316 A Sketch of the Laws of Chemical Combination. 



condition generally requisite for this process, is the slow and gradual 

 change from the fluid to the solid state ; the arrangement of the particles 

 being at the same time undisturbed by external motion. Thus sulphur, or 

 bismuth, assumes a crystalline form, when slowly cooling from a fused 

 mass 5 and saline substances are deposited in crystals by the spontaneous 

 evaporation of tlieir solutions, or by the cooling of solutions saturated at 

 the boiling point. It has, however, been recently discovered, that some 

 solids can undergo a change in the form of their crystals, without being 

 reduced to a fluid state. A great variety of external form prevails amongst 

 crystals of different substances ; and the study of them has now become so 

 extended, as to form a distinct science, termed Crystallography, of which 

 a few of the leading principles may be here stated. 



It was pointed out by Hauy, that in every crystal there is & primitive 

 form, or nucleus, which may be procured by cleaving it in the direction of 

 its laminee; and that this is susceptible of modification into various se- 

 condary forms, geometrically allied to it, in one or other of which the 

 crystal may occur. Thus the cube is easily convertible into the octo- 

 hedron, tetrahedron, or rhombic dodecahedron ; the six-sided prism is al- 

 lied to the six-sided pyramid, and so on. On these views is founded the 

 classification of Mohs, who arranges all the forms of crystals under six 

 systems, with which it is important for the chemist to be acquainted, as 

 the various forms which a salt or any substance may resume, almost always 

 belong to the same system. It was considered until lately, that the pri- 

 mitive form was constant in each substance j but late observation has 

 shown that sulphur, and probably other bodies, are capable of affording 

 two varieties of crystals, whose primary forms are incompatible with one 

 another. Such substances are said to be dimorphous. Notwithstanding 

 the general correspondence in forra, there is so much difference in the 

 proportions of the sides and angles of crystals of different substances, that 

 most salts have one or more forms peculiar to themselves , and identity of 

 form was formerly thought to indicate, in most cases, similarity of com- 

 position. But in 1819, it was discovered by Mitscherlich of Berlin, that 

 certain substances have the power of assuming the same crystalline form, 

 and may be substituted for one another in combination, without affecting 

 the external character of the compound. Such substances are said to be 

 isomorphous ; and se\eral distinct groups have been already described. 

 One of the most remarkable of these includes the salts of arsenic and 

 phosphoric acids, which will be j)resently adverted to more particularly ; 

 another contains the salts of sulphuric, selenic, chromic, and manganic 

 acids ; and in another are included the sesquioxides of aluminium, iron, 

 chromium, and manganese. Several other groups might be enumerated, did 

 space permit. In comparing together isomorphous bodies of the same 

 group, we are not to expect identity of forra, unless there is similarity of 

 composition. The variety which may be produced in the constitution of 



