INTRODUCTION. 21 



cement, varnish, or white lead, allowed to dr}- nearl_y to hardness, 

 upon which a cover-glass fits firmly-, and is retained by a second 

 ring of the same cement. If the precaution is talien to have the 

 cover-glass fit evenly- to the first laj-er of cement, tliere is little 

 danger that the subsequent layer, which is to hold the cover in 

 place, will creep under it and into the cell. 



101. Glycerin, pure water, calcic chloride solution, potassic 

 acetate, and like liquids may be used as mounting-media in cells 

 prepared iu the manner just mentioned, but made of greater 

 thickness. Care must be observed to avoid touching the upper 

 edge of the cement ring with the liquid ; and yet the cell must 

 be completel}- filled, in order to exclude air. 



102. If a specimen has been prei)ared in glycerin, and it is 

 not considered well to disturb the cover-glass, a cement ring or 

 square can be built up around the cover at a little distance from 

 it, provided the glass slide is thoroughly cleaned at the place 

 where the cement is to be put. After the requisite number of 

 laj-ers have hardened sufficiently, a ring of the same or, better, 

 of a more quicklj- drjMng cement may be placed across from the 

 edge of the cell to the cover-glass, to hold it in place. As this, 

 in dr3"ing, will contract somewhat, it is a good plan to place two 

 or three fragments of thin glnss under the cover, that these 

 may receive the pressure and prevent crushing the specimen. 



103. Of the mounting-media, one of the best is glj'cerin and 

 acetic acid in equal parts, boiled and filtered. It serves well for 

 thin-walled specimens (especiall3- in the lower plants). 



104. Specimens of fresh cells or of juicy tissues which are to 

 be mounted in gl3'cerin are best treated in the manner recom- 

 mended In- Beale.' " The specimen is first immersed in weak 

 glycerin, and the density of the fluid is gradually- increased, 

 either by adding from time to time a few drops of strong gly- 

 cerin, until it bears the strongest, or bj- allowing the original 

 weak solution to become gradually' concentrated by slow evapo- 

 ration. In this way, in the course of two or three da3-s the 

 softest and most delicate tissues may be made to swell out 

 almost to their original volume in the densest glycerin or sj-rup. 

 They become more transparent, but no chemical alteration is 

 produced, and the addition of water will at any time cause the 

 specimen to assume its ordinary' characters." 



105. It is plain that mounts in anj' liquid must be liable to 

 injur}' from displacement of the cover-glass ; but this can be 



1 How to Work with the Microscope, p. 360. 



