72 Answers to Queries. 
trace of acid by repeated washings. Boil finally each density in a 
weak solution of bicarbonate of soda in a test-tube of large capa- 
city foran hour. ‘This will remove all flocculent matter. After 
repeated washings, perfectly clean Polycistina will be obtained. 
The heaviest density, consisting of sand and the largest Polycis- 
tina, should be placed in a test-tube with about three inches of 
water, and subjected to rotatory motion. ‘This will cause the 
Polycistinous shells to rise above and to free themselves from 
sand, They are then poured off in a wide-mouthed bottle or 
beaker, the sand being left at the bottom of the test-tube. This. 
must be very carefully conducted, and repeated to see that no 
large and perfect shells may be left in the sand. Each density 
should then be bottled in distilled water. Polycistina may be 
mounted dry, as diatoms or in balsam. 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 cleaned, and 
a sufficient quantity of balsam, thinned with turpentine, put upon 
it. The Polycistina are then to be placed in the balsam in suffi- 
cient quantity to form a surface over the cover when evenly 
spread upon it. The cover, with the Polycistina, is then to be 
put aside for from twelve to twenty-four hours, in order that the 
balsam may thoroughly permeate the forms and all air escape. 
Now harden the balsam by gentle heat, and the cover is to be laid 
on the slip, balsam side upwards, and held under the flame of a 
Bunsen burner until the balsam is liquified. The Polycistina are 
then to be evenly spread upon the cover by means of a needle. 
When the balsam is cold, a small drop of benzole or balsam is to 
be placed upon its surface, and a clean half-inch cover to be 
warmed and carefully lowered upon the balsam. The Polycistina 
is thus between ¢wo covers. Now turn the mount over, so that 
the under surface of the lower cover shall be upwards. Coat this 
over with asphalt, and put aside for a day or two to harden. Then 
upon the asphalt disc on the slip place 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 zinc cement. V. As de 
326.—Preparing Rock Sections.—How far G. H. B. can dis- 
pense with lathe and diamond-powder in preparing rock-sections 
depends very much upon whether the rocks he wishes to prepare 
are silicious or not. Silicious rocks or fossils are extremely 
difficult to tackle without proper apparatus. Limestones, and the 
softer rocks and fossils, are very easily ground with emery or 
even pumice-stone powder. I have some reason to believe that 
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