INTRODUCTION 23 



ment of the filtering candle. The best, certainly that which com- 

 bines the greatest assurance of sterility with the highest degree of 

 simplicity, is the scheme of Martin (fig. 1). This arrangement makes 

 use of a cylinder with a neck, of a separating funnel and of a tube to 

 receive the filtrate. The candle is mounted in the tube in advance 

 by rolhng a narrow band of non-absorbent cotton* about the varnished 

 portion of the candle in such a manner that a plug is formed which fits 

 firmly to the ground rim at the neck of the tube. In this way at one 

 time a number of candles may be prepared and sterilized by dry heat. 



For the filtration, upon the long tube of the funnel are placed, first, 

 a large rubber stopper which fits into the neck of the cylinder, and 

 then, a small stopper, also of rubber, which is introduced tightly into 

 the opening of the candle. The apparatus being assembled, the fluid 

 to be filtered is poured into the funnel, and in the cylinder a moderate 

 vacuum (a few centimeters of mercury is adequate) is estabhshed. 



When the filtration is complete, the large stopper is removed, lifting 

 out in this way the candle adapted to its tube, and the funnel is re- 

 moved. It only remains then to collect the filtrate with a pipette. 

 If sufficient tubes are available, and if it is desired to preserve the 

 filtrate, it is possible, instead of transferring the filtrate with a pipette, 

 to remove the candle and to insert a sterile cotton plug withdrawn 

 from a large test-tube. 



I have tried a great many types of filtering apparatus, but of all of 

 those which have been recommended, that of Martin is the most con- 

 venient and the most trustworthy. I have effected thousands of filtra- 

 tions without a single filtrate being contaminated. The apparatus 

 for assembling this type of filter is made in three sizes, the smallest 

 utilizing the candles mentioned above, and serving well for the filtra- 

 tion of quantities of from 10 to 15 cc, the middle size is the usual 

 laboratory size, and is adapted to the recovery of 40 to 50 cc. of filtrate, 

 and the largest makes use of the same candles as the latter but is 

 adapted to tubes having a capacity of about 150 cc. This last serves 

 particularly well for the preparation of relatively large quantities of 

 bacteriophage suspension intended for use in the treatment of patients. 



* It is strange that a great many laboratories employ absorbent cotton for all 

 purposes, even for plugging culture tubes. This is certainly illogical, since this 

 cotton, quite true to its name, is hydrophile, i.e., it absorbs water. It is pref- 

 erable to use a hydrophobe cotton, as a non-absorbent cotton, which has not 

 undergone a special treatment converting it to the condition where it absorbs 

 water. 



