316 MANIPULATION. 



wood at each end a strip of metal a very little thicker than 

 the two thin glasses ; the saw cuts are present, but the screws 

 are applied between the strips of metal and the thin glass, and 

 not near the ends, as seen in fig. 212. The strips of metal 

 keep the ends of the wood open, and the screws pinch the 

 middle more firmly down on the thia glasses, which are there- 

 fore more securely fixed than by the former method. The 

 wood employed for the purpose of making these slides should 

 not be either cedar, wainscot, or any other of the kinds that 

 are continually giving oif volatile matter, but should be some 

 close-grained wood that has no smeU whatever ; a piece of 

 zinc is perhaps the best thing that can be used. In the 

 preceding description it was stated that the thin glasses were 

 kept together by a little piece of wax at each corner; if 

 necessary, however, Canada balsam may be employed to 

 mount many of the specimens, such as several of the species 

 of naviculae that are used as tests, the balsam having the great 

 advantage of rendering the risk of fracture much less frequent. 

 When thicker objects, such as sections of bone, teeth, or 

 wood, require to be mounted dry, some thin form of cell 

 should be employed ; this may be made out of writing-paper 

 or cardboard, by selecting a piece of the same size as the 

 cover about to be used, and cutting out a hole in it of the 

 shape required, and cementing this to a slide by sealing- 

 wax varnish ; when the spirit has evaporated and the cell is 

 firmly fixed to the glass, a coating of the same cement may 

 be employed to cover the entire upper surface of the cell, and 

 to thoroughly saturate the paper. When this coating is dry, 

 the cell is fit for use; the object being laid in it, the thin 

 glass cover may be put on by first touching the edges of the 

 cell with some very thick sealing-wax cement, and then 

 dropping the cover on it ; the cover will be held in its place 

 by the varnish, and the slide should be put away untH the 

 varnish is dry, when another small quantity of the same ma- 

 terial is to be applied round the edges of the cover, but not 

 enough to run far under it ; as soon as the last coat is dry, 

 another may be laid on until the cover is firmly fixed. 

 Cells may also be made of the electrical cement, described at 



