ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 463 



stone. These may be finished with putty-powder, applied with a bit 

 of woollen rag. 



The desired flatness and polish having been secured, and all traces 

 of scratches and dirt having been completely removed, we proceed to 

 grind down the opposite side, and reduce the chip to the requisite 

 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 

 thin 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 centre of the glass, and allowed to remain until it has acquired 

 the necessary consistency. To test this condition, the point 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 possible, for if it becomes 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 forceps, and its polished 

 side is laid gently down upon the balsam. It is well to let one end 

 touch the balsam first, and then gradually to lower the other, as in 

 this way the air is driven out. With the point of a knife the chip 

 should be moved about a little, so as to expel any bubbles of air, and 

 promote a firm cohesion between the glass and the stone. The glass 

 is now removed with the forceps from the plate, and put upon the 

 table, and a lead weight or any other small heavy object is placed 

 upon the chip, so as to keep it pressed down until the balsam has 

 cooled and hardened. If the operation has been successful, the slide 

 ought to be ready for further treatment as soon as the balsam has 

 become cold. If, however, the balsam is still soft, the glass must be 

 again placed on the plate and gently heated, until on cooling the 

 balsam resists the pressure of the finger-nail. 



Having now produced a firm union of the chip and the glass, we 

 proceed to rub down the remaining side of the stone with coarse emery 

 on the iron plate as before. If the glass cannot be held in the hand, 

 or moved by the simple pressure of the fingers, which usually suffices, 

 it may be fastened to the end of the bobbin with the rosin cement as 

 before. When the chip has thus been reduced until it is tolerably 

 thin — until, for example, light begins to appear through it when held 

 between the eye and the window — we may, as before, wash it clear of 

 the coarse emery, and continue the reduction of it on the glass plate 

 with fine emery. Crystalline rocks, such as granite, gneiss, diorite, 

 dolerite, and modern lavas, can be reduced to the required thinness on 



