558 



FILTERABILITY OF MICRO-ORGANISMS 



Figure 2 visualizes the same thing with the gypsum filter carrying a positive 

 charge. The arrows indicate the direction of the flow of the liquid being filtered. 



A filter of any material will of course hold back any rigid particle that is larger 

 than the pores of the filter; but for those particles that are smaller than the pores of 

 the filter, chemical or electrical forces come into play. One may speak of a filter when 

 it is in use as a suspension of the material of which the filter is composed, in the fluid 

 which is being filtered. Silica has a definite negative charge, and it might be that if one 





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Fig. I. — Siliceous filter with 

 pore of negative charge. 



Fig. 2. — Plaster of Paris filter 

 with pore of positive charge. 



constructs a filter of a material of charge opposite to that of silica, one might find that 

 bacteria or colloids which are filterable through silica filters are non-filterable through 

 such other filters, and vice versa. 



Accordingly, filters were made of plaster of Paris, and experiments with various 

 dyes and viruses were made. Thus Victoria blue B or methyl green, which do not pass 

 a Berkefeld or siliceous filter, readily pass through a filter made of plaster of Paris; 

 Congo red, which readily passes through the Berkefeld filter, does not pass through a 

 plaster of Paris filter. Acidic and basic dyes should be used to test filters. In this 

 work any Berkefeld filter which does not completely remove Victoria blue B from a 

 1-20,000 solution is rejected. The gypsum filter is tested with a similar solution of 

 Congo red. 



