Crystallization. 351 



combine with each other, and thus form a new body called racemic acid. 

 (Cooke's Chem.) The salts of sesquioxide of chromium, as well as the 

 oxide itself, occur in two isomeric conditions, one known as the green 

 and the other the violet modification. The compounds of the latter 

 usually crj'stallize readily, while the green compounds do not. If a solu- 

 tion of the violet salt is heated nearly to the boiling point it is apt to pass 

 into the green sort and become un-crystallizable. Calcic Carbonate is 

 formed into two very distinct and different forms of crystals, one of 

 which is calcite and the other aragonite ; which differ in densit} T , hard- 

 ness and other qualities, but are the same chemically. In like manner 

 there are three natural forms of Titanic Oxide answering to the three 

 minerals, rutile, brookite and octahedrite. 



Ethylic Formate is very different from Methylic Acetate, but they are 

 composed of the same C EL (X. 



* t> D & 



Arsenious Acid. AS 2 3 exists in an amorphous glassy state, but 

 upon contact with air it becomes changed into minute octahedral crys- 

 tals, opaque and porcelain-like. It also takes a third state, forming in 

 right rhombic prisms. The amorphous and crystalline varieties differ 

 in chemical as well as physical properties, but are absolutely identical 

 as to kinds and numbers of component atoms. 



The following essences are composed of exactly the same atoms and 

 expressed by the same formula, C 10 H 16 ; viz., Lemon, Bergamot, Cu- 

 bebs, Neroli, Juniper, Lavender, Gilliflower, Pepper. 



C 4 H g O 2 forms Butyric Acid, which is of rank odor, boils at 156 C. 

 and does not easily inflame. C 4 H g 2 also forms Acetic Ether, which 

 is a clear liquid with a pleasant fruity smell, highly volatile, boiling at 

 74 C., and inflaming with the greatest ease. 



One variety of isomerism is called polymerism, by which it is meant 

 that a body possesses the same sort of atoms as another body and in the 

 same proportion, but that the absolute number of atoms is multiplied 

 two or more times. For example, butyric acid is polymeric with respect 

 to oxide of ethylene which has a formula of C 2 H 4 0. This is just half 

 the molecule of butyric acid. Acetic ether is also said to be a polymer 

 of aldehyde, which has the same molecule as ethylene. Then there is 

 another compound body called paraldehyde whose molecule is C 6 H 12 3 , 

 which is three molecules of aldehyde condensed into one. Compounds 

 when left to themselves often undergo transformations into other sub- 

 stances, and these are frequently multiples of the first, the molecules 

 either doubling, trebling, or halving, &c. Cyanimide, C N 2 H 2 , at 150 

 temperature centigrade is converted into cyanuramide, C 3 N 6 H 6 , a mole- 

 cule multiplied by three. Anhydrous sulphuric acid, S O 3 , is a white 

 substance composed of glistening crystalline fibres and is something like 

 asbestos. There are two varieties of it which pass into each other, the 



