FILTERS. 



43 



O 



2 3 



Fig. 39.f 



when the medium has a retarding action, it may not 

 occur until after two or three weeks. Of course, the 

 rapidity of the clouding depends to a considerable extent 

 on the size of the loop and on whether the inoculation 

 was from a young or old, a fluid or a solid culture. 



Among other tools, the student should be provided 

 with five platinum-iridium wires set into glass handles, 

 three of which are bent at the free end into loops of a 

 definite size, i. e., with an inside diameter of i, 2, and 

 3 mm. These are made by bending around wires of the 

 given size, and will enable one to measure out approxi- 

 mately uniform quantities of fluids and solids. Smaller 

 quantities may be transferred on the extreme tip of a 

 straight platinum needle. It is also convenient to have 

 a platinum* needle bent at the end into a short hook 

 (see fig. 39). In comparing rates of growth in fluid 

 cultures it is best to inoculate them from other fluid 

 cultures of a given age and not from solids. 



If there is any reason to think that boiling changes 

 the nature of any of these fluids, they should be steril- 

 ized cold by forcing them through a Chamberland or 

 Berkefeld filter. The Chamberland has the finer pores, 

 the Berkefeld filters quicker. The simplest way of using 

 such a filter is that first described by Dr. Theobald 

 Smith, viz, to put the fluid inside and force it out by 

 means of clean compressed air. For this purpose select 

 a flat-bottomed cylindrical glass vessel (a round-bottomed 

 one is less convenient, but may be set into a hole bored 

 in a block of wood) of a larger diameter and 5 or 10 

 centimeters longer than the bougie, which should be 

 clean (previously unused), but washed by having had 

 some liters of distilled or filtered water forced through 

 it. Wrap the nipple-end of the filtering cylinder firmly 

 with clean cotton for a distance of 5 or 10 cm. down. 

 Thrust the wrapped bougie into the glass vessel securely, 

 so that only the nipple and the cap or shoulder projects. 

 The top of the bougie should also be wired so that it 

 can not possibly slip down during the filtering. This 

 apparatus should now be sterilized by putting it into 

 the dry oven for two hours at 145 C. Wrap in clean 

 Manila paper and heat at the same time a large cotton 

 plug, /. e., one which has been made to fit the mouth 



*PlatinurrHiridium is preferred to pure platinum because it bends less easily. The wire used by 

 the writer has a diameter of 0.48 mm. The alloy as usually found on the market is said to contain 

 about 10 per cent of iridium, sometimes less, but never more. The ware shown in fig. 39 was made 

 to order and contains 20 per cent iridium. 



t FIG. 39. Platinum-iridium wires set into glass rods, for bacteriological work. I, needle; 

 2, hook ; 3, one-millimeter loop ; 4, two-millimeter loop; 5, three-millimeter loop. The size of this 

 wire is about one fifty-fifth inch. 



