VARNISHES AND CEMENTS 443 



purposes, and perhaps three or four for special purposes, and the re- 

 mainder will be in practice neglected. We do not hesitate to say 

 that the two cements on which the most complete trust may be re- 

 posed are jajxmner's gold size and Bell's cement. This opinion is the 

 result of over twenty years of special observation. 



A good varnish may easily, in a general way. be tested : when it is 

 thoroughly hard and old. if scraped off it comes away in shreds ; un- 

 safe varnishes break under the scraper in flakes and dust. To those 

 who put up valuable preparations and objects of value the risk 

 should never be run of using a new and unknown varnish or cement. 

 Neither appearance nor facility nor cheapness in use should for one 

 moment weigh against a varnish or cement of known and tested 

 worth. 



Japanner's gold size may be obtained from the colour shops. It 

 may be used for closing-in mounted objects of almost any description. 

 It takes a peculiarly firm hold of gla--. and when dry it becomes 

 extremely tough without brittleness. When new it is very liquid 

 and ' runs ' rather too freely ; so that it is often advantageous to leave 

 open for a time the bottle containing it until the varnish is some- 

 what thickened. By keeping it still longer, with occasional exposure 

 to air, it is rendered much more viscid, and though such ' old' gold- 

 size is not fit for ordinary use, yet one or two coats of it may be ad- 

 vantageously laid over the films of newer varnish, for securing the 

 thicker covers of large cells. Whenever any other varnish or cement 

 is used, either in making a cell or in closing it in, the rings of these 

 should be covered with one or two layers of gold-size extending 

 beyond it on either side, so as to form a continuous film extending 

 from the marginal ring of the cover to the adjacent portion of the 

 glass slide. 



Asphalte Varnish. This is a black varnish made by dissolving 

 half a drachm of caoutchouc in mineral naphtha, and then adding 4 oz. 

 of asphaltum, using heat if necessary for its solution. It is very 

 important that the asphaltum should be genuine, and the other 

 materials of the best quality. Some use asphalte as a substitute for 

 gold size ; but the Author's experience leads him to recommend that 

 it should only be employed either for making shallow ' cement cells 

 or for finishing off preparations already secured with gold-size. For 

 the former purpose it may advantageously be slightly thickened by 

 evaporation. 



Bell's cement is sold by J. Bell and Co., chemists, Oxford Street, 

 London; they are the sole makers, and retain the secret of its com- 

 position. It is of great service for glycerin mounts ; but the edge 

 of the cover should be ringed with glycerin jelly before this cement 

 is applied. It is an extremely useful and reliable varnish, which is 

 extremely easy of manipulation. It can be readily dissolved in 

 either ether or chloroform. 



Canada balsam is the oleo-resin from Abies balsamea and Pinus 

 canadensis; it is so brittle when hardened by time that it cannot 

 be safely used as a cement, except for the special purpose of attaching 

 hard specimens to glass, in order that they may be reduced by 

 grinding, &c. Although fresh, soft balsam may be hardened by heating 



