ill!Ill

74
PREPARATION OF DIATOMS.
The liquid having cooled, the diatoms are washed and then boiled
in a solution of sodium carbonate of a strength depending upon the
circumstances of the case. This operation, which is carried out in a
porcelain dish, is carried on for ten minutes.
After again cooling, the liquid is decanted and water, containing a
certain quantity of nitric or hydrochloric acid, is added. After again
decanting and washing, the deposit is poured into a long narrow flask,
so as to fill only a third of it and the whole is thoroughly shaken for one
or two minutes. The shaking separates the material ; water is then
added,
after which it is allowed to rest for a tew minutes. The clear
portion is then decanted by means of a pipette and the deposit is again
shaken and the subsequent operations repeated, and so on as long as they
continue to give a good result, i.e., enable fresh diatoms to be separated.
The mass which is ultimately obtained from these successive operations
is separated by decantation, if necessary, from any sand which may have
been left behind, and placed in a large flat-bottomed measure glass which
is filled up to the top and decanted into a measure No. 2 at the end
of a minute, this is again decanted into a measure No. 3 at the end of
three minutes, then again after five and ten minutes. The operation is
performed six times in succession, and timed by a watch, then the
diatoms will be sufficiently well separated according to their size.
The diatoms are then placed in a small test tube, and as much water
as possible is drained off, which should be replaced by pure alcohol.
This alcohol is then drained off again and twice replaced by isobutylic alcohol.
The diatoms, intended to be used for type-slides, are preserved permanently
in this isobutylic alcohol. When preparations are made the diatomiferous
liquid is placed, by means of a pipette, upon cover-glasses, which may then
be used either for immediate mounting or for the purpose of selection.
The isobutylic alcohol cannot be mixed with water ;
it has the advan-
tage of drying very slowly taking at least several hours and of avoiding
the accumulation of the diatoms during evaporation : the valves remain
absolutely in the same position in which they are placed at the moment
when the drop falls, but they do not stick to the cover-glass as is the
case with ordinary alcohol.
Professor Brntis method of preparation.
Professor Brun, the enthusiastic
diatomist of Geneva, described in 1887 a method of preparing diatoms,
which, according to experiments I have made, gives excellent results. He
has thus described his operations :
" A large number of recent microscopical writers," says Mr. Brun
" have devoted their attention to pelagic gatherings, to those from lacustrine