86 PREPARATION AND MOUNTING 



oblong; and the surface being flat, the cover is easily fastened 

 upon it. These are now cheap, and are very safe as to leakage. 



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 fre mentioned do away with all 

 necessity of it. 



GLASS RINGS. Where any 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 done away 

 with. These rings have both edges left roughened, and conse- 

 quently adhere very well to the slide, this adherence being gene- 

 rally 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 to be perfect. This method has the advantage of 

 requiring no heat, but the gold-size must be perfectly dry, and 

 the ring must have been fixed upon the slide some time before 

 use. Canada balsam has also been used for the same purpose, but 

 cannot be recommended, as when it is perfectly dry it becomes so 

 brittle as to bear no shock to which the slide may be ordinarily 

 exposed. 



These are the cells which are mostly used in this branch of 

 microscopic mounting. The mode of using them, and the dif- 

 ferent treatment which certain objects require w r hen intended to 

 be preserved in the before-mentioned liquids, may now be inquired 

 into. 



I may mention, however, that this class of objects is looked 

 upon by many with great mistrust, owing to the danger there is 

 of bubbles arising in the cells after the mounting has been com- 

 pleted, even for years. I know some excellent microscopists who 

 exclude all objects in cells and preservative liquids from their 

 cabinets, because they say that eventually almost all become dry 

 and worthless ; and this is no matter of surprise, for many of them 

 do really become so. Perhaps this is owing to the slides being 



