yo ee! «On Selecting and Mounting Diatoms. 309 
beautiful foreign forms abounding in the general dirt and débris, 
and, not having the slightest idea how to proceed, I consulted the 
well-known little work ‘On Preparing and Mounting Microscopic 
Objects, by Mr. Davis. This, however, did not give the informa- 
tion required. Left, then, to my own resources, I have found that 
the best implement for picking out diatoms is a finely-pointed 
badger’s hair whipped on to a light handle, so that the hair does 
not protrude more than 2ths of an inch, thus ensuring sufficient 
stiffness, whilst undue elasticity is avoided. If this is dipped into 
a weak solution of gum, and, being allowed to dry, is then breathed 
upon, the most delicate form from a dried gathering may be taken 
up, and will remain on the hair if it is held steadily, whilst the slide 
containing the general gathering is shifted, and that on which it 
is to be placed substituted. On the latter, near the centre, which 
of course must be marked, has been placed a drop of fluid, to be 
hereafter described, and into this the hair is lowered, when the 
diatom adhering to it is immediately freed, and can then be pushed 
into position in the centre, and thus any number can be arranged 
as desired. It is preferable to mount all diatoms on the cover and 
not on the slip; the method of doing this is described farther on. 
In either case a few minutes’ exposure on the hot plate evaporates 
any moisture, when the collection is ready for the mounting process ; 
and here the beginner encounters two difficulties,—the wave of 
balsam is apt to float away the diatoms, or should this luckily not 
be the case, they are often crushed by the contraction of the medium 
as it dries. Observing that Mr. Davis states that, to avoid the 
first cause of failure in mounting general gatherings, Professor 
Ryland adds a drop or two of gum to the last washing of his col- 
lection, I availed myself of the hint, modifying the plan to suit my 
own purpose. Of course, for selection, no gum must be mixed 
with the last washing, as it is imperative that the diatoms should 
be perfectly loose and free for lifting ; but one or two drops of gum 
in a quarter of an ounce of distilled water makes a capital fluid, into 
which the selected forms may be dropped previously to their being 
pushed into place, and this is amply sufficient to fix them firmly.* 
A shallow cell formed of paper, card, or tea-lead will obviate the 
second cause of failure, but this necessitates the trouble and time of 
papering the slide, whilst Moller’s thin-glass cells are difficult to 
procure, and costly. Here I was for a long time at fault, but hap- 
pening to make the acquaintance of Captain Haig, who is an adept 
at mounting diatoms, he kindly showed me that a ring of gold-size, 
if subjected to great and prolonged heat, becomes perfectly black 
* A grain and a half of dry gum-arabic to an ounce of distilled water forms 
the best solution for this purpose, though for the Campylodisci, Aulacodisci, and 
some species of Coscinoidisci, &ce., which rest on a small portion only of their 
surfaces, a little more gum is requisite to retain them in their places. 
i, Oe 
fond 
