ill!Ill

76
PREPARATION OF DIATOMS.
" This dry deposit is then put into a phial and twice its volume of
concentrated sulphuric acid poured on it. This is allowed to act for
several hours, during which it is frequently shaken. The mass will grow
black. Guanos require from five to six times their volume of sulphuric
acid. This acid is the only efficient solvent of excrementary debris, and
even then most of it can be removed by decanting three-fourths of the
sulphuric liquid after leaving
it to rest for a sufficient time. On this
thick gruel-like and blackened matter some coarsely-powdered potassium
bichromate is then added. It should be added by small successive
doses, giving it a good
shake each time. The mass becomes heated
and oxygen is often given off. It should be stopped
as soon as its
colour has changed from black to red, or when crystals of chromic
acid have been formed. In this treatment the organic matter is
carbonized by the sulphuric acid, and the nascent chromic acid effects
its combustion. The preliminary washing with hydrochloric acid is
made so as to avoid the formation of lime sulphate.
" To the above liquid is added water little by little. The mass again
becomes heated. An abundant supply of water is then added. The
colour of the resulting deposit is now comparatively white. It is then
carefully washed by decanting. The last decantings are made with dis-
tilled water. It is then ready for use. For this purpose it is diluted
with distilled water, and the mixture dropped on to large cover-glasses,
on which it is dried. From these cover-glasses the selection of species
is made."
Preparations with Styrax and Liquidambar. Towards the middle of
the year 1883 I disclosed a new method of preparation, which I had
myself practised
for some considerable time.
I have given up Canada Balsam and I use Styrax instead, which,
while being easier to manage, has a considerably greater
index of
refraction and shows the details of diatoms much more clearly. Since
I introduced this medium to the public I have had the satisfaction of
seeing it adopted by the most competent diatomists. I will therefore
shortly explain the method of preparing and using Styrax. Styrax
is a
natural balsam which exudes from the Styrax Orientalis Miller, a native
of Asia Minor.
It must be bought in the raw state as sold commercially, when it
appears in the form of a soft, greyish mass. A thin layer
of it should
be spread on a plate and exposed to the air and light until it becomes
sufficiently hard and has lost all the water which it contained.
This is then dissolved in a mixture of equal parts of alcohol and