246 CEMENTS, VARNISHES, AND CELLS. 



If desired, the bleached lac may be used when the light 

 orange tint of the other is an objection. This makes a colourless 

 solution, but its adhesive powers are much less than those of the 

 orange lac. It is prepared as follows : — From the centre of a 

 stick of bleached lac select a few small pieces and dissolve them 

 in absolute alcohol, by the aid of a gentle heat. The central 

 portion is chosen as it has been less exposed to atmospheric 

 influences, and the fresher the specimen the stronger will be the 

 resulting solution. If the bleached lac can be procured fresh 

 enough, it makes a very fair cement, but if old it may be insoluble. 



Bell's cement and liquid marine glue are shellac cements, and 

 seem to be no better than the simple alcoholic solution. 



Shellac works well on glycerine mounts, as a varnish for the 

 inside of wax cells, to attach the cover-glass, and especially is it 

 valuable as the first coat, when it is desired to " ring " a balsam 

 mount. Being of the same colour as the balsam, if any should 

 run in, it mixes readily with it and is not noticeable. As it dries 

 so rapidly when thin this seldom occurs. I always finish balsam 

 mounts with a ring of varnish, as they look neater and last much 

 longer. By the use of shellac no time need be lost in allowing 

 the balsam to harden, but the mount can at once be put on the 

 turn-table and a ring of shellac run around the edge of the cover. 

 In ten minutes this will be hard enough to receive a coating of 

 white zinc, Brunswick black, or some other quick-drying varnish, 

 and in twenty-four hours the mount can be safely entrusted to the 

 mails. 



Shellac is too brittle to be used alo?ie as a cejnent, a7id must 

 ahuays be backed up with some more tenacious varnish. 



White zinc is my favourite in glycerine mounts, but it is far 

 from being perfect. Glycerine is almost a universal solvent, and 

 after a time the white zinc will crumble. This can be greatly 

 retarded by giving the mount a fresh coating every two years. 

 Unless well made this cement is worthless. It is the oxide of 

 zinc suspended in a solution of dammar in benzole. The bottle 

 must frequently be forcibly shaken to keep the mixture uniform. 

 It should be used as thick as possible, or on drying there will not 

 be enough of the damar to prevent crumbling. As the benzole is 

 very volatile the solution must be used rapidly. The secret of 



