280 BACTERIOLOGY. 



while the cut organ is drawn over the surface of the glass. 

 The piece of tissue should be held by a pair of wide-pointed 

 forceps. Not infrequently, the cut surface of an organ is 

 very soft and pulpy, and if applied to the cover-glass will 

 leave a thick daub of material. This can be avoided by 

 first touching the tissue to a clean slide, or to the bottom 

 of a Petri dish. In this way the excess of fluid is removed, 

 and it becomes possible to spread a very thin, even film 

 over each cover-glass. 



Cover-glass streaks should also be made from the heart- 

 blood. A good preparation of the blood should show the 

 corpuscles spread out in a single layer and separated from 

 one another. This cannot be done if a platinum wire is 

 employed to spread the material. A minute drop of the 

 blood should be placed on the glass-slip and covered with 

 another one. The two slips should then be drawn apart, 

 when a thin layer will be left on each cover-glass. 



An easier procedure is to place the droplet of the blood on a 

 cover-g-lass. A large square cover-glass (20 mm.) should be cut into 

 halves. The edge of one of these should be touched to the droplet of 

 blood and then drawn across the cover-glass. The same result can 

 be obtained with a glass-slide. The drop of blood, placed on one of 

 these, is touched with the edge of another slide which is then drawn 

 along the surface. The slide is then covered with alcohol, for a min- 

 ute or two. This is then washed away and the stain applied for a few 

 seconds. The slide is then washed and examined. For this purpose, 

 a cover-glass can be dropped upon the moist slide. Or, the latter 

 may be dried between filter-papers, and a drop of cedar-oil can then 

 be placed directly on the specimen. If a permanent mount is desired 

 the slide is freed of water or oil by means of paper; a drop of balsam 

 is then added and covered with a glass-slip. 



The fixing of the blood preparations requires spe- 

 cial care in order to prevent alteration of the cor- 

 puscles. A minimum of heat should be used. The best 

 specimens are obtained by immersing, for a few minutes, in 

 absolute alcohol or in a mixture of equal parts of absolute 

 alcohol and ether. When fixed in this way the organisms 



