MICROSCOPICAL EXAMINATION. 67 
bubbles be formed, it is difficult to get rid of them, and if 
not removed they will prevent the complete adhesion of the 
two surfaces when applied to each other ; the heat of the 
metal should never be so great that the fingers may not be 
held in contact with it for a few seconds without inconve- 
nience. When air bubbles are formed, they should be dis- 
placed by a small piece of wood tapering to a point ; when the 
balsam is thought to be sufficiently concentrated, and all the 
air bubbles have disappeared, the slice and glass may be 
taken from the heated metal, and pressed closely together ; 
a slight degree of pressure will suffice to expel the super- 
abundant balsam, and this will be facilitated by gently 
sliding the specimen to and fro on the glass; by this kind 
of motion any air that may have got entangled when the 
two surfaces were brought in contact, will also be removed. 
When the whole is cooled down to the temperature of the 
air, and the balsam has become solid, that part which 
adheres to the surface of the glass surrounding the slice 
should be scraped off with the point of a penknife; and by 
this operation, it will at once be seen whether the balsam 
has undergone the requisite concentration ; for if it flakes 
off before the knife, it will be found that the slice and glass 
will cohere so firmly, that in the subsequent grinding, there 
will be no risk of their separating from each other ; but if 
the balsam has not been sufficiently concentrated, it will 
slide before the knife, and in that case the two bodies will 
not adhere with requisite firmness. If the layer of balsam 
applied to the two surfaces be not too thick, its due concen- 
tration will be accomplished in four or five minutes, provided 
the application of the heat be properly regulated. The slice 
must now be ground to that degree of thinness which will 
permit its structure to be seen by the help of a microscope. 
This will be accomplished by rubbing the slice, by a rapid 
circular motion with the hand, on a piece of sheet lead, sup- 
plied with a little emery (size Vo. 1.) moistened with water ; 
