46 CONCLUDING REMARKS. 
The slice and the glass are now to be laid on any thin plate of metal, as a 
common fire-shovel, and gradually heated over a slow fire with a view to 
concentrate the balsam. In performing this operation, it will be requisite 
to prevent the heat from becoming so great as to throw the balsam into a 
state of ebullition ; for, if air-bubbles be once formed in it, it will be diffi- 
cult to remove them, and if they are not removed, they will prevent the 
complete adhesion of the two surfaces when applied to each other. The 
heat of the shovel should never become so great that the fingers may not be 
held in contact with it, without inconvenience, for a few seconds. With 
every precaution, some few air-bubbles will sometimes make their appear- 
ance, but these may be removed by a small piece of wood tapering to a point. 
When the balsam is thought to be sufficiently concentrated, and all air- 
bubbles completely removed, the slice and the glass may be taken from the 
shovel, and applied to each other. A slight degree of pressure will be ne- 
cessary to expel the superabundant balsam, and this will be facilitated by 
gently sliding the one on the other. By this kind of motion, any air that 
might have got entangled in the balsam, when the surfaces were brought 
into contact, will also be removed. 
When the whole is cooled down to the temperature of the air, and the 
balsam becomes solid, that part of the balsam adhering to the surface of the 
glass surrounding the slice should be removed by the point of a pen-knife ; 
and it may be right to remark, that, in this operation, it will at once be 
seen whether the balsam has undergone the requisite concentration. If, for 
instance, it has entirely lost its sectility, and starts off in flakes before the 
knife, it will be found that the slice and the glass will cohere so firmly, that, 
in the subsequent grinding, there will be no risk of their separating from 
each other. 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 
sufficient firmness. A very few trials, however, will enable any one to con- 
duct the process with success; and it may be right to add, that, if the layer 
of balsam applied to the two surfaces be not too thick, its due concentration 
may be accomplished in four or five minutes, provided the application of the 
heat be duly regulated. 
