
Bee eS 
' ParrIl.§ vii] DETERMINATION OF ROCKS. —185 
_ the Water of Ayr stone. These may be finished with putty powder, 
appled with a bit of woollen rag. 
- The desired flatness and polish having been secured, and all 
trace of scratches and dirt having been completely removed, we 
proceed to a further stage, which consists in grinding down the 
opposite side and reducing the chip to the requisite degree of thin- 
ness. The first step is now 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 slice is to be affixed, together with the © 
slice 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 enough to resist the pressure of the finger nail, 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 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 needle or 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 other small heavy object is placed upon the chip, so 
as to keep it pressed down until the balsam has cooled and hardened. 
_ Ifthe 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 fulfils the 
condition of resisting 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 
1 A piece of wire-gauze placed over the flame, with an interval of an inch or more 
___ between it and the overlying thin iron plate, tends to diffuse the heat and prevent the 
4 _ balsam from being unequally heated. 
