THE COLLOIDAL STATE 91 



dialyzed, and two end chambers containing water. The latter 

 contain the metal electrodes and are separated from the central 

 chamber by parchment membranes. (A platinum electrode is 

 used at the anode, as acid is formed here.) Current is applied, 

 the water changed frequently, and dialysis continued until the 

 desired purity is reached. In this manner Pauli has dialyzed 

 the serum of horse blood for seven weeks and obtained what is 

 probably the purest seralbumin yet produced. 



Fig. 68. — Electrodialyzer of Pauli. 



The Two Major Groups of Colloids. — "A dominating quality 

 of colloids is the tendency of their particles to adhere"; thus did 

 Graham characterize the "gluelike" substances. But there is 

 nothing gluelike about such fine suspensions as water droplets 

 in air (clouds), carbon in air (smoke), clay and gold in water, 

 and similar systems which are colloidal because they fulfill other 

 prerequisites of the colloidal state, viz., fine dispersion and 

 permanent suspension. The question therefore arises whether 

 the gluelike properties of such colloids as gelatin, albumin, and 

 rubber or the minuteness in size and permanency of suspension 

 of particles in such systems as clouds, smoke, and the like shall 

 be the criterion of the colloidal state. If failure to pass through 

 a parchment-paper membrane, i.e., to dialyze, is the test, then 

 both types of systems, the gluelike ones and the suspensions, are 

 colloids. The French chemist Duclaux has attempted to 

 restrict the term colloid to the gluelike substances, but colloidal 



