INVISIBLE MICROORGANISMS. 



a 



FIG. 42. Apparatus for fractional filtration, designed for use with Pasteur-Cham- 

 berland or Berkefeld filters, a, glass mantle surrounding filter; b, Chamberland filter; 

 c, paraffin joint; d and e, rubber stoppers;/, double side-arm suction flask; g, pinchcock 

 controlling outlet from suction flask; h, outlet tube surrounded by glass shield and 

 attached to lower end of suction flask by means of short rubber tubing; i, glass shield 

 fused to and surrounding outlet tube as a protection against contamination when the 

 filtrates are drawn off; j, glass inlet tube plugged with cotton, for admitting air into 

 suction flask; k, pinchcock governing the admission of air into flask; /, vacuum gauge; 

 m, stopcock connected with vacuum pump. (U. S. Dept. of Agriculture, Bureau of 

 Animal Industry, Bui. 113.) 



