162 



SUMMARY OF CUERENT RESEAEOHES RELATING TO 



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Moulding. — This must be done while the suhstance is quite hot 

 and plastic. First put a piece of common flat glass, three times the 

 size of the cavity of the mould, upon the hot plate, with the 

 mould on the top and the notched end downwards, so 

 as to have it perfectly flat on the glass, and when 

 quite hot, place the substance in the mould and press 

 it firmly in with the presser (fig. 39), taking care to let 

 as little as possible of the substance escape from the 

 bottom. This may be to a great extent prevented by 

 holding the mould down with the back of a knife with 

 the left hand, and pressing in the substance with the 

 right. 



This done, take the whole off the plate smartly, 

 with the glass attached, and press it on another flat 

 slab or iron plate with the left hand, and with the 

 right pour on a little cold water, when it will im- 

 mediately set hard. Next place the whole in cold 

 water for two or three minutes, after which the piece 

 of glass at the bottom can be knocked or broken off; 

 then loosen the wire which fastens the mould together, 

 and open it a little (fig. 38, h); the moulded substance 

 will then drop out in the form of a very hard mass, 

 and is ready to be cut into sections. After a little 

 practice, the whole operation can be done in an hour. 



Preparation of the Sections. — Eub down and polish 

 one end of the moulded substance, first upon a common 

 hone, with a slow, equable motion, and a steady pres- 

 sure, so as to produce the desired flatness of surface, 

 and afterwards upon a Water-of-Ayr stone to give a 

 fine polish. It must be held quite flat, so as to pre- 

 vent the stones from getting worn into a hollow, when 

 it will be impossible to get a perfectly flat surface. 



The desired flatness and polish being secured, 

 proceed to cement with Canada balsam the polished 

 surface on an ordinary glass slide 8x1 in., or 

 according to the size of the sections required. This 

 is done in the same way as with hard rocks, but great 

 care must at first be taken not to have the slide too 

 hot, or the balsam will become too brittle. After 

 having been properly mounted, it should be cemented round with 

 a composition formed of four parts of resin and one of beeswax, 

 melted together in a crucible on a hot plate," and put round the 

 preparation with a glass pipette ; when quite cold it may be 

 cut with a lapidary's wheel, or ground down on a metal plate with 

 emery powder. The slice remaining on the slide should be well 

 cleaned and rubbed down on the hone to the required thinness. 

 This part of the process is most difficult. The slides should be kept 

 as flat as possible, and looked at frequently with the Microscope, so 

 that the first indication of disruption may be detected. The proper 

 thinness having been obtained, the sections should be at once covered 



