EXAMPLES OF THE EXPLANATION OF LAWS. 531 



entering into chemical combination with any other substance 

 whatever. 



2. Some interesting cases of the explanation of old uni- 

 formities by newly ascertained laws are afforded by the re- 

 searches of Professor Graham. That eminent chemist was 

 the first who drew attention to the distinction which may be 

 made of all substances into two classes, termed by him crystal- 

 loids and colloids ; or rather, of all states of matter into the 

 crystalloid and the colloidal states, for many substances are 

 capable of existing in either. When in the colloidal state, 

 their sensible properties are very different from those of the 

 same substance when crystallized, or when in a state easily 

 susceptible of crystallization. Colloid substances pass with 

 extreme difficulty and slowness into the crystalline state, and 

 are extremely inert in all the ordinary chemical relations. Sub- 

 stances in the colloid state are almost always, when combined 

 with water, more or less viscous or gelatinous. The most 

 prominent examples of the state are certain animal and vege- 

 table substances, particularly gelatine, albumen, starch, the 

 gums, caramel, tannin, and some others. Among substances 

 not of organic origin, the most notable instances are hydrated 

 silicic acid, and hydrated alumina, with other metallic per- 

 oxides of the aluminous class. 



Now it is found, that while colloidal substances are easily 

 penetrated by water, and by the solutions of crystalloid sub- 

 stances, they are very little penetrable by one another : which 

 enabled Professor Graham to introduce a highly effective 

 process (termed dialysis) for separating the crystalloid sub- 

 stances contained in any liquid mixture, by passing them 

 through a thin septum of colloidal matter, which does not 

 suffer anything colloidal to pass, or suffers it only in very 

 minute quantity. This property of colloids enabled Mr. 

 Graham to account for a number of special results of obser- 

 vation, not previously explained. 



For instance, " while soluble crystalloids are always highly 

 sapid, soluble colloids are singularly insipid," as might be ex- 

 pected ; for, as the sentient extremities of the nerves of the 



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