VARNISHES AND CTEMENTS. 219 



tenacity, being very apt to lose their hold upon the glass after a 

 time ; and the Author, having suffered much injury to his pre- 

 parations from trusting to them, would recommend that, even in 

 mounting objects dry, some other cement should be first used, 

 by which the glass cover should be attached to the slide, the 

 sealing-wax varnish being only laid on as a finish. If a black 

 varnish be desired for such a purpose, it may be readily made by 

 mixing gold-size with a small quantity of lamp-black ; this dries 

 quickly, and is free from brittleness ; but, for the reason already 

 mentioned, it should not be used in the first instance to mount 

 objects in fluid, although it may belaid as a finish over gold-size 

 or asphalte. For making cement-cells ( 134), either asphalte, 

 gold-size, or liquid glue may be employed, the first being on the 

 whole preferable ; the varnish termed Black Japan also makes 

 very good and durable cells, if the glasses to which it has been 

 applied be exposed to the heat of an oven, not raised so high as 

 to cause them to " blister." 



121. Although Canada balsam has been sometimes used as 

 a cement, and has the advantage of being worked with extreme 

 convenience, yet it is so apt to crack when hardened by time, 

 that a slight "jar" will cause the cell to spring away from the 

 glass to which it has been attached. Hence, if employed at all 

 for fixing cells to glass slides, its use should be limited to those 

 plate-cells which afford a large surface of attachment ( 136), or 

 to those very thin tube-cells ( 135), which cannot be so con- 

 veniently attached with marine glue, and of which the cover 

 may be secured to the slide by spreading the ring of gold-size 

 round the margin of the cell itself ( 138). Care should be taken, 

 in applying the Canada balsam, that it be sufficiently hardened 

 by heat, but that it be not so heated as to become brittle ( 109) : 

 the general method of using it for this purpose, is the same as 

 that which must be practised in the case of marine glue. The 

 superfluous balsam left after pressing down the cell, is to be re- 

 moved, first by scraping with a heated knife, and then with a rag 

 dipped in oil of turpentine, after which it is desirable to give the 

 glass surface a final cleansing with alcohol. For all kinds of cells 

 ( 135, 137) except those just mentioned, the proper cement is 

 Marine Grlue, which is a mixture of shell-lac, caoutchouc, and 

 naphtha, now extensively employed ; being distinguished by its 

 extraordinary tenacity, and by its power of resisting solvents of 

 almost every kind. Different qualities of this substance are 

 made for the several purposes to which it is applied ; that which 

 is most suitable to the wants of the Microscopist, is known in 

 commerce as GK 4. As this cement can only be applied hot, and 

 as it is a great saving of trouble to attach a considerable number 

 of cells at the same time, a Mounting -Plate should be provided, 

 which will furnish the requisite heat to several slides at once. 

 Such a surface may be afforded by the top of a stove ; but it is 

 better to have one which can be used at all seasons, and the heat 



