighter, floats, and thus 

 the cooler water and ice remain at the surface. Similar phe- 

 nomena occur in the cooling of some other substances. In freez- 

 L. s. s 



