22 EFFECT OF WATER ON ROCK POWDERS. 



precipitate which had been coagulated with barium chlorid was digested 

 with a cold dilute ammonic-chlorid solution, after which the barium 

 which had been absorbed was again found in the solution. The 

 cause of such action is not at present understood, but it appears 

 to be physical rather than chemical in its nature. It is possible that 

 it is due in some way to the electrostatic conditions that exist between 

 the ions and the colloidal precipitates, but, whatever the cause, if 

 such actions take place in the disintegration of rocks, it is quite cer- 

 tain that it is of very great importance in th economy of nature. 

 If, indeed, this is true, we must distinguish three kinds of alkali 

 which will be present in wet-ground rock powders (whether ground 

 by the processes of nature or in a laboratory mill), namely, that which 

 goes freely into solution, that which is occluded by pectoids, and that 

 in the rock particles which has not been acted upon. The inference is 

 plain that in soil formation a certain proportion of the potash is con- 

 tinually being conserved, waiting to be made available by subsequent 

 treatment with solutions of electrolytes. 



The work of Way and other experimenters on the absorptive power 

 of soils has been thoroughh r reviewed by S. W. Johnson in the well- 

 known book, How Crops Feed. a The whole review might be used as 

 un excellent argument to support the colloid theory of rock decompo- 

 sition and clay formation. 



PRACTICAL APPLICATIONS. 



Although it has been necessary in this paper to discuss the questions 

 raised from the theoretical and chemical point of view, many interest- 

 ing practical problems are presented, and certain effects that have long 

 been noted, but whose causes have seemed obscure, appear to find an 

 explanation. The clay worker has always known that kneading and 

 working his clay bodies increases both the plasticity and binding 

 power, and it has lately been discovered that in many cases the addi- 

 tion of such substances as ammonia, alum, tannic acid, etc., produces 

 a still greater increase in these properties. In the same way the road 

 engineer has known that wet rolling will cause a macadam surface to 

 bind. It now appears that the more wet rolling the better, and the 

 question arises whether the addition of suitable substances in dilute 

 solution would be of benefit. Of the great number of different kinds 

 of rocks used in road building only a comparatively few will furnish 

 such a compacted surface as to be practically mud and dust free under 

 average traffic. 



A number of investigators have noted that even in moist air water 

 is absorbed by finely powdered minerals that can not thereafter be 

 driven off entirely except at a high temperature. This has been 



Orange Judd Co., New York, 1900. 



