of
preservation, some of the material is boiled in acid for the purpose of
cleaning it. The acids usually employed are hydrochloric, nitric or
sulphuric, according as circumstances require. When the operator
considers that by this process all foreign matter has been eliminated,
the residuum is put into a precipitating jar of a conical shape, broader
at the bottom than at the top, and covered to the brim with filtered or
distilled water. When the diatoms have settled in the bottom of the jar,
the supernatant fluid is carefully removed by a syringe or some similar
instrument, so that the sediment be not disturbed. The jar is again
filled with water, and the process repeated till the acid has been
completely removed. It is desirable afterwards to boil the sediment for
a short time with supercarbonate of soda, the alkali being removed in
the same manner as the acid. A small portion may then be placed with a
pipette upon a slip of glass, and, when the moisture has been thoroughly
evaporated, the film that remains should be covered with dilute Canada
balsam, and, a thin glass cover having been gently laid over the balsam,
the preparation should be laid aside for a short time to harden, and
then is ready for observation.

_General Remarks._--Diatoms are most abundant in cold latitudes, having
a general preference for cold water. In the pelagic waters of lakes and
of the oceans they are often very abundant, and in the cold waters of
the Arctic and Antarctic Oceans they exist in prodigious numbers. They
thus form a large proportion of both the marine and the fresh-water
plankton.

Large numbers of fossil diatoms are known. Not only are these minute
plants assisting at the present time in the accumulation of oceanic and
lake deposits, but in former ages they have been sufficiently active to
give rise to considerable deposits of diatomaceous earths. When the
plant has fulfilled its natural course the siliceous covering sinks to
the bottom of the water in which it had lived, and there forms part of
the sediment. When in the process of ages, as it has often happened, the
accumulated sediment has been hardened into solid rock, the siliceous
frustules of the diatoms remain unaltered, and, if the rock be
disintegrated by natural or artificial means, may be removed from the
enveloping matrix and subjected to examination under the microscope. The
forms found may from their character help in some degree to illustrate
the conditions und