810 On Selecting and Mounting Diatoms. — [Monthly Microscopical 
and charred, and is then not affected by balsam, and thus forms the 
very cell I was searching for. Captain Haig burns these on the 
hot plate by means of a Bunsen burner, at a great expenditure of 
gas and much tedious watching, but I find the same result is 
obtained by giving the prepared slides to my cook to be placed in 
the oven when she is baking. He uses very thick balsam, but 
working, as I prefer, with that medium very much thinned with 
chloroform, it is just possible that even such cells may, in the course 
of time, be acted upon by it; and so I have since hit upon the 
plan of forming my rings of gum-dammar dissolved in benzole. 
Of course I am aware that the chloroform of the balsam will affect 
these also, but this is of little consequence, as the one material is 
as transparent as the other, and should they entirely mix, the neces- 
sary thickness of medium to prevent crushing is gained. I may 
here mention that a thin solution of gum-dammar is equally as 
good as balsam for mounting diatoms ; it must, however, be filtered 
through blotting-paper to get rid of impurities in the gum. For 
this reason Mr. Tatem, in conjunction with whom I have been 
carrying on these experiments, objected to the gum-dammar cells, 
not being aware that it could be so easily filtered, and suggested 
that they should be formed of a solution of pure gum-arabic, which 
is not acted upon by balsam. We have both tried such cells, and 
find they answer admirably. Practically, however, I find it matters 
little whether the cell is formed of gum-arabic or gum-dammar, and 
in either case the superfluous portion of the ring beyond the cover- 
ing glass is easily cleaned off, if desired, in the usual manner, when 
it would be almost impossible for anyone to tell that a cell had 
been employed. ‘The pointed end of a small stick (an expended 
lucifer match will do) is better than a brush to form these ring-like 
cells, as 1t can be thrown away and a fresh one used on each occasion. 
Some delicate diatoms are almost obliterated in balsam, and 
should be mounted dry. ‘This is not such an easy task as it may 
appear, as such slides are apt to become spoiled by the penetration 
of damp, which eventually forms in beads of moisture on the mner 
surface of the covering glass. Captain Haig for such purposes still 
uses his gold-size rings, which in this case are not completely 
charred, but are removed from the hot plate, whilst the pomt of a 
needle will still leave its impression on the ring as long as it is hot, 
but will not do so when it cools. By replacing the slide on the hot 
plate at the moment of mounting, the ring is once more softened, 
and the cover applied and pressed down to ensure adhesion. I have, 
however, found that the fumes from the heated gold-size are liable 
to form crystals, which are as unsightly, if not so injurious, as the 
beads of moisture; and so I prefer using his charred ring, which I 
smear with my gum-dammar solution, on which, whilst it is still 
slightly viscid, I press down the covering glass; and afterwards, if 
the adhesion has been made perfect round the whole periphery, a 
