OF MICROSCOPIC OBJECTS. 67 
As an occasional aid, it may be remarked, that in some 
cases liquor anvmonie may be used in one of the later 
washings in place of water, as it often separates fine dirt, 
which is not otherwise easily removed. Ammonia also dis- 
solves a flocculent matter which sometimes remains; and 
this method does not injure diatoms like some strong alkalies. 
Some fossil deposits require to be treated with a boiling 
solution of carbonate of soda to disintegrate them; but 
this operation requires great care, lest the alkali should 
destroy the diatoms. Vegetable silicates also sometimes 
require to be removed by a solution of carbonate of soda; 
but as the frustules of the diatoms themselves are but 
_ vegetable silica, even more care is required in this case. It 
may be well to mention, that some diatoms are so imperfectly 
siliceous that they will not bear boiling in acid at all. Some 
of these may be allowed to stand in cold nitric acid some 
time, whilst others of a smaller and more delicate character 
should, when possible, be treated with distilled water 
alone. 
We will now consider the mode of mounting the prepared. 
’ diatoms, which, if used dry (as described in this chapter), 
should be carefully washed two or three times with the 
purest distilled water. In this branch, as in every other, 
each collector gives preference to that method in which he 
is an adept. Thus the diatoms may be placed on the 
under side of the cover, to be as near to the object-glass 
as possible, or upon the slide itself; and each plan has 
its advocates. Whichsoever of these is used, nothing seems 
more simple to the novice than a tolerably equal dispersion 
of the objects upon the slide or cover; but this is by no 
means so readily accomplished; consequently I give Mr. 
Rylands’ method, as his slides are perfect in this respect 
also. _ He always places the diatoms upon the thin glass 
cover. It is not sufficient, as is frequently thought, to 
take a drop of liquid containing the cleansed material and 
spread it upon the cover or slide, as without some additional 
precaution that uniform and regular distribution of the 
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