58 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES 



colloidal substances. On the one hand we have the 

 power of the colloid gel to absorb water, and the power 

 of both gels and sols to adsorb a great variety of 

 substances as a result of their immense internal surfaces. 

 On the other we have the fact that the gel membrane, 

 especially the protoplasmic membrane, is semi-perme- 

 able, so that it does not allow of the escape of osmotic 

 substances within the cell, while it does allow water 

 to enter freely, and other substances, such as salts, 

 at various rates. 



The colloidal nature of complex organic substances 

 largely depends on the great size of their molecules, 

 and this is most conspicuously seen in the proteins 

 which form the basis of protoplasm. The great size 

 of these complex molecules is often associated with 

 instability of particular arrangements of the atoms 

 which compose it. This makes possible the existence 

 of very numerous chemical compounds of distinct 

 though closely similar molecular structure. A given 

 group of atoms within the molecule is replaced by 

 another of somewhat different arrangement. This is 

 the cause of the extraordinary richness and variety 

 of the chemical and physical changes within the cell 

 which are the basis of the varied manifestations of 

 life and of organic form. 



Though we are still very far from understanding 

 exactly how what we call living protoplasm actually 

 carries out all the complex processes which take place 

 within it, we can already trace the direct dependance 

 of many of them on the chemical and physical 

 structure and relations of the substances of which 

 protoplasm is composed. 



