188 Mr Witham's Observations on Fossil Vegetables. 



and gradually heated over a slow fire, with a view to concen- 

 trate the balsam. In performing this operation, it will be re- 

 quisite 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 difficult 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 se- 

 conds. With every precaution, some few air-bubbles will some- 

 times make their appearance, 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 com- 

 pletely removed, the slice and the glass may be taken from the 

 shovel, and applied to each other. A slight degree of pressure 

 will be necessary 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 ad- 

 hering 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 whe- 

 ther the balsam has undergone the requisite concentration. If, 

 for instance, it has entirely lost its sectiiity, 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 be- 

 fore the knife, and, in that case, the two bodies will not adhere 

 with sufficient firmness. A very few trials, however, will en- 

 able any one to conduct 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 accom- 

 plished in four or five minutes, provided the application of the 

 heal be duly regulated. 



" The slice must now be ground down to that degree of 

 thinness which will permit its structure to be seen by help of a 



