Crystallization. 349 



in a particular gas converts it into the second variety called beta phos- 

 phorus. This is red and forms into acute rhombohedral crystals of a 

 specific gravity of 2. 34. It does not take fire in the air till heated to 

 260 and it is not poisonous. It melts at 260 and is then reconverted 

 into the alpha variety. 



Sulphur is an element which exists in three distinct forms. In the 

 first variety, called alpha sulphur, its crystals are forms of orthorhombic 

 octahedrons like numbers 1 and 2, fig. 150. Its specific gravity in this 

 form is 2.05. The beta variety is in monoclinic crystals like 3, fig. 150. 

 The specific gravity of this is 1.95. The first variety is soluble in car- 

 bon di-sulphide, but the second is not. The second tends with the aid of 

 heat to pass into the first. The third variety or gamma sulphur is formed 

 into a waxy, sticky amorphous mass by pouring melted sulphur at 250* 

 temperature into cold water. This variety has a specific gravity of 1.95 

 and like the second is insoluble in carbon disulphide. The third sort 

 passes slowly into the first. But it passes quickly if heated to 100 and 

 in doing so becomes ten degrees hotter. Experiments with sulphur gas 

 show that its molecule at a temperature of 500 contains six atoms while 

 at 1000 it contains but two. 



FIG. 151. 

 Different Crystalline forms of Diamond. 



FIG. 152. 

 Graphite Crystal. 



Carbon has three allotropic states : diamond, plumbago or graphite y 

 and charcoal, lamp-black, &c. It is well known that in any of its solid 

 states, as charcoal, &c. , carbon does not unite with oxygen at ordinary 

 temperatures, and is in fact one of the least oxydizable substances in 

 nature. It is known to have remained unchanged for 2, 000 years. But 

 when heated it rapidly unites with oxygen forming carbonic dioxide. 



Silicon, which next to oxygen is the most abundant substance on 

 earth, exists in three allotropic states much like those of carbon ; that 

 is, it is diamond-like, graphitoidal, or amorphous. The first variety is 

 in the form of regular octahedral metallic-like crystals with specific grav- 

 ity of 2. 49. The second variety crystallizes in bright hexagonal plates. 

 At a very high temperature it passes into the diamond form. The 

 amorphous variety is a brown powder. At about 2000 temperature it 

 melts ; and if mixed with salt at a heat sufficient to vaporize the latter, 

 it passes into the graphitoidal sort. 



Selenium has two or three allotropic states. The first, alpha, is a 

 dark, grayish solid, not soluble in carbon di-sulphide ; with specific 



