Properties and Activities of Living Protoplasm 75 



B 



E 



D 



C 



Fig. 24. — Diagram to illustrate different theories of the physical structure of 

 protoplasm. A, Granular theory; B, fibrillar or filar theory; C, reticular theory; 

 D, alveolar theory; E, colloidal theory. 



Colloidal Systems 



The results of recent scientific investigations suggest that the chemical 

 constituents of protoplasm are in a finely divided, colloidal state, thus 

 forming a complex colloidal system. A colloid (Gr. kolla, glue) is a 

 mixture of invisible, submicroscopic particles of comparatively large size 

 (usually larger than molecules) which are suspended in a liquid medium. 

 Colloids often have a sticky, gluelike property; hence the name. The 

 sizes of colloidal particles may vary from one-millionth (0.000,001) to 

 one-ten-thousandth (0.0001) of a millimeter (mm.) in diameter. Col- 

 loids do not diffuse through a parchment or similar membrane, while 

 crystalloid solutions, like those of sugar or salt, do. When a colloid is 

 evaporated, it leaves a formless mass, while a crystalloid solution leaves 

 crystals of definite form. Possibly a better understanding of a colloid 

 can be gained by stating that such familiar materials as milk, ink, e^g 

 white, gelatin in water, etc., are colloids. In the language of the chemist, 

 the particles (solid, liquid, or gas) of a colloid and the medium (solid. 



