1138 General Notes. [November, 
of water, and the chitinous parts to be studied immersed (either 
fresh or hardened) for twenty-four hours or more, the chitin is 
rendered permeable to staining fluids. Nematodes and their eggs 
may be successfully treated in the same manner. It is remark- 
able that the underlying soft parts do not suffer, the finest struc- 
tural conditions being preserved. The potassium compound acts 
with more energy than the sodium compound. 
WHITE Zinc Cement.—This cement is recommended, by Dr. 
Frank L. James, as superior to those in general use for inclosing 
preparations mounted in glycerine. The following are his direc- 
tions for preparing it: 
Dissolve gum damar in pure benzol sufficient to make a solu- 
tion of the consistency of a thin syrup, and filter through absorb- 
ent cotton. Into a porcelain capsule put a small quantity of 
chemically pure oxide of zinc, free from moisture (a precaution 
which is best assured by heating the zinc in a muffle for a short 
time prior to using it), and, having previously wet it with 
a small quantity of benzol, add sufficient of the damar solution to 
make a paste the consistency of cream orathick paint. Rub 
with the muller or pestle until perfectly smooth, and then pour 
into a stock bottle. Repeat the operation until a sufficient 
amount of the cement is obtained. The fluid should now be 
filtered through absorbent cotton to remove all of the grosser 
particles of the zinc which escaped the action of the muller. It 
may now be allowed to stand until the zinc subsides to the bot- 
tom. If the fluid has been used in proper proportion the zinc 
will occupy about half of the entire mass. In other words, the 
fluid and zinc should be in about equal proportions. If there be 
too much fluid, a portion may be decanted, while if there be too 
little the requisite amount may be added from the damar solu- 
tion. The operation is finisued by adding sufficient drying oil 
(boiled linseed or nut oil) to give the cement a proper toughness. 
The cause of the so-called “ creeping ” of hea leakage of 
cells, etc., is explained by the same author, as follow 
All of the cements which I have enumerated and Jobi in 
the preceding chapters, with the exception of gold size, consist of 
some solid material or materials dissolved or held in on o wc 
c 
SENA principall y upon AE In the process of harden- 
ing or Te the bulk or mass of the cement is very materially 
altere crease in volume occurring which is proportionate to 
the eaten of volatile matter lost in drying. The cement shrinks. 
Now, when a cell is properly finished it must be entirely filled 
mounting medium. If it is not so filled we are bound 
air cere the béte noir of microscopists, which are not 
