COHESION. 



301 



sion is established between the particles, and the body 

 passes into the aeriform state. 



The same effects are produced by the exertion of 

 that attraction winch unites the particles of one body 

 with those of another. If a liquid be poured on a 

 solid, it often happens that their mutual attraction is 

 sufficiently powerful to overcome the cohesion of the 

 solid : its particles are consequently disunited, to 

 combine with those of the liquid, and it entirely dis- 

 appears. This forms the chemical process of solution . 

 A similar effect is sometimes produced by the chemi- 

 cal action of an aeriform body. 



When these powers, whether of heat or of chemi- 

 cal attraction, are withdrawn, cohesion resumes its 

 force, but with results which are different, according 

 10 the circumstances under which this happens. 

 When the attraction of aggregation is suddenly and 

 forcibly exerted, the particles are united, in general, 

 indiscriminately, and according to no regular law. 

 If a body, which has been melted, is suddenly cooled 

 to a sufficient extent, it becomes solid, and forms a 

 mass of no regular structure or figure ; or, if its co- 

 hesion has been suspended by the chemical attraction 

 exerted by another body towards it, and if this attrac- 

 tion suddenly cease to operate, the force of cohesion 

 is resumed, and the solid substance appears in the 

 form of a powder. This latter case forms the chemi- 

 cal operation denominated precipitation. But, if the 

 force of cohesion is exerted more slowly, the particles 

 are united, not indiscriminately, but usually with re- 

 gularity, so as to form masses of regular structure 

 and figure, bounded by plane surfaces and determi- 

 nate angles. This forms the operation of crystalliza- 

 tion ; and such masses are denominated crystals. 



Crystallization takes place from fluidity, produced 

 either by heat or by the exertion of a chemical at- 

 traction. Ice is an example of the first, which shoots 

 in long, slender crystals, when water is cooled to a 

 sufficient extent ; and salts, which, when they have 

 been dissolved in water, separate in crystals, on with- 

 drawing a part of their water by evaporation, or re- 

 ducing its solvent power by a diminution of its tem- 

 perature, is an example of crystallization from flu- 

 idity, produced by affinity. In either of these cases, 

 if the operation is conducted slowly, so as to admit 

 of the particles uniting by those faces most disposed 

 to union, crystals are formed ; and these are, in ge- 

 neral, larger, more transparent, and more regular in 

 their form, the slower the crystallization has taken 

 place. The production of these regular forms is fa- 

 voured by the introduction of an already formed 

 crystal, or of some foreign substance, into the solu- 

 tion, which operates as a nucleus, and upon which 

 the crystallization commences. The access of air 

 and light exerts an important influence, also, on the 

 crystallization of certain salts. An enlargement of 

 volume is often produced by crystallization, as in the 

 examples of ice, of several metals, and of a number 

 of salts ; while, in other cases, the reverse is the fact, 

 the volume of the crystallized substance being less 

 than while it existed in the liquid state differences 

 evidently depending on the mode in which the parti- 

 cles unite. Crystals formed from a watery solution 

 generally retain a portion of water in a combined 

 state ; and this is the case not only with those salts 

 which are formed by the chemist, and in the arts, but 

 with nearly all of the earthy and saline crystals found 

 in nature. This water is named their water of crys- 

 tallization. When deprived of it, they lose their 

 transparency and density. Some part with it from 

 mere exposure to the. air, and suffer these changes ; 

 they are then said to effloresce. If they attract water 

 and become humid, they are said to deliquesce. In 

 some salts, the water of crystallization is in such large 

 quantity, that, on the application of a moderate heat, 



it causes them to melt a change called the watery 

 fusion. Water, which has dissolved one salt to the 

 point of saturation, will still take up a considerable 

 proportion of a second, and even of a third. Sea- 

 water contains several well-known saline compounds. 

 In such cases, as the salts have different degrees of 

 solubility, they mayoften be obtained separately, by 

 a gradual evaporation of the water, the least soluble 

 being the first to separate. The water of the ocean, 

 evaporated to a certain degree, yields common salt ; 

 evaporated still further, it deposits Glauber's salts, 

 and the remaining liquid holds dissolved a compound 

 containing magnesia. Crystallization also takes place 

 in the transition from the aerial form, as is well ex- 

 emplified in the arrangement of a flake of snow. 

 Every substance in crystallizing is disposed to as- 

 sume a certain regular figure : sea-salt, for example, 

 takes the form of the cube ; nitre, that of a prism. 

 Carbonate of lime is found crystallized in rhomboids, a 

 particular class of prisms and pyramids ; and garnet in 

 regular dodecahedrons. The important application, 

 therefore, of this law becomes at onee obvious. The 

 form of the crystal, in mineralogy, enables us to 

 determine the species to which it belongs. The 

 same is true of pharmaceutical preparations ; their 

 crystalline forms furnish a certain test of the nature 

 of the crystallized body. 



The theory of crystallization is still obscure. It 

 may be conceived that the particles of bodies are of 

 certain regular figures, and that in uniting, they may 

 be disposed to approach by certain sides, in prefer- 

 ence to others, probably by those which admit of the 

 most extensive contact. Hence a regular structure 

 and figure, uniform with regard to each substance, 

 will be produced. The numerous diversified figures of 

 crystals may be reduced to others more simple ; thus 

 the equilateral, six-sided prisms, and the double six- 

 sided pyramid of calc-spar, or carbonate of lime, 

 may be reduced by successive sections (parallel to 

 natural joints hi these crystals) to the rhomboid. 

 The figure thus arrived at by mechanical division, 

 and which is supposed to constitute the nucleus of 

 the crystal, is called the primitive form. The num- 

 ber of original forms thus obtained, according to M. 

 Hauy, amounts to six ; 1. the regular tetrahe- 

 dron; 2. the parallelopipedon, which includes 

 the cube, the rhomboid, and all the solids, whicn 

 have six faces parallel, two and two; 3. the oc- 

 tohedron, the surfaces of which are triangles, and, 

 according to the species, equilateral isosceles, or sca- 

 lene ; 4. the hexagonal prism ; 5. the dodecahe- 

 dron with rhombic faces ; 6. the dodecahedron, 

 with triangular faces. The secondary forms 

 of crystals, or such as are usually exhibited by 

 nature, are supposed to grow out of the primitive 

 forms hi the following manner : The particles first 

 unite to produce the primitive form, and from this 

 proceeds the secondary form by the application of 

 successive layers of particles parallel to its faces ; 

 which layers are denominated laminae of superposi- 

 tion. The modification of figure is the consequence of 

 the abstraction of one, two, or more rows, or ranges 

 of particles, from the planes or angles of each of these 

 laminae, by which a decreasing series of particles 

 will be formed. Thus, supposing that upon one side 

 of a cube successive layers of cubic particles be plac- 

 ed, and each layer be less, by one range of particles, 

 than the surface upon which it rests, it is obvious 

 that the lines wliich bound the sides must be continu- 

 ally approaching each other, and that the last layer 

 must consist of a single cube. It follows, then, that 

 a four-sided pyramid will be raised upon one of the 

 surfaces of the cube ; and that, if the same thing 

 happen upon the five other sides, the cube must 

 be converted into a dodecahedron, with rhombic 



