282 THe Microscope. 
the sand, and they can be poured off into a wide-mouthed bottle 
or beaker, the sand being left at the bottom of the test-tube. 
This latter process should be carefully conducted and repeated 
several times in order that no large and perfect shells may be 
_ left among the sand. Each density should then be bottled in 
~ distilled water. Polycystina may be mounted in Balsam—in 
the same manner as Diatoms—or “dry.” Beautiful slides can 
be produced by calcining the shells upon a piece of thick 
platinum foil, or in a small platinum capsule, and mounting 
them “opaque.” The neatest and best method of preparing 
such slides is to make a disc half an inch in diameter in the 
centre of a slip, allowing it to harden for some days. A half- 
inch cover is then to be cleaned and a sufficient quantity of 
Balsam, thinned with turpentine, put upon it; the Polycystina 
are then to be placed in the Balsam in sufficient quantity to 
form a surface over the cover when evenly spread upon it; the 
cover with the Polycystina is then to be put aside (as recom- 
mended with Diatoms) for 12 or 24 hours in order that the 
Balsam may thoroughly permeate the forms and all air escape; 
the Balsam is then to be hardened by gentle heat, as already 
described, and the cover is to be laid upon a slip, with the Bal- 
sam upwards, and held over the flame of a Bunsen burner until 
the Balsam is liquified; the Polycystina are then to be evenly 
spread upon the cover by means of a needle; when the Balsam 
is cold a small drop of Benzole-Balsam is to be placed upon its 
surface and a clean half-inch cover to be warmed and carefully 
lowered upon the Balsam. The Polycystina will thus be 
mounted between two covers; now, turn the “‘ mount” over, so 
that the under surface of the lower cover shall be upwards, and 
coat this with asphalt, put the mount aside for a day or two, in 
order to allow the asphalt to become thoroughly hard, then put 
upon the asphalt dise on the slip a small drop of (cold) “ French 
Liquid Glue,” place upon this the asphalted under surface of 
the “mount,” and when the glue is quite dry finish off the slide 
with either asphalt or white zinc cement. 
