CHAP, ii.] ANORGANIC AND ORGANIC SUBSTANCES. 15 



crystallisable or volatile without decomposition, but they are found 

 in the organism itself, and come forth from it direct as excre- 

 mentitial bodies. They are acids, for instance, the acids tartaric, 

 lactic, uric, citric ; vegetal and animal alkaloids : creatine, creati- 

 nine, urea, caffeine, and so on ; fat or resinous bodies ; sugars 

 of the liver, of grape, of milk, of cane, and the like. 



3. In the third class of immediate principles we find bodies 

 not crystallisable or coagulable. They are formed in the 

 organism itself ; then, decomposed there, give birth to the imme- 

 diate principles of the second class. The organic substances, 

 properly so called, the substances of the third class, constitute 

 the most important part of the body of organised beings (globu- 

 line, musculine, fibrine, albumine, caseine, cellulose, starch, 

 dextrine, gum, and some colouring matters, such as hsematine, 

 biliverdine.) 



It is from the intense union, molecule by molecule, of substances 

 appertaining to these three groups that organised substance 

 results, formed of multiple elements, but constituted in great 

 part of bodies complex, inert, unstable, easily decomposed, either 

 through the play of chemical affinities, or the action of undula- 

 tions calorific, luminous, electric. 



So far we have occupied ourselves with the materials of 

 organised bodies only from the point of view of their chemical 

 composition ; but it is quite as needful to take into consideration 

 their physical state and physical properties. 



An English chemist, justly celebrated, Graham, fell on the 

 happy idea of grouping all bodies, according to their characteristic 

 physical state, into two grand classes, that of crystalloids and 

 that of the colloids.* 



The crystalloids comprehend all the bodies which ordinarily 

 form solutions sapid and free from viscosity. These bodies have 

 furthermore the property of traversing by diffusion porous 

 partitions. 



The colloids have a consistency more or less gelatinous (gum, 

 starch, tannin, gelatine, albumine). They diffuse themselves 

 1 Phil. Transactions, 1861, p. 183 ; Moigno, Physique Moliculaire. 



