26 JOURNAL OF THE [February, 



varnishes are made of resins, copal, or amber. 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 fiat, disk-shaped pieces, varying in dimensions 

 from the size of a pea to .the size of the human hand ; is color- 

 less, yellow, or of a dark red-brown color, and transparent ; the 

 surface, rough. Bombay copal comes in larger pieces, is of a 

 yellowish-red color, has, when broken, a smooth, glassy surface, 

 and is but very slightly inferior in quality to the copal of Zan- 

 zibar. Sierre-Leone copal comes in small, ball-shaped pieces, 

 about one inch 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 etherial 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 advanta- 



