COLLOIDAL MIXTURES. 279 



coagulation of the negative colloid, and that this is true of that of the 

 positive ion in the case of the positive colloid. On the other hand, by 

 comparing the numbers in the same row, it will be seen that the coagu- 

 lation of the negative colloid takes place at a much lower concentration 

 when the salt produces positive ions with a higher electric charge, and 

 that an increase of the electric charge upon the negative ion has a sim- 

 ilar effect upon the coagulation of the positive colloid. 



It is evident from these facts that it is the ion with a charge op- 

 posite to that of the colloid particles that is mainly responsible for 

 their coagulation, but what the mechanism of this coagulation is, is not 

 yet understood, though it has been the subject of much discussion. 

 Interesting though they are, it is not worth while to describe the expla- 

 nations that have been suggested; for, in the opinion of the writer, 

 mere speculative hypotheses, that is, hypotheses which have not been 

 shown to facilitate to an important extent a knowledge of the actual 

 phenomena, are of little value except to the investigators of them, and 

 to them only because of the possibility of their future development 

 into really useful conceptions. The recent literature of colloids fur- 

 nishes a striking example of the unfortunate tendency even of our 

 modern investigators and text-book writers to attach greater impor- 

 tance to hypothetical interpretations of imperfectly known phenomena 

 than to a determination and presentation of the laws in regard to 

 them. 



Even an elementary consideration of the properties of colloids 

 should include a discussion of the absorption or coprecipitation of 

 other substances with them when they are gelatinized or coagulated — 

 a phenomenon which is of great importance in analytical chemistry, 

 as well as in other directions. But, for lack of space, this side of the 

 subject will have to be entirely omitted. Moreover, only a mere refer- 

 ence can be made to the importance of a knowledge of the properties 

 of colloids, not only in the industrial applications of chemistry, but also 

 in many other sciences and arts. It must suffice to mention that the 

 industries of dyeing, of tanning, of glass-making and coloring, and 

 of the manufacture of photographic materials and of modern explo- 

 sives have to deal primarily with substances in this peculiar state of 

 aggregation; that the clarification of syrups and other liquors by char- 

 coal, and that of water and sewage by precipitation, are based on the 

 phenomena of absorption by colloidal substances; that it is with these 

 substances as constituents of living bodies that physiology is mainly 

 concerned; that they constitute the culture-media of the bacteriologist, 

 to the employment of which the development of his science is largely 

 due; and that to the geologist the phenomenon of the sedimentation 

 of mud and slimes, which is closely related to that of the coagulation 

 of colloidal suspensions, is one of much interest. 



