330 PREPARATION AND MOUNTING 



another piece is to be attached should be laid thin strips of 

 the glue; both pieces must then be heated upon a small 

 brass table, with the aid of the spirit-lamp, until the strips 

 become melted ; the small piece is then to be taken up and 

 placed upon the spot to which it is to be attached, and so 

 on until the cell is completed. It will be found necessary to 

 spread the glue over the surface required with a needle or 

 some other instrument, so that an unbroken line may be 

 presented to the wall of the cell, and no bubbles formed. 

 Too great a heat will burn the marine-glue, and render it 

 brittle ; care must be therefore taken to avoid this. 



When shallow cells are required, those which are made 

 by grinding a concavity in the middle of an ordinary slide 

 will be found very convenient. The concavities are cut both 

 circular and oblong ; and the surface being flat, the cover is 

 easily fastened upon it. These are now cheap, and are very 

 safe as to leakage. It is a very great improvement, where 

 it can be done, to turn a shallow ring outside the concavity 

 of the slide, but close to it. This prevents the cement with 

 which the cover is fastened from running in. 



Circular cells with a flat bottom used to be made by 

 drilling a hole through glass of the required thickness, and 

 fixing this upon an ordinary slide with marine-glue; but 

 the danger of breakage and the labour were so great that 

 this method is seldom used now, and, indeed, the rings about 

 to be mentioned do away with all necessity for it. 



GLASS RINGS. AVhere anv depth is required, no method 

 of making a cell for liquids is so convenient as the use of 

 glass rings, which are now easily and cheaply procurable. 

 They are made of almost every size and depth, and, except 

 in very extraordinary cases, the necessity for building cells 

 is completely obviated. These rings have both edges left 

 rou<*h, and consequently adhere very well to the slide, this 

 adherence being generally accomplished by the aid of 

 marine-glue, as before noticed with the glass cells. Gold- 

 size has been occasionally used for this purpose ; and the 

 adherence, even with liquid in the cell, I have always found 



