69 



substance suitable for such a purpose. The substance should be in 

 a very fine state of division, composed of particles grading gradually 

 in size from the ultramicroscopic to those of definite microscopic size 

 The substance should be insoluble in the menstrum, so that a particle 

 recognized in the filtrate would not represent a precipitate formed 

 after passing the filter. 



^ We finally selected carbon on account of its insolubility, the very 

 fine .state of division into which it can be brought, and the ease 

 with which the small particles may be recognized because of their 

 black color and violent Brownian movement. 



To 60 cc. of distilled water we added 40 drops of Higgins's Amer- 

 ican india ink, bought on the market, and this suspension was placed 

 into each of the four Chamberland B filters and drawn through the 

 walls from within outward by a vaccuum in a reverse manner to fig. 2. 

 The first water to come through was pale, but gradually it became 

 slightly brown and later the surface of the filter took on a distinctly 

 dark color. About one and a quarter hours was required for the 

 60 cc. to pass through each filter. 



The filtrates of the four filters were examined with Zeiss micro- 

 scopes, using objectives of 1.5 and 2 mm. and oculars 4 to 12, and there 

 was not the slightest difficulty to see in the hanging drops small parti- 

 cles of carbon in active Brownian movement. Dried specimens of 

 the filtrate showed small particles plainly visible. 



These filters were new and were tested under pressure beneath the 

 surface of water and found free from cracks and pin holes. Before 

 testing with the india ink they were washed with distilled water, 

 about 200 cc. being put through each one. 



The filters first became black in disseminated points on the surface. 

 The black areas were of irregular shape, having a diameter of one- 

 eighth to one-fourth inch. As the filtration continued these areas 

 became larger. At the end of the filtration the filter had a distinctly 

 mottled appearance, showing streaks of white, small circumscribed 

 areas of deep black, and larger areas less deeply stained. That these 

 areas of black are carbon may be demonstrated by burning them in 

 the flame. 



This shows a lack of uniformity in the structure of each individual 

 filter, which only confirms what may be seen after breaking a filter 

 into pieces. At places air spaces may be seen which may extend 

 through almost the entire thickness of the wall, thus reducing the real 

 thickness of the filter to a mere shell. 



Two new Berkefeld filters, 2£ by f inches, were tested with the 

 dilute ink solution. Neither pressure nor vacuum was used. The 

 filtrate came in drops in rapid succession and was as black as the test 

 fluid and showed the particles of carbon under the microscope. 



