COLLOIDS 53 



When a fluid solution exhibits both of the above properties (i. e., the 

 Tyndall phenomenon and indiffusibility) , there can be no doubt as to its 

 being in a true colloidal state, but there are substances, such as congo 

 red or protein solutions of certain strengths, which may exhibit a very 

 slight diffusibility in a dialyzer but not show the Tyndall phenomenon. 

 Substances of this group constitute transitional types between molecular 

 and colloidal solutions, and to determine their true nature it is neces- 



Fig. 13. To show diffusion into gelatin of a crystalloid stain in b and the nondiffusion of a 

 colloid stain in a. (From W. Ostwald.) 



sary to employ refined methods such as those of ultramicroscopy, ultra- 

 filtration, etc., which can not be described here. 



3. The Size of Colloidal Particles. It will be apparent that the essential 

 property upon which the above-mentioned phenomena depend is the size 

 of the particle. Particles which can still be seen under the microscope 

 are called microns. They have been computed to have a dimension of 

 0.1 /u, (0.001 mm.) or more, and they form suspensions. Particles which 

 are invisible microscopically under the ordinary conditions of illumina- 



