AND HOW TO USE IT. 45 



salt in I oz. of distilled water." He recommends that "this 

 application of potash should be made in the daytime, as the 

 action of daylight upon it, in conjunction with the gelatine, is to 

 render the latter insoluble in water." Wash well in methylated 

 spirit, to remove all the glycerine, and then run on a ring of zinc 

 white, which may be repeated until a good firm ring is made. To 

 keep the brushes clean, I always have a little phial of turpentine 

 or benzole for zinc white, and warm water for the size, glue, or 

 gelatine. When zinc white becomes too thick to run readily, 

 dilute with benzole, but only when absolutely necessary. 



Dammar Cement — Dissolve gum dammar in benzole, and add 

 one-third gold-size. It dries quickly, and is preferably used as a 

 first coat for fixing the cover-glass when glycerine is used for 

 mounting. If the square covers are employed, they may be fixed 

 by a simple method, much in vogue in Germany. A thin wax 

 taper is to be lighted, and being partially inverted for a few 

 seconds, the wax surrounding the wick will become melted. 

 After the slide has been freed from excess of glycerine, a drop of 

 this heated wax is allowed to fall upon each corner of the cover, 

 and a line of the melted wax run along the margins of the cover 

 between these points so as to perfectly surround it. If a good 

 coat of white zinc cement be subsequently laid over the wax, a 

 very durable and not unornamental line of union will have been 

 formed. 



Coloured Cement. — Carefully evaporate shellac- varnish to con- 

 sistence of thin mucilage, and colour with a filtered, concen- 

 trated solution of aniline blue or gamboge in absolute alcohol. 

 Finally, about a scruple of castor-oil is to be added to each ounce 

 of the mixture, and, after some further evaporation, it is to be 

 preserved in a w«ll-closed vessel. If necessary, it may after a 

 time be diluted by a few drops of absolute alcohol. 



Gum-Water, thickened with gilder's whiting until as thick as 

 treacle ; to each quarter of an ounce add two drops of glycerine. 



Cement for Fixing Metals to Glass or Earthenware.— Mix 

 alum and plaster of Paris (finest kind) with water to a liquid 

 state or to a convenient paste. It forms an excellent cement for 

 many purposes, and resists the action of water for a considerable 

 time. 



