THE MICROTOMIST'S VADE-MECUM

wanted for use. A stove, or the sun, may be employed for
drying, but it is best to dry slowly at the normal tempera-
ture. The block may be preserved in good condition almost
indefinitely, the gum, when mixed with a sufficient quantity of
glycerin, never becoming hard or brittle. It is generally
better to wait till the blocks have assumed such a consistency
that they cannot be easily bent. It is after having waited
almost a week that the author always obtained the best sec-     I,
tions. The gum is dissolved out from the sections by means
of a drop of water on the slide. The sections are then
covered, and a drop of glycerin being added, the preparation
is complete as soon as the water has evaporated.               _AC(
An infiltration-mass. It has the advantage of being trans-
parent. Joliet employs it for Pyrosom. A similar mass was
employed by Hertwig for Ctenophora ('Jen. Zeitsch.,' xiv
(1880), pp. 313-314; 'Journ. Roy. Mic. Soc.' (N.S.), ii, COlo
p. 278).                                                       iY
It would probably be advantageous to add some preserva-
tive substance to this mass.
252. Gum-Imbedding (Stricker's method ').-A concentrated
solution of gum    arabic. The object may be prepared in
alcohol and imbedded in the gum in a paper case. The
whole is thrown into alcohol, and after two or three days may  1O
be cut.                                                        1k
(It is better to add a, little glycerin to this mass.-AUTwoR.)
253. Gelatin (Kaiser's formula 2).-One part by weight of
the finest French gelatin is left for about two hours in 6 parts
by weight of water; 7 goa:    of glycerin are added, and for
every 100 grammes of the mixture 1 gramme of concentrated
carbolic acid. The whole is warmed for ten to fifteen minutes,
stirring all the while, until the whole of the flakes produced a
by the carbolic acid have disappeared. Filter whilst warm      t
' ' Hdb. d. Gewebel.,' p. xxiv.
2 ' Bot. Centralb.,' i (1880), p. 25. 'Journ. Roy. Mic. Soc.,' iii (1880),
p. 504.

192


F