70 THE AMERICAN MONTHLY [April, 
to the estimated amount of organic matter in the material, exclusive of the 
sand; stir until mixed. A slip of glass one-half inch wide and of convenient 
length, with rounded corners and smoothed edges, is more effective to. stir 
with than a glass rod. Now take strong commercial sulphuric acid ina bot: 
tle convenient for pouring, and drop in at first slowly and afterward more 
rapidly, with constant stirring, until brisk effervescence sets in with copious 
disengagement of carbon dioxide. Continue the operation until the further 
act neon of sulphuric acid produces no effect. Then, after waiting a few 
minutes for the completion of the reaction, pour the whole material into a stiita- 
ble vessel containing the requisite quantity of cold-water. Rinse the opera- 
ting vessel and add, and stir the whole briskly for some minutes; then leave 
at rest to settle thoroughly. The diatoms should now be nearly clean, requir- 
ing only the usual alkaline treatment and thorough washing. After the addi- 
tion of the bichromate see that the temperature of the material, and of the 
acid to be used also, is not lower than 70 degrees Fahr. If the acid is not 
added fast enough, or too much water is present, the action may not be ener- 
getic enough to dissolve refractory material, and if too fast, or too much at a 
time, the diatoms may be injured. If there is very little organic matter 
present and too much bichromate is used, the addition of sulphuric acid may 
cause a deep red precipitate, in which case very little action will take place. 
If mistakes have been made and the diatoms are not sufficiently cleaned, the 
operation may be repeated with less bichromate, or nitric acid can now be 
used without ‘fuming much or much danger of injury. If the material has 
been dried, it will be better to soak or boil it in water for some time before 
using acid. Marine muds should be washed in fresh water to remove the 
salt, otherwise acid fumes would be generated. They are apt to contain more 
refractory material than fresh-water gatherings, and the action should be pro- 
portionately energetic. Fossil marine earths would require a considerable 
modification of above process, as the organic matter they contain is so changed 
and compacted with silicates as not to be readily acted upon by acids. They 
should be thoroughly softened by long soaking and boiling before being acted 
on by acids, as otherwise the gases disengaged would tear and fracture very 
many of the forms. I would Advise against boiling in alkalies in all cases ~ 
where it can be avoided. Recent fresh-water gatherings do not generally 
require it; vigorous shaking or stirring in a weak solution of sodium car- 
bonate is generally sufficient with thorough washing. If any one wishes to 
see the action of alkalies on diatoms let him place a drop of potash solution 
on a slip, and on it a cover prepared with Prxxularia, Stauroneis, and Cym- 
bella and watch it a short time under the microscope. The shells will soon 
be softened and distorted and very soon dissolved more or less entirely, and 
this in a cold and comparatively weak solution, or even in potash soap. It 
is common to find preparations containing, say, Cosc¢zodiscus, in which one 
layer of the shell is more or less completely removed, causing a very difter-. 
ent appearance from its normal condition. I have been surprised to -find 
that, among fresh-water varieties, some of the heavier and more glossy shells 
were the first attacked; the larger Pinnularias, Stauroneis, Acuta, and a 
variety of Wavicula elliptica being destroyed by the chemicals, while many 
smaller and more delicate varieties were apparently uninjured. The marine 
varieties generally will bear rougher treatment than the fresh water. Inex- 
perienced operators should be reminded that the first washing from both acid 
and alkaline treatment settles very slowly, and should be given plenty of 
time; if poured off too quickly the lighter and more delicate varieties would 
be lost. I have used the above-described process exclusively for a consider- 
able time, and generally succeed in getting the diatoms beautifully white and 
clean at the first operation ; if not, it is easily repeated or supplemented, and 
A 
ay 
