18 
OF THE FREE DIATOM ACE^E. 
get rid of these, the objects to be cleaned should be placed 
in an evaporating dish of porcelain, or in test tubes (such 
as are used in chemical laboratories), into which some 
strong mineral acid — muriatic or nitric — has been pre- 
viously poured, and the whole suspended over a spirit- 
himp. By this means the fragments of Oscillatorise and 
other organic matter will be dissolved, leaving a residuum 
of the silicious coats of the . Diatomaceee (which are 
practically indestructible), and also whatever flinty par- 
ticles of sand may have been introduced : these latter 
can be got rid of by washing, in a way to be presently 
described. 
During the boiling which ensues, means must be taken 
to let the vapour escape that rises from the heated acid, 
by placing the apparatus, for instance, on the hob of a fire- 
grate, so that the steam may be carried up the chimney. 
If this precaution is neglected, serious damage is likely to 
ensue to articles in the room, the lenses of a microscope 
will be spoilt, and any metallic substance is sure to be 
tarnished, not to mention the injury that may be done 
to the observer himself, from imbibing the poisonous 
vapour. 
How long the boiling should continue can be determined 
only by practice and experience ; a few minutes are gene- 
rally sufficient. Sometimes, however, a second application 
of acid is needed, before the whole of the organisms are 
dissolved. The best test, perhaps, of this having been 
accomplished is the clear bright appearance of the acid 
that remains in the tube or saucer, and then the Diatoma- 
ceous valves may be considered as thoroughly cleansed. 
To clear them of the acid, empty the mass into a tolerably 
large glass of pure water, and allow the Diatomaceee to 
sink to the bottom. Immediately pour off the water, 
taking the utmost care not to disturb the sediment, and 
introduce a fresh supply, which must, in its turn, be 
poured off, and the glass refilled. This should continue 
until a strip of litmus paper ceases to be tinged with a 
