THE MICROSCOPE. OL 
must be thin, as if thick it does not adhere as well. It will in course 
of time become thick from the evaporation of a portion of the alco- 
hol, when it should be prepared afresh. The rule that cements and 
varnishes are deteriorated by being thinned after they have become 
thick by evaporation or oxydation has few, if any, exceptions. 
If desired, the bleached lac may be used when the light orange 
tint of the other is an objection. This makes a colorless solution, 
but its adhesive powers are much less than those of the orange lac. 
It is prepared as follows: From the center of a stick of bleached 
lac select a few small pieces and dissolve them in absolute alcohol, 
by the aid of a gentle heat. The central portion is chosen as it has 
been less exposed to atmospheric influences, and the fresher the 
specimen the stronger will be the resulting solution. If the bleached 
lac can be procured fresh enough, it makes a very fair cement, but if 
old it may be insoluble. 
Bell’s cement and liquid marine glue are shellac cements, and 
seem to be no better than the simple alcoholic solution. 
Shellac works well on glycerin mounts, as a varnish for the 
inside of wax cells, to attach the cover-glass, and especially is it val- 
uable as the first coat, when it is desired to “ring” a balsam mount. 
Being of the same color as the balsam, if any should run in, it mixes 
readily with itand is not noticeable. As it dries so rapidly when 
thin this seldom occurs. I always finish balsam mounts with a ring 
of varnish, as they look neater and last much longer. By the use of 
shellac no time need be lost in allowing the balsam to harden, but 
the mount can at once be put on the turn-table and a ring of shellac 
run around the edge of the cover. In ten minutes this will be hard 
enough to receive a coating of white zinc, Brunswick black, or some 
other quick-drying varnish, and in twenty-four hours the mount can 
be safely entrusted to the mails. 
Shellac is too brittle to be used alone as a cement, and must 
always be backed up with some more tenacious varnish. 
White zinc is my favorite in glycerin mounts, but it is far from 
being perfect. Glycerin is almost a universal solvent, and after a 
time the white zine will crumble. This can be greatly retarded by 
giving the mount a fresh coating every two years. Unless well made 
this cement is worthless. It is the oxide of zine suspended in a 
solution of damar in benzol. The bottle must frequently be forci- 
bly shaken to keep the mixture uniform. It should be used as thick 
as possible, or on drying there will not be enough of the damar to 
prevent crumbling.: As the benzol is very volatile the solution 
must be used rapidly. The secret of its use is that it must be used 
