20 STAINING METHODS. 



suspended in an albuminous medium it will be necessary, after the 

 film is dry, to heat the preparation sufficiently to coagulate the albu- 

 men, in order that it may not ba washed off in the subsequent stain- 

 ing process. This is best done, in accordance with Koch's directions 

 for the preparation of tuberculous sputum, by passing the cover 

 glass, held in slender forceps, rather quickly through the flame of an 

 alcohol lamp three times in succession. In this operation it must 

 be remembered that too much heat will destroy the preparation, 

 while too little will fail to accomplish the object in view coagu- 

 lation of the albumen. In passing the cover glass through the 

 flame the smeared side is to be held upward. The time required 

 will be about three seconds for passing it three times as directed ;. 

 but this will vary according to the intensity of the flame, and some 

 little experience is neoessary in order to .obtain the best results. 



The operation of " fixing," or coagulating the albumen, may also 

 be effected by exposure in a dry-air oven, heated to 120 to 130 C., 

 for a few minutes (two to ten minutes), as directed by Ehrlich. 



Bacteria simply suspended in distilled water adhere very well to 

 the cover glass when treated as directed, but if they have been taken 

 from a liquefied gelatin culture the film is very apt to be washed 

 away during the staining process. This is best avoided by taking as 

 little as possible of the gelatin medium and suspending the bacteria 

 to be examined in a drop of water, which dilutes the gelatin and 

 washes it away from the surface of the cells. 



Smear Preparations. In various infectious diseases bacteria are 

 found in the blood and tissues of the body, and their presence may 

 be demonstrated by making what is called a smear preparation. A 

 little drop of blood may be spread upon the thin glass cover, or it 

 may be brought in contact with the freshly cut surface of one of the 

 vascular organs, as the liver or spleen. It is especially desirable that 

 the material used for such a preparation be small in amount and dis- 

 tributed evenly in a very thin layer. In Germany it is the custom, 

 in making smear preparations, to press the material between two glass 

 covers, which are then separated by sliding them apart, thus leaving 

 a thin layer upon each. This answers very well, but the writer pre- 

 fers to spread the material by drawing across the face of the cover 

 glass the end of a well-ground and polished glass slide. This method 

 is especially useful for spreading blood in a uniform layer, in which 

 the corpuscles are evenly distributed and retain their normal form. 

 A very small drop of blood is placed near one edge of the cover glass, 

 which is placed upon a smooth surface ; the glass slide is held at a 

 very acute angle and is gently drawn across the cover glass, without 

 any pressure. 



Most bacteriologists make their preparations upon the cover glass, 



