SUSPENDED CHANGES IN NATURE 



27 



Certain glassy appearing minerals like obsidian are really under- 

 cooled substances, natural glasses, which have cooled without crystal- 

 lizing. In the course of time they begin to crystallize. It is not un- 

 usual to find in nature numerous samples of these minerals existing in 

 all stages : the metastable, par- 

 tially crystallized, and the stable 

 or completely crystallized mineral. 



The rate of the growth of 

 crystals is a specific property de- 

 pending on the substance used. 

 Much information has been ob- 

 tained on this subject by study- 

 ing the rate at which crystals 

 are deposited from undercooled 

 liquids. By filling a narrow glass 

 tube with the desired liquid, as 

 shown in figure 2, inoculating at 

 one end and measuring the time 

 necessary for the crystal surface 

 to travel the length of the tube, 

 the rate of growth of a crystal 

 can be measured. Undercooled 

 phosphorus crystallizes 200 feet 

 a minute, water at two degrees 

 below zero at the rate of 8 inches 



Fig. 6. Crystals of Needle-like 

 or Monoclinic Solphur. When first 

 formed they are bright yellow. After 

 standing for a few hours they change 

 to a dull yellow color and become very 

 brittle. A microscopic examination of 

 a fragment shows that it is now 

 made up of minute rhombic crystals 

 (Fig. 7). 



a minute, while one thirty-second of 

 an inch a minute is the velocity of 

 crystallization of betol. In nature 

 crystals often grow much more 

 slowly than this. 



The examples cited above deal 

 with undercooled liquids, but the 

 phenomenon of undercooling is by 

 no means restricted to this class of 

 substances. 



When molten sulphur is allowed 

 to cool slowly, long lustrous needle- 

 shaped (monoclinic) crystals sepa- 

 rate from the liquid. On standing 

 for a few days the appearance of the 

 crystals changes ; they lose their luster, and examination with the micro- 

 scope shows that their crystalline form is no longer needle-like, but con- 

 sists of so-called rhombic figures. The temperature at which the tran- 

 sition from needles to the rhombic form takes place is 96°. 



fig. 7. 



Crystals of Rhojibic 

 Sulphur. 



