OF MICROSCOPIC OBJECTS. 61 
diatoms upon it. This may be held over the flame of the 
spirit-lamp until all the surrounding matter is burnt away, 
and a tolerable idea may thus be obtained as to the quality . 
of our treasure. 
In some cases it is well to use this burning operation 
alone in mounting specimens of diatoms, when they may be 
placed in their natural state upon thin glass, burnt for 
awhile upon the platinum plate, hereinafter described, and 
mounted dry or in balsam. 
In the preparation and cleaning of Diatomaces, there is 
little satisfaction unless these operations have been success- 
fully performed, as a very small portion of foreign matter 
seriously interferes with the object. The mode of preparing 
them varies even amongst the most experienced. It will be 
found, therefore, most satisfactory to examine the principal 
of these separately, although it may be at the risk of some 
little repetition. 
The method which is most frequently employed is the 
following :—Place the gathering containing the Diato- 
maceze in a small glass or porcelain vessel, add strong 
nitric acid, and, by the aid of a Bunsen’s burner or spirit- 
lamp, boil for some minutes. From time to time a drop of 
the mixture may be put upon a slide, and examined under 
the microscope to see if all foreign matter be got rid of. 
When the valves are clean, the vessel containing them must 
be filled with water, and the whole left for an hour or two, 
so that all the diatoms may settle perfectly. The liquid 
must then be poured off carefully, or drained away by the 
aid of a syphon, so that none of the diatoms are removed 
with it. Indeed, it is well to examine the liquid drained off 
each time with the microscope, as the finer forms are 
frequently lost in the washings. The vessel must then be 
refilled with pure water, allowed to settle, and drained as 
before. This washing must be repeated until a drop being 
placed upon a slide and evaporated leaves no crystals. 
When it is desirable to preserve the diatoms in this state 
before mounting (which process will be described in another 
