14 MEDICAL BACTERIOLOGY 



the dried matter or bacteria picked up with a sterile platinum loop and the loop 

 lightly touched to the drop of water until a slight cloudiness appears. The 

 excess is then burned off the loop and when it is cool, the bacteria are mixed with 

 the drop of water and it is spread out in a thin film with the loop. 



When the matter to be examined has been spread on the slide or cover glass, 

 whether fluid as bouillon, viscid as sputum, or solid mixed with water, it is fixed 

 by drying. This is commonly done by gently heating over a flame; should the 

 slides be placed nearer the flame than one can hold one's hand with comfort, 

 the organisms may be destroyed or so altered as to become invisible. Some 

 experienced observers state that of two sets of slides smeared from one sample 

 of sputum, one set dried or fixed by gently heating over a flame, the other set 

 fixed by simply permitting them to stand at room temperature until dry, often 

 those fixed without heating will show tubercle bacilli when those fixed by flame 

 do not. Some workers prefer fixing their preparations by placing them in a 

 thermostat until dry. 



Whether smears shall be made upon cover glasses or slides is a matter of 

 personal choice, determined by convenience. Slides are less fragile and easier 

 to handle than cover glasses while making the smears; on the other hand, cover 

 glasses are easier to handle when staining. 



If smears are made on slides, when fixed, stained and dried they can be ex- 

 amined without mounting a cover glass; cover glasses when ready for examina- 

 tion must, of course, be mounted on slides. 



The essential points in making smears for microscopic examination are as 

 follows : 



1. Use absolutely clean slides and covers. 



2. Spread the material in a thin, even, transparent film. 



3. Protect it from contamination with dust, etc. 



4. Guard against overheating during fixation. 



Cover glasses are usually attached to slides with Canada balsam which 

 should be free of acid. A drop of balsam is placed on the slide and the cover 

 glass placed upon it. The balsam should be free from air bubbles and dirt. If 

 it has the proper consistency it will spread out in a thin film. There should be 

 no appreciable elevation of the cover above the slide. When balsam becomes so 

 thick that it will not spread unless pressure is used, it should be thinned out 

 by adding to it xylol. 



Slides can be used repeatedly, if thoroughly cleaned. A satisfactory method 

 is as follows: 



Keep a jar containing a piece of toilet soap and water on the microscope 

 table; when through with slides, place them in jar. After 48 hours wipe them 

 clean and place in running water for Y hour, wipe dry and store in a jar con- 

 taining spiritus aetheris nitrosi. 



After a time slides that have been used and cleaned repeatedly become 

 scratched; scratched slides should never be used when differential staining is to 

 be done. 



In a general laboratory new slides should be used for bacteriological work; 

 afterward they should be set aside for tissue and other work. 



