120 PROTOPLASM 



the metal. It is said that only metals which do not unite with 

 oxygen directly, and are therefore stable in the presence of tis- 

 sues, are germicidal; silver, gold, and platinum would, therefore, 

 qualify, and copper not, though copper is highly toxic and was 

 the first oligodynamic metal. Colloidal gold has been introduced 

 as a germicide for use in tuberculosis. 



Conclusion. — The colloidal world is unique. It has, in the 

 main, its own laws. Classical laws formulated for gases, liquids, 

 and solids may or may not apply to colloidal systems, and when 

 they do apply, they usually do so only in a modified form. That 

 property primarily responsible for colloidal behavior is surface. 

 We have seen how great the increase in surface may be when 

 solid matter is finely dispersed. With this tremendous increase 

 in surface, the system takes on new and unique properties ; thus, 

 capillarity and Brownian movement belong to the colloidal world 

 alone. If the dispersion is carried too far — to that of molecules — 

 the system ceases to be colloidal, and new properties again 

 characterize it, viz., those of solutions. We must, therefore, be 

 cautious in applying the known laws of one kind of system to 

 another distinctive system. For example, colloidal particles 

 and the bodies of the solar system have much in common; both 

 are freely suspended and both possess an electric charge on 

 their surface; yet how different are the laws that govern them. 

 In the former case, gravity predominates, while electric forces, 

 important to us who live upon the earth {e.g., as lightning), are 

 to all other (celestial) purposes nonexistent, and the astronomer 

 pays no attention to them. In the colloidal world, the situation 

 is reversed; surface charge is the significant factor, and gravity 

 is a negligible quantity. The two systems are superficially 

 alike — they look alike when one is seen with the naked eye and 

 the other with a microscope — and they have certain properties 

 in common, yet they are two distinct worlds; because of this, 

 they acquire new properties and are governed by different laws. 



