APPENDIX A 



123 



Aside from the limited capacity afforded by the use of the 

 above method, it is open to another objection : namely, the residuum 

 accumulates upon the inside of the bougie and is hard to remove. 



An extremely satisfactory filtering apparatus is that 

 shown in Fig. 41, which was made for the author after C^^^^ 

 the filters used in the Bureau of Soils of the United 

 States Department of Agricidture and described by ^^s~5 

 Schreiner and Failyer in Bulletin No. 31 of that bureau. 

 The metal cylinder which contains the bougie is large ^^=2> 

 enough to hold 600 ec. or more of solution and to take 

 a bougie of any length. This last feature is not with- 

 out value, since it is at times impossible to obtain 

 bougies short enough to fit the apparatus of European 

 manufacturers. The metal filtering cylinders are lined 

 with porcelain enamel and are thus entirely nontoxic 

 — an advantage over the older brass cylinders or those 

 which were silver-plated inside. 



The construction of the metal filtering cylinder is 

 evident from the accompanying illustration (Fig. 42). 

 Both ends are accurately threaded and receive screw 

 caps. The lower end has a rubber gasket which fits 

 on the nipple of the bougie. This gasket goes into 

 the lower end of the cylinder and is held in place 

 by the nipple plate and screw collar. The upper end 

 of the cylinder is closed with a cylinder head and a 

 screw collar. A good rubber washer lies between the 

 cylinder and the cylinder head. The cylinder head 

 has a flanged opening at the center, which is threaded 

 to fit the pipe from the air tank and usually needs a 

 rubber washer. 



The air pressure needed for filtering, which is from 

 1 to 2 atmospheres, is supplied by any convenient 

 form of pump. The air tank, which should hold from 

 10 to 20 gal., serves to store compressed air and to 

 equalize the pressure for filtration. The opening on 

 the side of the tank is fitted with a stopcock and a bicycle valve, 

 so as to hold the pressure in the tank. The bougie is wrapped in 

 manUa paper and sterilized in the hot-air sterilizer. A plug of cotton 

 should be inserted in the nipple end and the whole carefully wrapped 

 ia manila paper. When sterilized and cool, the bougie is put into 



Fig. 42. Cyl- 

 inder for the 

 Chamberland 

 Filter (after 

 Schreiner and 

 Failyer) 



