MOUNTING WITH CANADA BALSAM.
The mounting material is Canada balsam ; as sold at the chemist's this is a thick fluid of honey - like con. sistency. It is placed in a saucer and baked in the oven until it is quite hard when cold ; it is then broken up, placed in a wide-mouth bottle, and covered with benzol or zylol, and next day it is placed in a water bath and gently heated until it is fluid. A slip of glass, 3 in. x 1 in., is taken and carefully cleaned ; it is next placed upon an iron plate, which is gently heated ; then a drop of the melted balsam is placed on the centre of the slide, and the thin glass circle is in verted over it so that the film containing the bacteria is toward the balsam. It is gently lowered by means of the needle (Fig. 719), and slight pressure applied with a clip until the balsam has set. The ex cess of balsam is removed from the edges of the cover with a little benzol and a rag, and the slide carefully cleaned. Objects mounted in balsam require no further preparation, because they are practically permanent ; but it is a very usual plan to finish off all slides with a ring of black varnish or of coloured enamel to give a better appearance. To do this the slide
is centred on a turntable (Fig. 720), which is spun round at a rapid rate, and a camel-hair brush dipped in the black varnish or enamel is brought down vertically until it just touches the edge of the cover glass ; the rotation of the slide causes the formation of a ring of the material which entirely covers the edge of the cover glass and seals it up. Fig. 721 shows a self-centring turntable, which, immediately it is rotated, brings the slide and are mounted in Canada balsam under considerable pressure. Portions of insects are obtained by dissection, which is a rather delicate operation, and needs to be done under a low power lens to magnify the parts. Vegetable preparations are mounted in different ways ; the seeds, pollen, etc., may be mounted dry, a ring of black varnish first being made on the slide in which the specimens are placed, and a cover glass is then cemented on with another ring of black varnish round it.