BACTERIA. 



II. Fixing Methods. 



251 



i. Cover-glass Preparations. 



a. Fixing by Dry Heat. 



459. For fixing Bacteria from fluids containing them, 

 from gelatine cultures, or the like, dry heat is almost 

 exclusively used at present, and commonly, according to 

 R. Koch's method, as follows : 



A small drop of fluid containing Bacteria is transferred, 

 by means of a platinum wire sterilized by heat, to a carefully 

 cleaned cover-glass,* and spread out over it as evenly as 

 possible. If the Bacteria are taken from a solid substratum, 

 a drop of water is first placed upon the cover and a very 

 small particle of the material is rubbed up in it as completely 

 as possible with a platinum wire. 



The fluid is now allowed to evaporate from the cover- 

 glasses thus smeared with Bacteria, at the ordinary tempera- 

 ture, until the preparation is " air-dry." 



The Bacteria are then fixed by passing the cover-glasses, 

 with their Bacteria-sides upward, three times through the 

 non-luminous flame of a Bunsen burner.f Johne's rule may 

 give an idea of the rapidity with which this should be done. 

 According to this, the hand should describe an horizontal 

 circle a foot in diameter in a second, moving at an equal 

 rate throughout the course, and passing through the flame 

 at one part of it. 



460. In this way the Bacteria are so fastened to the cover- 

 glass that they may be treated with staining-fluids and other 

 media without fear of separation. It is thus possible to 

 stain or restain, at any time, preparations which have been 

 mounted for a long time in Canada balsam. For this pur- 



*The cleaning of cover-glasses may be accomplished by the method pro- 

 posed by Giinther (I, 40, note), which consists in heating the glasses, after 

 cleaning with alcohol, in the non-luminous flame of a Bunsen burner. 



f In these and the following manipulations the so-called Kuehne forceps 

 are very convenient. Their arms are bent at about i cm. from their tips, 

 and end in broad surfaces. 



