42 JOURNAL OF THE [April, 



should be then moistened with a mere film of liquid marine glue; 

 the object and preservative then introduced ; the cover applied 

 and pressed down into contact with the sticky film of glue; the 

 excess of preservative which has exuded cleaned away with rolled 

 bits of absorbent paper; and a thin seal of paraffin then spun around 

 the joint between the cover and the cell. Both to strengthen the 

 seal, to protect the paraffin with a hard coat, and for appearance, 

 a thin coat of some finish is then spun over the whole surface of 

 the paraffin. This may be colored according to taste, such as 

 red sealing-wax varnish or black asphalt ; but in our labora- 

 tory a colorless finish is preferred, imparting a porcelain glaze 

 to the cell, such as gold size, liquid marine glue. King's colorless 

 ■cement, rubber cement, etc. With a paraffin cell one may thus 

 finish the entire mount at once, without the necessity of waiting 

 .at any point for cell or seal to dry. 



A limitation of the use of paraffin cells lies in the necessary 

 avoidance of oils as preservatives, as in the case of mounting 

 ■crystals in kerosene or castor oil ; for this a cell of shellac varnish 

 or King's cement is best suited. Nor can liquid Canada balsam 

 or gum dammar be used with safety in a paraffin cell, on account 

 -of the attack of the turpentine, as a ready solvent of the paraffin 

 wall. 



Colored varieties of balsam-paraffin are also of use, especially 

 black, white, and blue, made, respectively, by intermixture with 

 lampblack, with zinc oxide, and with Prussian blue, each thor- 

 oughly dried and carefully sifted through a fine lawn sieve. Heavy 

 powders, like white lead and vermillion, cannot be well used, on 

 account of rapid settling to the bottom of the melted paraffin. 

 Even with the three colors above mentioned, the mixture in the 

 capsule must be rapidly stirred just before the brush is loaded. 



Only cells of some thickness can thus be made from colored 

 paraffin ; but, when the mount has been finally varnished with 

 one of the finishes already stated, the black paraffin assumes a 

 jet-like glaze, and the zinc paraffin a white enamel of great 

 beauty. The latter seems preferable to zinc cement, on account 

 of its uniformity, constant insolubility and impermeability toward 

 most preservatives. 



Of course, in the use of paraffin mounts with a projection mi- 

 croscope, the insertion of the alum-cell is desirable, to prevent ele- 



