110 BACTERIOLOGY FOR NURSES 



solution of sodium chloride add sufficient bichloride of mercury 

 to make a saturated solution. Small pieces of tissue are immersed 

 in the solution for about ten hours ; for larger pieces twenty-four 

 hours is necessary. They are then tied in a piece of gauze and 

 placed in a stream of running water for from twelve to twenty 

 hours according to their size in order to wash out the excess of 

 bichloride of mercury. 



Hardening. The pieces of tissue are placed for twenty-four 

 hours successively in each of the following strengths of ethyl 

 alcohol ; 30 per cent, 60 per cent, 90 per cent, and finally absolute 

 alcohol. They are then ready to be embedded in paraffin. If 

 only a minute particle of tissue is to be examined for diagnosis 

 all the stages may be compressed into twenty-four hours. 



Embedding in Paraffin. The tissue is transferred to (1) cedar 

 oil or xylol until translucent; (2) equal parts of cedar oil and 

 paraffin at 37 C. for two hours ; (3) melted paraffin at 52 C. 

 for four hours ; (4) each piece of tissue is removed from the hot 

 paraffin by means of forceps and placed in very small tin or paper 

 box, after which it is surrounded with the melted paraffin. The 

 paraffin used should be one that has a melting point about 52 C. 

 When the block is cold the edges are pared and the preparation is 

 ready for sectioning. 



Cutting of Paraffin Sections. A microtome is generally used 

 for this purpose. Each section should be as thin as possible. 

 When cut the sections are floated on the surface of a glass of warm 

 water kept at about 40 C., where in a short time they become 

 perfectly flat. 



Fixation on Slides. A clean slide is thrust obliquely into 

 the water below the section, a corner of the section is held on to 

 it with a needle, and the slide is withdrawn. The surplus water 

 is wiped off with a cloth, the section carefully adjusted by means 

 of a camel's hair brush, and the slide placed on a support with 

 the section side downward in an incubator for from twelve to fifteen 

 hours ; it will then be sufficiently fixed for staining. Before stain- 

 ing, however, the paraffin must be removed by dropping on to it 

 a little xylol. When the paraffin is dissolved the xylol is removed 



