12 EFFECT OF WATER ON ROCK POWDERS. 



reactions show but little alkali in solution and vice versa, it is fur- 

 ther pointed out that the cause of these discrepancies remains to be 

 ascertained. 



Reduced to the simplest language, the condition of affairs may be 

 expressed as follows: The action of water on certain rock powders is 

 in the nature of a hydroh T sis with an accompan} T ing formation of 

 hydroxyl ions if alkaline bases are present. After filtration or clear 

 subsidence the number of hydroxyl ions in the clear liquid is found to 

 be reduced. 



THE COLLOID THEORY. 



After this discussion it is fair to inquire if there is any theory that 

 will account not only for the increased Binding power produced in 

 rock powders by the prolonged action of water and b} T wet grinding, 

 but also for just such a phenomenon as has just been described. 



Investigations carried on by Mr. L. W. Page and the writer for 

 several years have led to the belief that the action of water on many 

 rock powders results in the formation of colloid films on the surfaces 

 of the particles. The word "colloid" is not altogether. a satisfactory 

 one to express this stick} 7 , gumnry condition that is being studied so 

 much of late in relation to binding material in the various problems 

 of road building and street paving. The word "colloid," as scientific- 

 ally used, refers generally to those bodies which pass into solution in 

 water, but which will not, while in solution, pass through a parch- 

 ment diaphragm as substances in true molecular solution always will 

 do. The word "hydrogel," given by Graham, is used to refer to the 

 jellied colloids as they .originally coagulate from the hydrosols. It 

 does not property describe the condition of the surface of crystalline 

 rock particles after the}" have been acted on by water and thereafter 

 air dried. Moreover, it is possible that these theories may be applied 

 to the binding power of certain substances that have formed in an oil 

 instead of in water, as is not unlikely to be the case with blown oils, 

 petroleum residues, and asphalts. 



Ostwald says: "Colloidal substances exist in two modifications, 

 the soluble and the coagulated or ' pectinized."' We need another word 

 to express this latter condition, and the word pectoid is truer to the 

 Greek root (TT^/CTOS) than pectini&ed. The fact that this pectoid con- 

 dition is superinduced on the surface of fine crystalline powders by 

 wet grinding can be demonstrated by microscopic preparations. It 

 has even been possible to stain the pectoid matter with methylene blue, 

 owing to the well-known faculty of colloidal stuff of absorbing dye- 

 stuffs. Ordinary photographic prints in .black and w^hite do not suffice 

 to show the action that has taken place. The endeavor has been made 



Solutions, p. 153. Translated by M. M. P. Muir. Longmans, Green & Co., New 

 York. 



