14 EFFECT OF WATER ON ROCK POWDERS. 



that it was possible to obtain. After grinding for many hours in 

 the mechanical agate mill, the dust was sifted by beating it through a 

 fine, close-woven linen cloth. This sifted dust was then shaken up 

 with air in a dry flask and the portion collected which remained the 

 longest time in air suspension. In spite of the extreme fineness of 

 the particles, there is no doubt of their crystalline integrity when 

 examined under the microscope. 



PI. Ill, fig. 2, shows the same powder that was prepared for the 

 preceding figure after it had been ground wet for one-half hour in the 

 agate mortar and subsequently dried and repulverized. The bunching 

 and clogging effect, due to the coagulation of the pectoid matter, is 

 very clearly shown in this preparation, and in fact it is difficult to 

 recognize the persistence of any crystalline particles whatever. 



PL IV, fig. 1, is precisely the same as PI. Ill, fig. 2, except that the 

 material has been stained with methylene blue in the manner already 

 described. 



In some respects PL IV, fig. 2, is the most instructive illustration 

 of the series. It represents material obtained as follows: Ordinary wet 

 ground orthoclase powder, such as is shown in PL I, fig. 2, was thor- 

 oughly shaken with a very large excess of water and allowed to settle 

 as much as it would during several days' standing. A certain portion 

 of the water which still contained matter in colloid suspension was 

 pipetted off, evaporated to complete dryness on a steam bath, and the 

 residual materal lightly pulverized. The illustration shows quite 

 faithfully the clumps of this matter which have taken up stain from 

 the balsam. It is not possible to discern in this preparation the slight- 

 est indication of crystalline matter; in fact, we have what might per- 

 haps be compared to a mineral glue. 



All the evidence so far obtained points to the fact that the binding 

 power and plasticity of rock dusts, clays, and soils are due to the for- 

 mation and presence of just such matter existing either free or as 

 cloggy films on the surface of crystalline or amorphous granules. 



If suspensions in water of clays and rock powders are allowed to 

 stand in a strong light facing a window it will be found that in some 

 cases the sedimentation will show a tendency to collect upon the illu- 

 minated side of the glass vessel, in other cases upon the side in shadow. 

 Quincke a studied this action, and spoke of such suspensions as being 

 positively or negatively photodromic. Quincke accounts for this 

 action as well as for the phenomenon of flocculation by supposing 

 that the kaolin particles are coated with a thin, viscous, oily fluid 

 Ia3 r er of colloidal silicic acid. On the surface of separation of this 

 viscous fluid from the surrounding aqueous fluid, surface tension acts. 

 Probably changes of the surface tension will excite movements among 



Chem. News, 84 : 174. 



