128 PREPARATION AND MOUNTING 
preserved with a thin coating of oil. It may be necessary 
sometimes to spread it with a needle or other instrument. 
The thin glass should then be carefully placed upon it, so 
that all air may be excluded; and should any oil be forced 
out, owing to the quantity used being too great, it must 
be removed with blotting-paper. When the edge of the 
thin glass cover and the surrounding parts_of the slide are 
as clean as possible, a coating of sealing-wax varnish or 
liquid glue must be applied and allowed to dry. A second 
or even a third coating may be required, but not before the 
previous cover is quite dry. These varnishes, however, are 
very brittle, and it is much safer, as a finish, to use one of 
the tougher cements—gold-size, for instance—which will 
render it doubly secure. 
The above are the principal liquids, &., used for pre- 
serving objects in cells. The different cells may be here 
mentioned ; and it is recommended that these should always 
be kept some time before use in order that the cement may 
become perfectly dry; and care must be taken that no 
cement be used on which the preservative liquid employed 
has any action whatever. 
Cement Crtis.—Where the object is not very thick, this 
kind of cell is generally used. They are easily made with 
the turntable before described ; but when the objects to be 
preserved are very minute, these cells need not be much 
deeper than the ordinary circle of cement on the slide. 
When, however, a comparatively great depth is required, it 
is sometimes necessary to make the wall of the cell as deep 
as possible, then allow it to dry and make another addition. 
Of these cements gold-size is one of the most trustworthy, 
and may be readily used for the shallow cells. The as- 
phaltum and india-rubber, before noticed, I have found very 
durable when well baked, and exceedingly pleasant to work 
with. It may be used of such a thickness as to give space 
for tolerably large objects. Black japan also is much used. 
Many cements, however, which are recommended by some 
writers, are worse than useless, owing to the brittleness which 
