CHANGES WROUGHT IN PATHOLOGY, 109 



represents a suspension of weakly charged electronegative 

 particles. In consequence of electrical attraction, the 

 negative colloidal particles will collect about the electro- 

 positive ions, until through the heaping up of a suffi- 

 cient number of such particles the collecting ion will 

 be electrically neutralized. When the aggregates thus 

 formed have reached a sufficient size, the solution 

 becomes turbid, and finally a precipitate drops to the 

 bottom. 



We are able to foretell the possibilities which result 

 from a change in the number, size, and electrical charge 

 of the particles of a colloid, all of which can be proved by 

 experiment. When the number of particles is decreased, 

 the probability that a sufficient number will be collected 

 together through added ions for the formation of large 

 aggregates is also decreased, and finally a stage is 

 reached in the concentration of the colloid below which 

 precipitation does not occur. The precipitation is also 

 rendered difficult when the particles are very small and 

 carry but a weak charge, because under these circum- 

 stances too large a number of the particles have to be 

 collected together. If, on the other hand, the electrical 

 charge of the particles is too great, then too few suffice 

 to neutralize the oppositely charged ions, and the aggre- 

 gates formed are too small to settle to the bottom. A 

 medium charge, a sufficient number, and a sufficiently 

 large size of the colloidal particles constitute, therefore, 

 the optimum for precipitation. Just as we have electro- 

 negative metallic colloids, we have also electropositive 

 colloids. Oppositely charged colloids precipitate each 

 other in the same way as the ions of salts precipitate a 

 colloid, only in consequence of the size of the reacting 



