Of all resins, amber and some kinds of copal are the hardest. Copal varnish is both hard and elastic; amber varnish is harder than copal, but not so elastic, and is, consequently, more brittle; hence, for a cover-glass cement, a mixture composed of both should be used. Only the best and clearest kinds of amber (the opaque pieces contain various kinds of minerals), and only the hardest kind of copal - that is, the East-India or Zanzibar copal - should be selected for cover-glass cements. Zanzibar copal is taken from the earth in flat, disc-shaped pieces, varying in dimensions from the size of a pea to the size of the human hand; is colourless, yellow, or of a dark red-brown colour, and transparent; the surface, rough. Bombay copal comes in larger pieces, is of a yellowish-red colour, has, when broken, a smooth, glassy surface, and is but very slightly inferior ill quality to the copal of Zanzibar. Sierra-Leone copal comes in small, ball-shaped pieces, about 1 in. in diameter, or in pieces resembling drops in shape. All the other kinds are softer than those just described. The best solvent for resin, and the one which possesses the most adhesive quality, is linseed-oil varnish, made of pure, old linseed oil.
Neither alcohol, ether, chloroform, nor any other quickly evaporating menstruum should be used. In order to hasten desiccation of the resin, and to obtain for the cement the proper consistency, an ethereal oil which, upon drying, will leave a surface perfectly even, should be added to the mixture; and oil of lavender, either alone or mixed with linseed-oil varnish, is suitable for these purposes. The resins being thus dissolved in linseed-oil varnish until the solution attains the consistency of syrup, oil of lavender should be added until the mixture becomes thin enough to use in mounting microscopical objects - and the cement is finished. The property of adhering to glass is increased in the cement by adding to it a small quantity of cinnabar; but such addition causes it to dry less rapidly. In a week from the time of using it the cement becomes dry, and so firm that the finger-nail will make but a slight impression on it. For months it remains in this condition. At the expiration of a year it is very hard, and has a glassy surface.
So much for the component parts. The preparation of this cement being somewhat difficult it would perhaps be advantageous to buy the varnishes ready made, and then proceed as follows: - taking equal parts of the best, clearest, and hardest amber-varnish and copal-varnish, mix them and heat until all the turpentine has disappeared. This will require a temperature of 100° to 150c R. (257°-370° Fahr.). As soon as all the turpentine has evaporated, remove the dish from the flame, allow it to cool somewhat, and then add oil of lavender to the liquid in proportion of 1/2 to 1; mix well, and allow the entire mass to cool thoroughly. The process is terminated by adding from 20 per cent, to 40 per cent, of artificial cinnabar (rosin with cinnabar), which should be very carefully and thoroughly rubbed in. The best method for rubbing in the cinnabar is that employed in the preparation of fine oil-paints. Should the cement when finished be too thick for use, as much oil of lavender as will give the required fluidity may be added.
The component parts and their proportions would then be as follows :-
Amber... 25 parts
Copal ... 25 „
Linseed-oil varnish50 „ Oil of lavender . 50-60 „ Artificial cinnabar . 40-60 „
Dr. Heydenreich continues his article by describing the manner in which the cement should be applied, but as his method is the same as that employed in the use of Canada balsam and other cover-glass cements, and, consequently, familiar to all microscopists, it is not necessary to make a note of it. However he advises, in order to secure a perfect mount, that a second ring be made after the first or second week from the time of mounting ; and a third, after the first or second month ; each additional ring to be slightly wider than the preceding one.
For cementing rubber or guttapercha to metal, Moritz Grossman, in his " Year Book " for 1883, gives the following receipt: - Pulverised shellac, dissolved in ten times its weight of pure ammonia. In three days the mixture will be of the required consistency. The ammonia penetrates the rubber and enables the shellac to take a firm hold, but as it all evaporates in time, the rubber is immovably fastened to the metal, and neither gas nor water will remove it.