20 GENERAL BACTERIOLOGY 



plugs, dipping same in melted paraffin (melting point about 50 C.) 

 and then replacing them; (3) by cutting off the projecting cotton 

 and drawing over the mouth of the vessel a rubber cap (made for the 

 purpose) which has been sterilized in a solution of mercuric bi- 

 chloride, or rubber dam, easily obtained from dentists, fastened on 

 with a rubber band, may also be used ; or (4) by use of a cap of tin- 

 foil. In this case the foil should be put on as soon as the tubes are 

 filled, and sterilized with the medium. 



All media should be carefully examined every day for a week or 

 more, and if spots or the least cloudiness appears, the medium is not 

 sterile and the process of sterilization must be repeated. 



All receptacles containing media should be labeled after steriliza- 

 tion. For this purpose labels can be purchased, the size used for 

 glass slides, or gummed paper in sheets can be cut into squares 

 ( 2 cm. ) . The labels are to be attached to each ves- 

 sel 1 cm. from the lip. The name of the student, 

 the kind of medium and the date of preparation 

 should be written across the top, leaving the rest of 

 the label to be filled in when the medium is inocu- 

 lated. Rule VII. 



EXERCISE 12. PLATINUM NEEDLES. 



GENERAL DIRECTIONS. These are made by fusing a piece of No. 

 27 platinum wire (5 cm. long) into a glass rod or tube (18 cm. 

 f ^ long). (Fig. 8.) The dan- 

 ger of having the wire crack 



O- c ] off when the needle is heated 



FIG. 8. Platinum Needles. is lessened if a little piece 



(1/2 cm.) of fusible glass is soldered on the glass rod before the 

 wire is melted in. Each student should have two such needles ; in 

 one the wire should be straight (designated " needle") and the other 

 bent to form a ' ' loop. ' ' This loop should be formed around a No. 10 

 wire. These instruments must be sterilized shortly before and im- 

 mediately after use by heating the wire to a glow in the gas flame. 

 The handle should also be passed through the flame two or three 

 times. Cool before using. If the habit of sterilizing is thoroughly 

 acquired much trouble will be avoided and possible danger prevented. 

 These needles will be in constant use. 



REFERENCES. A. 131 ; H. 42 ; M. & R. 51 ; McF. 196 ; P. B. C. 33, 

 foot note. 



