. xvii MICROSCOPICAL 1NVEST1 141 



unscratched surface even with the \Vater-of-Ayr stone. 

 These may be finished with putty powder, applied with a 

 : woollen rag. 



desired flatness and polish having been secured, 

 and all trace of scratches and dirt having been completely 

 removed, we proceed to grind down the opposite side 

 and reduce the cl i site degree of thinness. 



The first step at this stage is to cement the polished 

 surface of the chip to one of the pieces of common glass. 

 A tlnn piece of iron (a common shovel does quite well) 

 is heated over a fire, or is placed between two supports 

 over a gas-flame. On this plate must be laid the piece 

 of glass to which the specimen is to be affixed, and the 

 specimen itself. A little Canada balsam is dropped on 

 the c he glass and allowed to remain until it has 



red the necessary consistency. To test this con 

 dition, the j>oim of a knife should be inserted into the 

 balsam, and on being removed should be rapidly cooled 

 by being pressed against some cold surface. If it soon 

 becomes hard it has been sufficiently heated. Care, 

 however, must be observed not to let it remain too long 

 on the hot plate ; for it will then become brittle and start 

 from the glass at some future stage, or at least will break 

 away from the edges of the chip and leave them exposed 

 to the risk of being frayed off. The heat should be kept 

 as moderate as posv : it Incomes too great it may 



injure some portions of the rock. Chlorite, for example, 

 is rendered quite opaque if the heat is so great as to drive 

 off its water. 



When the balsam is found to be ready, the chip, which 

 has been warmed on the same plate, is lifted with the 



R 



