44 OF THE CHARACTERISTICS OF THE DIATOMACEjE. 
quantity of the Diatomaceaa on a thin sheet of platinum, and 
holding it over the flame of a spirit lamp. As soon as the 
foreign matter is consumed, the frustules are to be removed 
from the platinum, and spread out on the stage plate for 
examination. 
It is sometimes more convenient, if the individual plants 
are very minute and diffused through water, to employ scales 
of mica in lieu of platinum. But care must be taken in the 
selection of' the mica, as not every piece that comes to hand 
is suitable for the purpose. A piece must be chosen, which 
separates easily into flakes, is clean and pellucid, and does 
not grow dim when exposed to extreme heat. Moreover, it 
must not be too thick, otherwise the scales are apt to fly apart 
during the process of incineration ; nor too thin, because, in 
that case, it becomes dull and opaque under the influence 
of the heat. Experience alone can guide the operator in 
choosing the right thickness. 
When the frustules have been laid in a drop of water on 
a scale of this description, the latter is to be held, by the 
aid of a forceps, at some little distance above the flame, so 
that the moisture may evaporate without boiling. Strict 
attention must be paid to this point ; for, should the water 
be allowed to boil, the frustules are thrown on their edges 
by the action of the air, as it is set free, and long dark 
heaps are formed on the surface of the mica, like Lilliputian 
moraines. So soon as the moisture is entirely dissipated, 
the plate may be lowered into the flame, and there held. 
At first the preparation assumes a black colour in conse- 
quence of the organic matter becoming carbonised : in the 
next stage it turns red, and finally white ; a token that all 
the foreign bodies have been got rid of, and the empty 
valves alone remain. 
The period during which the frustules should be exposed 
to this great heat is a matter of considerable moment, and 
must be carefully watched. Too short a time is insufficient 
to destroy the organic substances, and the valves come out 
dark and impervious to light. If the heating continues too 
