22 MANUAL OF BACTERtOtOGV. 



SO large that it will not cool rapidly after heating. A good size 

 for most purposes is number 23, American wire gauge (Brown 

 & Sharp). The wire may be straight throughout its length, or 

 the tip may be bent to form a loop (German, Oese). It is well 

 to follow, from the beginning, certain rules which make the use 

 of the platinum wire safe and accurate. Every time it is taken 

 into the hand and before using it for any manipulation heat it in 

 the flame of a Bunsen burner or an alcohol lamp to a red heat; 

 and always, after using, and before putting it down, heat it again 

 to a red heat. If the needle is wet, it should be dried by holding 

 it near the flame in order to avoid the "sputtering" which occurs 

 if it is plunged at once into the flame. This precaution is 



Fig. 5. — Straight Platinum Wire and Platinum Wire Loop. 



especially called for when the wire has been dipped in milk or 

 other substances containing oil. When the needle "sputters," 

 as it is called, from too rapid heating, particles that have not yet 

 been sterilized may be thrown some distance. On no account 

 should the needle touch any object other than that which it is 

 intended it should touch. With such a platinum wire, which 

 has been properly sterilized, one can easily remove portions 

 from a culture of bacteria, or from a fluid in which bacteria are 

 supposed to be present. The glass rod in which the platinum 

 wire is fixed should be held between the thumb and forefinger 

 of the right hand like a pen. (For the manner of holding test- 

 tubes, see page 73.) 

 The Hanging-drop. — Living bacteria may be studied with 



