35 



Quite distinct in character is a crystalline mass of 

 Ferro-cyanide of Potassium (Yellow Prussiate of Potash), a 

 substance used in the manufacture of Prussian blue. 



Its form is definite, and built up geometrically about its 

 axes ; it is transparent, and if we attempt to cut it, it differs 

 in an important respect from the amorphous mass of clay. 

 It cuts easily in one direction, splitting into thin plates, like 

 mica. This is said to be the direction of the plane of 

 cleavage of the crystal, and by exertion of much force the 

 crystal may be cut in other directions ; not, however, splitting 

 readily into thin plates as before, but being merely pulverised. 

 We may note, in passing, that these crystals have been grown 

 on a piece of string, as sugar-candy is ; the string is lowered 

 into a strong solution of the substance, and greatly facili- 

 tates crystallisation by affording a suitable bed or growing 

 ground. 



The essential and distinctive features of a crystal are 

 that it is geometrically built up about certain lines (the axes) ; 

 exhibits a tendency to split more easily in one direction than 

 another, this direction being termed the plane of cleavage, 

 and the tendency is held to be evidence of regularity of 

 internal structure, confirmation being obtained by the optical 

 properties and mode of expansion by heat, which are 

 generally found to be similar for crystals of the same class. 



The occurrence of the same substance in a constant 

 geometrical shape has enabled us to arrange crystals under 

 six general classes. 



Each class includes many substances, differing widely 

 in chemical composition, but exhibing the same geometrical 

 shape, and such substances are called, in a general sense, 

 Isomorphotis (Greek = same shape). 



One and the same substance may occur in two or more 

 distinct crystalline forms, such difference can generally be 

 clearly attributed to the presence of a larger or smaller 

 quantity of water in combination with the substance, or to 

 the formation of the crystal under differing conditions of 

 pressure, temperature, etc. 



