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CRYSTALLOGRAPHY. 21 



fizing solid. In a second case,, the substance is fused by 

 heat ; here again the particles are free to move as long as 

 the heat remains ; and when it passes off solidification com 

 mences, under the power of crystallization. In a third case, 

 the substance is reduced to a vapor by heat ; and from thi? 

 state — also one of freedom of motion among the particles — 

 it crystallizes as the heated condition is removed. 



In the hardening of steel, it is well known that the coarse, 

 ness of the grains varies with the temperature used, and the 

 manner in which the process is conducted. An increased 

 coarseness of structure, implies that certain of the crystal- 

 line grains were enlarged at the expense of others. It 

 teaches us that in some cases the powers of crystallization 

 may act at certain temperatures, even without fusion or so- 

 lution. The long continued vibration of iron, especially 

 when under pressure, produces a similar change from a fine 

 to a coarse texture ; and this fact has been the cause of ac- 

 cidents in machinery, by rendering the iron brittle : it has 

 led to the fracture of the axles of rail cars and of grind- 

 stones, and even the iron rails of a road may thus become 

 weak and useless. 



By these several processes, the various minerals and very 

 many of the widely extended rocks of our globe, have been 

 brought to their present state. 



Perfect crystals are usually of moderate size, and gems of 

 the finest water are quite small. As they enlarge they be- 

 come less clear, or even opaque, and the faces lose their 

 smoothness and much of their luster. The emerald, suffi- 

 ciently pure for jewelry, seldom exceeds an inch in length, 

 and is rarely as large as this ; but a crystal of this species 

 (of the variety beryl) was obtained a few years since at 

 Acworth, New Hampshire, which measured 4 feet in length 

 and 2£ feet in circumference ; it was regular in its form, yet, 

 except at the edges, opaque. The clear garnets, fit for set- 

 ting, are seldom half an inch through ; but coarse crystals 

 have been found 6 inches in diameter. Transparent sap- 

 phires also, over an inch in length, are of extreme rarity ; 

 but opaque crystals occur a foot or more long. 



Quartz crystals attain at times extraordinary dimensions. 

 There is one at Milan which is 3£ feet long and 5£ in cir- 

 cumference, and it weighs 870 pounds. From a single cav« 



Is fluidity essential to the process of crystallization 1 What is said 

 of steel and iron ? What is said of the size and perfection of crystals 1 



