OF MICROSCOPIC OBJECTS. 131] 
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. 
Iron Rines——Many have worked with these, having 
taken care to varnish thoroughly before using with any 
preservative liquid; but they are always untrustworthy, as 
they can never be guaranteed against the action of some salt 
in the liquid used. They can be procured beautifully made, 
and for dry cells cannot be surpassed. Zine and pure tin 
rings ray also be procured, and are excellent, especially the 
latter. 
Vuucanite.—This substance is a great favourite with 
some of our working microscopists, as it is very slightly in- 
fluenced by change of temperature. But my own opinion is 
that a glass cell is the safest and most satisfactory re- 
ceptacle for any object in liquid, and if carefully prepared 
will not deceive the operator. 
These are the cells which are mostly used in this branch 
of microscopic mounting. The mode of using them, and the 
different treatment which certain objects require when in- 
tended 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 completed, 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 sold 
before they could possibly be thoroughly dry. As to the 
air-bubbles, I shall have. something to say presently. 
ee 
