294 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
very defective in preparation and formula. In a former article I recom- 
mended Winsor and Newton’s picture varnish for finishing mounts. It 
makes a neat finish, but is not a durable coating. All the cements pre- 
pared from dammar, mastic, shellac, gum-arabic, and all the other gums 
or resins, to my knowledge (except copal, amber, and a resin or gum 
nearly as colourless and hard as glass, which resisted fifteen solvents, 
and for which I have not yet found a solvent or a name), all are too soft 
and brittle, and therefore unfit for cements or varnish. 
White zinc cement, according to my collection of mounts and all 
that I have purchased, and made myself, is the most defective of all 
cements ; it is not necessary to enter into an argument, or a controversy 
to prove this question with any pet theories. As a practical test, ‘ the 
eating is the proof of the pudding.’ 
\\ hen I examine the slides received from Europe and from every 
State in the Union, I find that the rings of white zinc, shellac, dammar, 
Brunswick black, marine glue, &c., have prismatic colours between them 
and the slide, an evidence that the cement has cracked loose from the 
glass. Some of my own preparations have also cracked and curled up 
in course of time. My slides are in a cool room seldom heated to 90° F. 
in the summer, and never below 4(U F., in the winter, and not exposed 
to sudden changes of temperature. 
I have resolved to put all cements for my own use to the following 
practical test : spin a ring on a clean slide and let it harden thoroughly, 
then push a sharp-pointed scratch-awl or a sharp brad-awl through the 
ring, cutting a groove just wide enough for the tool to pass. 
This repeated a dozen times on one ring should leave sections not 
less than 1/16 in. long intact, between each incision, and so hard that 
no impression can be made on the ring with the edge of a stout thumb- 
nail. This I consider a reliable test. But if large sections of the ring 
fly away, leaving no trace of the cement on the glass when tested in this 
manner, as do all the white zinc cements that I have bought of the 
opticians, it is, without any exception, a nuisance. 
It is impossible to prepare a good and reliable cement or varnish 
out of poor or improper materials ; nor is it always possible to prepare 
a good article out of the best material, if improperly proportioned, or 
if prepared in a hasty and careless manner. 
When the best glue or gelatin is soaked in cold water all night, and 
then boiled in a water-bath till thin, and thoroughly dissolved by frequent 
stirring and by adding to it prepared chalk, chloride of sodium, glycerin 
(C. P.), and acetic acid in the proper proportions, we have a reliable 
cement for mounts and labels that will never crack nor scale off. 
I have found the following original formula good ; but it may be 
improved, I think : — 
(1) Reduce 6 drm. of dry gelatin to a thin solution in distilled 
water — soaked overnight cold, and boiled in a water-bath. 
(2) Reduce 1 drm. of prepared chalk to a thin solution in distilled 
water, and add to it one fluid drm. of a strong solution of chloride 
of sodium (half that quantity of strong alum solution, or a little 
chloride of calcium may be better), and stir it well ; then pour it 
quickly into the gelatin solution ; and stir the whole thoroughly, boiling 
it until as thick as can be poured into a bottle with a large neck. 
