in sncli a medium, as the presence of the cell-contents would
prevent the access of the balsam to th e interior of the frustule.
To procure the valves in a separated state, it is necessary to boil
the frustules for a few minutes in muriatic or nitric acid ; thus
removing the softer parts, and dissolving the connexion between
the various portions of the frustule. The acid must afterwards
be removed by repeated washings, allowing the siliceous materials
to settle to the bottom of the vessel after each addition of
wa ter: Ave thus procure the specimens in a proper state.
Having poiu-ed off the superfluous AA'ater, a di-op of the fluid
containing the Diatomaceæ is to be placed on the slide, evaporated
to dryness, and the balsam and cover applied as in the
ordinai-y mode. To avoid the presence of air-bubbles, it is only
necessary to use th e balsam in a sufficiently fluid state ; a.nd tliis
may be secured by mixing it with ether or rectified spirits
of turpentine, taking care entirely to expel the latter by the
application of a gentle, b u t long-continued heat, before applying
the glass cover.
With some valves, more especially in those specimens the
resolution of whose cellular structure requires the application of
the highest powers of the instrument, a different mode of mounting
is necessary, as the application of balsam obliterates the more
delicate markings. Such objects must be preserved in a dry
state, Avithoiit being immersed in any medium, and must not
only be covered with glass snfEciently thin to allow the approach
of a lens of high power, but also placed upon a glass slide,
whose thickness will not impede the use of condensing lenses
of short focal lengths. These objects are accomplished by perforating
a wooden or metallic slide, and placing upon tbe perforations,
which ought to be as large as practicable, a portion of
thin glass, to receive the object, cementing this to the slide, and
placing over it a cover of glass not exceeding xiutli of an inch
in thickness, and thinner if possible. To prevent the admission
of moisture, Avhich would ultimately make its way to the object
and destroy its value, it is indispensable that the cover should
be cemented to the thin glass below. A neglect of this precaution
will, sooner or later, be followed by the loss, to all valuable
purpose, of the contained specimens.
In noticing the usual habitats of the Diatomaceæ, I have
omitted to mention a locality not ordinarily accessible— the
deeper parts of the ocean— which can only be reached by the
di’edgo of the more enthusiastic and laborious collector. Many
interesting species will reward a search conducted with this
implement. The magnificent Campylodisms lloroloyium, PL VI.
fig. 51, was^thus obtained. But these more hidden “ treasures
of the deep” may be reached in an indirect mode, by employing
those fishes or molluscs which frequent deep water, as our
substitutes for the dredge. Thus, in the stomachs of the crab
or lobster, sole or turbot, the scaUop or whelk, such organisms
are often found; and several species, rarely or never occurring
m my usual haunts, have been supplied in abundance by a
careful dissection of the above microphagists.
I have also failed to indicate any season as more suitable
than anotiier for the researches of the student. I have, however,
designedly omitted to do so, as every season is productive
of interesting forms. The notes appended to the descriptions
of each species, in which I have, when possible, recorded the
months in which the species were collected, will show how
perennial is the supply. Weather permitting, the collector may
always reckon upon well-filled bottles, provided his rambles are
by the sea-coast, or through a district supplied with ponds
and streams. I take my leave of him for the present, by
recommending to his pursuit a study pregnant with interest
and with beauty; which fills the mind with wonder in the contemplation
of forms at once so minute and perfect, so humble in
their individual capacity, and yet by their numbers and wide
diiiusion occupying an important position in the organic world,
and fulfilling the purposes of Power, Wisdom, and Benignity. ’