Mr Pearccy on Preparing Thin Sections of Friable Rocks, etc. 299 



hot place the substance in the mould and press it firmly in 

 with the presser (PL XI., Fig. 2), 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 immediately 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 (PI. XI., Fig. 1, a); 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. 



Peepakation 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 pressure, 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 prevent the 

 stones from getting worn into a hollow, when it will be im- 

 possible to get a perfectly flat surface. 



The desired flatness and polish having been secured, pro- 

 ceed to cement with Canada Balsam the polished surface on 

 an ordinary glass slide 3x1 inches, 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 bees-wax, melted together in a crucible on a hot plate, 

 and put round the preparation with a glass pipette ; when 

 quite cold it may then 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 



