COLLOID THEORY. 13 



to bring this out by means of illustrations colored by hand, in which 

 the appearance observed under the microscope with polarized light is 

 brought out as faithf ully as possible. Although the colors due to the 

 optical activity- of the crystalline particles in polarized light may 

 appear somewhat too bright in the colored plates, the results of the 

 action of water are not in the slightest degree exaggerated. In so far 

 as there has been failure to reproduce the exact appearance, it is 

 because the illustrations can not do justice to what the eye easily dis- 

 cerns with the aid of the polarizing microscope. In making the prep- 

 arations a drop of Canada balsam was placed on a glass slide, a little 

 of the rock powder to be examined was then stirred in thoroughly with 

 a needle, and, after warming to remove all bubbles, a cover glass was 

 pressed down on top in the usual way. 



PI. I, fig. 1, shows the appearance of an orthoclase feldspar dust 

 as it comes from the ball mill after having been ground dry for 

 three hours." A close inspection shows that although the cn^stals 

 exhibit every degree of illumination as the} 7 lie in different optical 

 planes to the direction of the ray of polarized light, each individual 

 crystal appears with clean, clear-cut surfaces and outlines. 



PI. I, fig. 2, shows the same orthoclase powder which has been 

 ground wet in the ball mill for the same length of time. Five hundred 

 grams of crushed rock and 90 cc of distilled water were used as this 

 mixture produced the desired past} T or doughy consistency. After the 

 grinding process was completed the dough was thoroughly dried on 

 the water bath, repulverized, and the microscopic slides prepared. 

 The pectoid films and gummy material are easily seen on and among 

 the crystals. The brown color of the pectoid matter is due to a stain 

 which is taken up from the Canada balsam. If the mount is made 

 with glycerin jelly the films appear colorless and almost transparent, 

 and can only be seen with difficulty. The ability to stain readily is 

 characteristic of this sort of matter, and serves to distinguish it from 

 the unchanged crystalline granules. 



PI. II, fig. 1, is precisely like PI. I, fig. 2, except that the wet grind- 

 ing was done in an agate mill. It is included in order to show that 

 the pectoid appearance is not due to iron hydroxid from the ball mill. 



PL II, fig. 2, shows wet ground orthoclase stained with methylene 

 blue. The dry powder is warmed with a dilute solution of the dye, 

 thrown upon a filter and washed with distilled water. Unchanged 

 crystalline grains will not absorb coloring matter, whereas pectoid 

 matter is very easily stained. It is not easy to show this in an illus- 

 tration as clearly as it can be seen under the microscope. 



PI. Ill, fig. 1, shows orthoclase dust in the finest state of subdivision 



The ball mill contains two chilled steel balls, which weigh 25 pounds each, and 

 is revolved at the rate of 2,000 revolutions per hour. It has been found that rock 

 thus ground is fine enough to pass entirely through a 0.25 mm. mesh. 



