ill!Ill

PREPARATION OF DIATOMS.
River, O., sent to me, but although it had been chlorated, still, when a
mounting was made, not more than one or two frustules would be in
the field of view, the great mass being either smaller forms, or fine frag-
ments of silex ; by careful watching and testing the time when the
different sizes would remain suspended, I have made from this a preparation,
which will show hundreds where before were scarcely any, and
which would never be recognised as the same gathering. Supposing now
a trial shows us the diatoms tolerably abundant, the trial being made
by heating in the manner presently to be described ; the phial is filled
with alcohol and water, half and half. Some samples
69
of alcohol leave
behind a scum after evaporation, especially noticeable after burning in
the mode presently to be described, and water which will leave crystals,
or any scum, must be avoided. The beauty of the preparation will
largely depend upon being particular in this matter.
"
For mounting diatoms I invariably place a drop of the fluid con-
taining them upon the cover, never on the slide. The alcohol and water
will spread out on the slide, but will remain heaped up on the round
cover, like a plane convex lens. I prepare a little stand, of quite fine
wire (so as not to conduct off too much heat), bent at right angles and
inserted into a base ; the free end is bent into a ring, and upon this
ring is placed a square plate of very thin iron, (') (such
as is used for
the so-called "tin-types" in photography, with the Japan burned off),
held in place by bending the corners of the square over the ring, loosely,
to allow expansion, without bending when heated ; upon this plate the
cleaned cover is placed, and then, by means of a pipette, a drop of the
alcoholic liquid with the diatoms is placed upon it, and the spirit lamp
applied below. The alcohol takes fire and is allowed to burn out ; the
flame of the lamp is then placed beneath, and the rest gently boiled,
the remaining alcohol escaping during this ebullition causes the diatoms,
by this very act, to distribute themselves very evenly over the cover, and
all matting is effectually prevented. It is better, after one perceives that
this even distribution has taken place, not to push the heat so as to
make large bubbles again, but to slowly evaporate until dry, after which
the full power of the flame must be applied until the iron plate and the
glass cover are red hot ; at first the mass of diatoms, etc., will become
black, but as the organic contents and debris burn away there will finally
remain only the silex nearly white. I invariably burn in this manner
on the cover; even the specimens which have been prepared with acids,
(
'
) I prefer to use a small square piece of platinum foil, with the corners folded back, and
held tightly with a pair of forceps.
H.V.H.