242 OUTLINES OF FIELD-GEOLO PART n 



forceps and its polished side is laid gently down upon the 

 balsam. It is well to let one end touc h t i first, 



and then gradually to lower the other, as in this \\-.\\ the 

 driven out. With the point of a knife the rhip 

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

 of air and promote a firm cohesion between the gl;i 

 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 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 

 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 the glass. Softer rocks may require 

 centle treatment with the Water-of-Ayr stone. 



