198 PRACTICAL MICROSCOPY. 



most solvents. A thin solution say xylol two parts to Canada balsam 

 one part should be first prepared, and, after filtering through paper, 

 it may be placed in an unstoppered bottle until, by evaporation, it be- 

 comes sufficiently thick for use. 



Xylol is miscible with strong alcohol, oil of cloves, etc., but not 

 with water. 



VARNISHES AND CEMENTS FOR RINGING MOUNTS. 



1. Dammar. I believe a clean dammar mount, with circular cover, 

 neatly labeled, cannot be improved in appearance by painting rings of 

 colored varnish around the specimen. Nevertheless, the beginner 

 will purchase and use a turn-table, and must be therefore directed in 

 its employment. 



A ring of dammar, thinned with turpentine so as to flow readily 

 from the brush, makes a very neat border to the cover-glass of a 

 specimen mounted in this medium. Let the layer be quite thin. 



2. Zinc Cement. To a small quantity of thin dammar varnish, add 

 q. s. of pure dry zinc white. Mix thoroughly on a glass plate with 

 paint-knife or spatula. The consistency should be such as to flow 

 readily from the brush. 



Before adding any cement containing pigment or color to a dammar 

 mount, a protecting covering should be applied; otherwise the cement 

 will eventually creep in between the cover and the slide and mix with 

 the original mounting varnish. It may proceed very slowly; but in 

 time the specimen will be surely ruined. This is best avoided by 

 painting a thin ring around the edge of the cover of liquid glue. 

 Cooper's and LePage's are both excellent. Let this dry, and any 

 amount of varnish may be subsequently applied without disaster. 



Aniline Colors. Red, blue, etc., maybe employed by dissolving the 

 dry color in a little thin shellac varnish. This dries quickly and may 

 be used over the zinc. 



Oil colors. The artists' colors which are sold in tubes may be thinned 

 with dammar. 



Black varnish. This is the ' ' Black Japan " of the paint shops. It 

 may be made by dissolving gum asphaltum in turpentine. The gen- 

 uine asphaltum is very difficult to procure, as coal tar is generally sold 

 under this name. It is therefore best to purchase the 'varnish, thin- 

 ning with turpentine or pure benzol if necessary. 



Shellac Varnish. The best orange shellac dissolved in strong 

 alcohol, in quantity sufficient to make a varnish of the consistency 

 of treacle. 



