148 Modern Microscopy 



twenty-four hours or more to a fresh quantity of the same 

 solution (to which a drop or two of formic acid may be 

 added). They may then be hardened with 50 per cent, 

 alcohol, and sections, which need not be thin, are cut either 

 from celloidin with a microtome or with the free hand. 

 The sections are mounted in Canada balsam, which is 

 allowed to dry on the slide. They must not be covered 

 with a cover-glass, but the balsam must remain exposed to 

 the air. 



(b) Instead of being slowly hardened in bichromate, the 

 tissue is placed at once in very small pieces in a mixture of 

 bichromate and osmic (3 parts of Miiller's fluid to one of 

 osmic acid). In this it remains from two to five days, after 

 which the pieces are treated with silver nitrate, as in the 

 other case. This method is not only more rapid than the 

 other, but is more sure in its results. 



Mounting in Canada Balsam. — Take 3 ounces of dried 

 Canada balsam and dissolve in 3 fluid ounces of pure benzol, 

 filter, and keep in an outside stoppered bottle. Clear the 

 section in clove-oil, and place it in turpentine. Clean a 

 cover- glass and a slide, place a few drops of balsam on the 

 centre of the latter, take the section from the turpentine on 

 a lifter, allow the excess of turpentine to drain away, and 

 with a needle-point pull the section off the lifter into the 

 balsam on the slide. Now take up the cover-glass with a 

 pair of forceps, and bring its edge in contact with the 

 balsam on the slide ; ease it down carefully, so that no air- 

 bubbles are enclosed, and with the points of the forceps 

 press on the surface of the cover until the section lies quite 

 flat, and the excess of balsam is squeezed out. The slide 

 must now be put aside for a day or two to allow the balsam 

 to harden ; the exuded medium may then be washed away 

 with some benzol and a soft camel's hair brush, after which 

 dry the slide carefully with a cloth and apply a ring of 

 cement. The above method answers well for mounting 

 sections quickly, but when time will admit the following is 

 a much better way. Clear the section and place it in turpen- 



