Organic Remains, Crystals, and Artificial Objects. 95 



fragment is to be selected, and ground down on a 

 hone till its surfaces are tolerably flat. When too 

 thin to be held longer in the fingers, one of its 

 sides should be fastened with Canada balsam to a glass 

 slide, and the grinding process gone on with till the 

 limestone becomes so thin as to be a pale grey and 

 partly transparent. 



Plate "VII., fig. 10, represents a portion of a sec- 

 tion of nearly black limestone, which I saw prepared 

 in this way ; and its resemblance to the slide of Atlan- 

 tic mud is interesting. 



From the consideration of coal, chalk, and lime- 

 stone, which, although they are in the mineral 

 domains, bear evidence of organic composition, we 

 may turn to the substances which are truly inorganic. 

 When these present any appearance of symmetry in 

 their forms, it is because their particles have arranged 

 themselves in that peculiar manner known as crystalli- 

 zation. Each substance which crystallizes at all, does 

 so after a certain type or plan. Not that all crystals 

 of the same substance will surely look alike; but it 

 will be found that the same plan of crystallization will 

 exhibit itself under a great variety of forms. Various 

 natural specimens are interesting, when shown in thin 

 sections; granits and agate may be mentioned as 

 examples ; and crystals can be obtained artificially by 

 making strong solutions of various salts, placing a drop 

 of ^ach on a glass slide, and allowing it to evaporate 

 slowly. Such crystals are in many cases splendid 

 objects for the polarizing apparatus. 



The actual process of crystallization may be ob- 



