80 THE MICROSCOPICAL NEWS. 
THE PREPARATION OF POLYCisi a 
By E. VAN DEN BROCK. 
‘are liquid containing the specimens is first of all strained 
through a small bag of fine cambric, which is then gently 
shaken in a vessel of pure water—the latter being removed and 
renewed several times. Three or four such washings leave a re$i- 
due, which is either very rich in Polycistina, or contains nothing 
else. This residue is then boiled in a glass flask with a dilute 
solution of caustic potash. Generally after five or six minutes of 
ebullition, the liquid appears full of minute air-bubbles which hold 
the frustules in suspension. The addition of a little nitric acid, 
however, immediately clears the liquid so that the Polycistina fall to 
the bottom of the flask, and the work of washing is rendered easy. 
After two or three washings in pure water, the residue is boiled 
anew in strong pure nitric acid for some minutes, the neck of the 
flask being inclined so as to avoid loss from projection. This 
operation leaves the Polycistina free from all impurity, and two or 
three washings with water remove all traces of acid. The whitish 
residue is then transferred to a watch-glass ‘containing a little dis- 
tilled water. 
If this residue should not be pure but contain grains of quartz, 
etc., they may easily be removed by levigation. 
With the aid of a pipette, or, better still, a little syringe, which 
can easily be made from a piece of glass tube with the end drawn 
out fine, and a rod with cotton wound round the end for a piston, 
a small quantity of the water in the watch-glass containing the 
residue is transferred to a slide. With a fine pencil, or a needle 
set in a handle, the Polycistina can be carefully spread out or set 
in some kind of regular order—provided the quantity of water put 
upon the slide has not been too considerable. When the frustules 
are arranged as wished, the slide is held above the flame of a spirit 
lamp, and the water evaporating leaves the specimens dry in the 
position in which they were placed on the glass. While the glass 
is still warm one or two drops of essence of turpentine are allowed 
to fall upon it from a very fine pipette. Before this dries a little 
very fluid Canada Balsam is dropped upon the slide. This 
spreads itself over the objects, and guided by the turpentine, makes 
its way so completely into the pores and crevices that not the least 
bubble of air is left in a single Polycistina. It only remains to add 
a cover and dry the slide, which made in this manner will always 
be perfectly successful. This mode of preparation was suggested 
by M. Rutot, who employed it successfully for diatoms and other 
silicious organisms. 
N.B. It is best to give the drop of water containing the frustules 
