HOW TO MOUNT SPECIMENS 43 
While melting the jelly, incline the slide in order to allow the 
melted glycerine jelly to flow gradually over the specimen, thus 
replacing the air contained in the cells and intercellular spaces. 
Finish the mounting as directed above, but under no conditions 
should you stir the glycerine jelly with the section. 
If specimens, after having been embedded in paraffin or 
collodion, are cut, cleared, stained, and dehydrated, they are 
usually mounted in Canada balsam. A small drop of this sub- 
stance, which may be obtained in collapsible tubes, is placed 
at one side of the specimen. While inclining the slide, gently 
heat until the Canada balsam covers the specimen. Secure a 
cover glass by the aid of pliers, pass it through the flame three 
times, and lower it slowly while holding it in an inclined position. 
Press gently on the cover glass with the needle-handle, and keep 
in a horizontal position for twenty-four hours, then place directly 
in a slide box or cabinet, since no sealing is required. 
Glycerine is sometimes used to make permanent mounts, but 
it is unsatisfactory, because the cover glass is easily removed 
and the specimen spoiled or lost, unless ringed—a procedure 
which is not easily accomplished. If the specimen is to be 
mounted in glycerine, it must first be placed in a mixture of 
alcohol, glycerine, and water, and then transferred to glycerine. 
Lactic acid is another permanent liquid-mounting medium, 
which is unsatisfactory in the same way as glycerine, but like 
glycerine, there are certain special cases where it is desirable 
to use it. When this is used, the slides should be kept in a 
horizontal position, unless ringed. 
COVER GLASSES 
_ Great care should be used in the selection of cover glasses, 
however, not only as regards their shape but as to their thickness. 
The standard tube length of the different manufacturers makes 
an allowance of a definite thickness for cover glasses. It is 
necessary, therefore, to use cover glasses made by the manu- 
facturer of the microscope in use. 
Cover glasses are either square or round. Of each there are 
four different thicknesses and two different sizes. The standard 
thicknesses are: 
