MICROSCOPICAL i:XAMINATION OF BACTERIA. 93 



and the cover-glass lifted off by means of forceps. It is then 

 allowed to dry, passed through the flame three times, and stained 

 as already described. In some oases of plate-cultures, especially 

 where no liquefaction has taken place, the growth is bodily trans- 

 ferred to the cover-glass, and a vacant area left on the " gelatine 

 or agar-agar, cori-esponding exactly with the form and size of the 

 cover-glass employed. 



Presebvation of Preparations. 



After examining a cover-glass preparation with an oil immersion 

 objective the cedar oil must be carefully wiped off, and the slide 

 set aside for tbe Canada balsam to set. At a convenient time all 

 preparations should be sealed with a ring of Hollis' glue ; the 

 cedar oil used at subsequent examinations of the specimen will 

 not be able to work its way under the cover-glass, and prevent 

 the balsam from hardening. When it is ringed cedar oil can be 

 readily wiped off, and the specimen cleaned without danger of 

 moving the cover-glass and injuring the preparation. 



(b) Bacteria in Sections op Tissues. 



Methods of Hardening and Decalcifying Tissues. — To harden small 

 organs, such as the viscera of a mouse, they should be placed on 

 a piece of filter-paper at the bottom of a small wide-mouthed glass 

 jar, and covered with about twenty times their volume of absolute 

 alcohol. Larger organs, pathological growths, etc., are treated in 

 the same way, but must first be cut into small pieces, or cubes, 

 varying from a quarter of an inch to an inch in size. Miiller's 

 fluid may also be employed, and methylated spirit may be sub- 

 stituted for alcohol, from motives of economy. Tissues hardened in 

 absolute alcohol are ready for cutting in two or three days, and 

 those hardened in Miiller's fluid in as many weeks. 



Teeth, or osseous structures, must first be placed in a decalcifying 

 solution, such as Kleinenberg's. When sufiiciently softened, they 

 are allowed to soak in water, to wash out the picric acid, and then 

 transferred through weak spirit to absolute alcohol. Ebner's solu- 

 tion also gives excellent results, especially when the structures to 

 be decalcified are placed in fresh solution from time to time. 



Methods of Embedding, Fixing, and Cutting. — The author 

 finds that freezing with ether combined with the method of em- 

 bedding in celloidin gives excellent results. The pieces of tissue 

 to be embedded are placed, after the process of hardening is com- 



