CONSTRUCTION OF MINERALS. 17 



ject has been already spoken of in 7. I introduce it 

 again here to notice the fact that the mineral matter is 

 never deposited in living substances in any thing like a 

 crystalline form. Though there is much silica "(quartz) 

 in the rushes, there are no quartz crystals ; and the phos- 

 phate of lime in bones, though its particles are deposited 

 after a certain definite plan (as seen in Fig. 84 in my 

 "Human Physiology"), exhibits no resemblance to the 

 crystals of this mineral. 



18. Crystallization not Confined to Minerals. Though 

 organized substances, vegetable or animal, never take on 

 a crystalline form, yet some of the products contained 

 within them may do so. This is the case with the sugar 

 of the sugar-cane and other plants. The " candying" 



of raisins is an example of the crystalli- 

 zation of sugar. The crystals which 

 sugar is disposed to form are of the 

 shape seen in Fig. 4, a six-sided prism, 

 as you may observe in what is called 

 Fig. 4. rock-candy. In the form of loaf sugar 



the crystals are huddled together, and are therefore im- 

 perfect, just as in the case of marble. Many of the veg- 

 etable alkaloids, morphine, caftein, etc. ( 590, Part II.), 

 are obtained in the crystalline form. In all these cases 

 the crystals are never formed so long as the substance 

 remains under the influence of the living agency. Sug- 

 ar, for example, never crystallizes in the living plant, but 

 only after it is taken from it. 



19. Modes of Crystallization. There are various modes 

 in which crystallization occurs. 1. One of the most com- 

 mon modes is by deposit from a solution. Thus alum, 

 salt, sugar, etc., may form crystals from a solution, the 

 water passing off by evaporation. When the substance 

 can be dissolved in larger amount by hot water than by 

 cold, as is the case with alum, but not with salt, consid- 

 erable crystallization can be obtained before evaporation 

 begins by introducing into hot water as much of the 



