70 PREPARATION AND MOUNTING 
now describe it. A circle of bitumen about one-third ~ 
smaller than the covering glass is drawn beforehand on my 
slides. When I wish to make a preparation, instead of 
coating, as formerly, the first circle with a second layer of 
bitumen, I form a second circle of it outside the first, and 
as near as possible to it, and each of the two circles has its 
own advantage; the first, in fact, while ferming the cell, 
serves aS a support for the covering glass, and thus pre- 
serves the Diatomacez from any breakage; it offers, besides, 
a serious obstacle to the spreading of the more liquid bitu- 
men of which the outer circle is composed; and the latter 
closes the cell by fixing the cover, which, when the prepara- 
tion is dry, may be covered with a final circle of bitumen 
At 1s of course understood that I am speaking of prepara- 
tions made in the dry way only, and not with balsam.” 
One of the most fertile as well as the most curious maga- 
zines of Diatomacez is guano. The siliceous forms con- 
tained therein have been devoured by sea-birds and passed 
through the stomach uninjured, and after lying for ages 
may be cleaned and classified. Many of these are not else- 
where met with, so that the student who is desirous to enter 
into the study of Diatomacez must be instructed as to the 
best mode of obtaining them from this source. The par- 
ticulars to be observed so closely resemble those before men- 
tioned in the treatment of the ordinary diatoms, that it will 
be sufficiently explicit to give the outlines of the process. 
The guano must be first washed in pure water, allowed to 
subside perfectly, and the liquid then poured off. This 
must be repeated until the top fluid is clear, and care taken 
not to decant the liquid until perfect subsidence has taken 
place. The deposit must then be treated with hydrochloric 
acid with a gentle heat for an hour or two, adding a little 
fresh acid at intervals as long as it excites any effervescence 
After this nitric acid must be substituted for the hydro- 
chloric, and the heat kept up to almost boiling-point for 
another hour at least, adding a little fresh acid as before. 
When this ceases to act, the deposit must be allowed to 
