118 



PROTOPLASM 



active. Metallic (solid) platinum is relatively inert. The 

 physics and chemistry of colloids are the physics and chemistry 

 of surfaces. Some conception of the extent of this surface can 

 be gained by considering the increase in surface which takes 

 place when a cube 1 cm. on an edge is subdivided. 



The same fact can be expressed by stating that if 1 cc. of water 

 is sprayed into droplets 0.01 /x (0.00001 mm.) in diameter, the 

 total area will be 6,000,000 sq. cm. 



While matter in the colloidal state owes its unique properties 

 to the tremendous surface exposed, there are other properties 

 which rather definitely characterize colloidal systems. The 

 most general of these is turbidity, as shown by the Tyndall cone. 

 The colloidal particle has also served as a basis of definition. 

 The word "particle" suffices for suspension (lyophobic) colloids, 

 but in gluelike (gel-forming lyophilic) colloids, the structural 

 unit is assumed to be of a special nature and has been given the 

 name micella or micelle. This term, with the theory of structure 

 underlying it, was the fundamental contribution of the German 

 botanist Karl von Nageli. Nageli believed that j ellies are built up 

 of discrete units larger than the molecule but too small to be seen 

 by direct microscopic observation, in other words, of colloidal 

 particles somewhat similar to those which characterize solid 

 suspensions. He believed the micelle of gels to be a minute 

 crystal. Present-day chemists, being of a similar opinion, have 

 called it a crystallite. The term micelle is generally reserved for 

 the structural units of gels and not usually applied to the colloidal 

 particles of suspension colloids. The micelle is characteristic 

 of colloidal systems, but it may not be a necessary distinguishing 

 feature. Small size of the structural unit when compared to 

 visible particles and large size in comparison to the average 



