280 THe Microscope. 
MOUNTING. 
MOUNTING THE DIATOMAC 2. 
For a considerable time experiments have been in progress 
in the hope of discovering a medium, in which to mount Dia- 
toms, which should be more satisfactory, in respect of its 
refractive index, than Canada Balsam, and equally reliable and 
permanent; many such media have been tried—phosphorus, 
monobromide of naphthaline, and others; but itis unnecessary 
to enter into details concerning them, since the desired effects 
have recently been obtained by Dr. Van Heurck, who has 
succeeded in providing an admirable substitute for Balsam in 
gum-styrax, which yields the best possible results. It will 
probably be found that Diatoms mounted in gum-styrax are 
less liable to accidents than Balsam “ mounts,” as the latter 
becomes resinous in time, and the covers are liable to “ spring,” 
the result of which is the appearance of prismatic colours in the 
Balsam, which are not only a great eyesore, but sadly deterior- 
ate the slide. Gum-styrax may be considered absolutely 
permanent and unalterable. Purified styrax contains a granu- 
lar substance, which must be removed by dissolving it in 
chloroform and filtering the solution. The solution thus 
obtained is used in the same manner as Canada Balsam. Styrax 
solution is even easier to work with than Balsam, and air-bubbles 
are not produced in it by the application of heat. The crude 
styrax, as purchased, should be exposed in a thin layer to the 
sun for several weeks. In this way much of its yellow color 
is discharged, the water it contains evaporates, and it becomes 
hard. It may then be dissolved in chloroform, as already 
described. Instead of chloroform, benzine, or a mixture of 
benzine and absolute alcohol may be employed in making the 
solution, while a solution in ether will prove valuable when 
rapid “ setting” of the medium is desired. 
CLEANING AND MOUNTING POLYCYSTINA. 
The siliceous shells of these lovely organisms, the beauty of 
which is only equalled by their variety, are found in profusion, 
