Crystallization. 353 



isomeric states of bodies are due to the agency of external energies. A 

 change from one state to another is always accompanied by a demonstration 

 of energy in some form heat, light, electricity, a blow, or even a stirring 

 with a stick. In passing from an amorphous to a crystalline state, it 

 generally appears that heat is given out. Thus sulphur gives up 10 of 

 heat in making this transfer. Antimony gives out great heat in doing 

 it. When arsenious acid of the amorphous glassy sort changes into the 

 opaque variety heat is given out, and if it is done rapidly a flash of light 

 goes with it. From this it would appear that the amorphous condition 

 is a position of potential energy, heat being used to bring it about, which 

 heat reappears when the body is let down again. 



There are two conditions in which different compound bodies appear, 

 called colloid and crystalloid. Bodies which, when put into water, rap- 

 idly diffuse through it like sugar or salt, and are capable of crystalliz- 

 ing, are called crystalloids, while those which are not capable of crys- 

 tallizing readily, or of diffusing through water, are colloids. The col- 

 loids are insipid or tasteless, and are apt to form into jellies by com- 

 bining with water. The word means like glue, and gelatine is a type of 

 this class of bodies. Among the colloids are silicic acid ( Si 2 ), alu- 

 mina ( A1 2 3 ), starch, gums, caramel, albumen, tannin, gelatine, and 

 the extractive matters of various animal and vegetable tissues. Crys- 

 talloids are separated from colloids by dialysis, which is a sort of sift- 

 ing process through a tray made of parchment paper, which is itself an 

 insoluble colloid. The tray is laid upon a vessel of water and the solu- 

 tion to be dialysed is poured on it. The crystalloid passes through the 

 membrane and diffuses in the water below, leaving the more sluggish 

 and inert colloid behind. There is no definite boundary between col- 

 loids and crystalloids, some crystalloid bodies being far more diffusible 

 than others. But a very significant fact is that a great many bodies- 

 pass from a colloid to a crystalloid state, and vice versa. Thus alu- 

 mina, ferric oxide, chromic oxide, silicic acid, stannic, metastannic, ti- 

 tanic, molybdic and tungstic acids, may exist in either condition ; al- 

 though they are commonly met with as crystalloids. 



It is said that the red globules of blood are often transformed into 

 oblique rhombic crystals, and chlorophyl has been found in a crystal- 

 line state b} r M. Trecul. Myeline also has been known to undergo the 

 same sort of change. According to Dr. Marcet, the blood contains 

 phosphoric anhydride and iron in a perfect colloid state, not diffusible. 

 Dr. Graham is the highest authority on this subject, and both he and 

 Dr. Marcet assert that there is a constant change in colloidal compounds 

 back to crystalloids, and from crystalloids to colloids. The difference 

 between the two appears to depend upon dynamical agencies external 

 to themselves ; which, by causing a polymeric condensation of the mole- 



