354: Dynamic Theory. 



* 



cules of a crystalloid, forms it into a colloid. Prof. Graham is inclined 

 to think that the colloid molecule is constituted by the grouping to- 

 gether of a number of smaller crystalloid molecules, and that the basis 

 of colloidality consists in this composite character of the molecule. 

 This condensation of molecules is only accomplished by some form of 

 energy, and generally this energy is given up in the shape of heat, when 

 the body returns to the crystalloid state. The importance of the rela- 

 tionship between these two states of bodies, consists in the fact that 

 vegetable and animal organisms are made up of combinations of the 

 two. The mass of the tissues are colloidal, but these colloids grade in 

 the same tissue into materials more or less crystalloid. The fact ap- 

 pears to be that the materials in the solutions from which nourishment 

 is drawn, are, in part, crystalloid and diffusible, and when incorporated 

 into the tissues are promoted to the colloidal state. Their initial dif- 

 fusibility is a useful and probably an essential quality, because by it 

 they are able to pass through colloidal membranes and tissues so as to 

 reach their destinations in the body. Colloids are not able to pass 

 through other colloids. The reason is found in the comparatively large 

 size of their molecules ; the small crystalline molecule getting through 

 a hole that stops its big brother, the colloid molecule. The difference 

 between the two, then, is physical, and it allows a variety of physical 

 reactions. A certain degree of temperature is required in some cases 

 to retain a constituent in solution, and when the temperature is either 

 raised or lowered from this normal degree, the constituent separates 

 from the solution. "Thus, when a hot saturated solution of alum or 

 nitre is allowed to cool, some of the salts crystallize out of the solution ; 

 when heat is applied to a solution of lime, some of it becomes precipi- 

 tated ; whilst when either heat or cold is applied to a solution of sodic 

 sulphate, already at a temperature of 33 C. , some of this salt separates 

 from the state of solution." 



Many bodies, both simple and compound, crystallize in forms which 

 belong to two or three different systems of crystallization, or to different 

 subdivisions of the same system. Such bodies are called dimorphous 

 and trimorphous, and they differ in their specific gravity, hardness, 

 color and other properties. The chief cause of these different modes of 

 crystallization lies in different states of temperature, but sometimes it 

 depends on the components of the solution from which the crystallizing 

 body is separated ; thus, arsenious anh}'dride crystallizes from water or 

 hydrochloric acid in regular octahedrons, but from alkaline solutions in 

 trimetric prisms. A hot solution of saltpetre yields, when slightly 

 cooled, nothing but prismatic crystals, but at 10 C. prismatic and rhom- 

 bohedral crystals appear together. Again, if a solution of carbonate of 

 calcium in water containing carbonic acid be left to evaporate at the or- 



