284 PRACTICAL MICROSCOPY. 



at a gentle heat over a long period, and this can be 



easily accomplished in the hot-air chamber shown in 



Fig. 237. 

 This piece of apparatus is not absolutely necessary, 



though it is of much assistance. By means of a spirit- 

 lamp or gas-burner almost any 

 temperature can be maintained 

 for lengthened periods, a thermo- 

 meter inserted into the air space 

 serving to measure the degree 

 of heat ; it serves many useful 

 purposes, and is certainly better 

 and more uniform than the " cool 



oven " so often recommended. 

 FIG. 237. 



The varnish of which a cell is 



constructed must vary according to the nature of the 

 medium it is to hold; for objects mounted dry and not 

 very thick, a brown varnish cell is all that is required (see 

 receipt, Chapter XIV.), or it may be made of asphaltum, 

 marine glue, or gold-size, at the fancy of the operator. 



The gold-size cell requires no baking, the hardening is 

 due to an oxidation process, and wherever used it forms a 

 most reliable cell. Varnish cells should be made some 

 time before they are required, in order that no change 

 takes place after the cover-glass has been put on. 



Deeper cells are constructed of glass, ebonite, tin, 

 ivory, brass, and several other substances; the glass slip 

 forming the base of the cell should be roughened as 

 before mentioned, the cell-ring roughened on each side, 

 and then cemented to a glass slip with gold-size or brown 

 varnish. 



The 'student is advised to eschew all paper, cardboard, 

 and wax-cells : varnish is the safest to use for thin cells, 

 and glass, pure tin, or ebonite for the deeper ones, and if 



