56 PREPARATION AND MOUNTING 
other substance preferred (as mentioned in Chapter IT.). 
Cardboard is easily procured of almost any thickness; but 
sometimes it is convenient to find a thinner substance even 
than this. When thin glass is laid upon a drop of any 
liquid upon a slide, every one must have observed how 
readily the liquid spreads betwixt the two: just so when 
any thin varnish is used to surround an object of little sub- 
stance, excessive care is needed lest the varnish should 
extend betwixt the cover and slide, and so render it worth- 
less. The slightest wall, however, prevents this from taking 
place, so that a ring of common paper may be used, and 
_ serve a double purpose where the objects require no deeper 
cell than this forms. 
Many objects, however, are of such tenuity—as the leaves 
of many mosses, some of the Diatomacee, scales of insects, 
&c.—that no cell is requisite excepting that which is neces- 
sarily formed by the medium used to attach the thin glass 
cover to the slide; and where the slide is covered by the 
ornamental papers mentioned in Chapter II., and pressure 
does not injure the object, even this is omitted, the thin 
glass being kept in position by the cover; but slides mounted 
in this manner are frequently injured by dampness, which _ 
soon condenses upon the inner surfaces and interferes both 
with the object and the clearness of its appearance. 
The thin glass, then, is to be united to the slide, so as to 
form a perfect protection from dust, dampness, or other 
injurious matter, and yet allow a thoroughly distinct view 
of the object. This is to be done by applying to the glass 
slide round the object some adhesive substance, and with the 
forceps placing the thin glass cover (quite dry and clean) 
upon it. A gentle pressure round the edge will then 
ensure a perfect adhesion, and with ordinary care there will 
be little or no danger of breakage. For this purpose gold- 
size is frequently used. The asphalt and india-rubber 
varnish also will be found both durable and serviceable. 
Whatever cement may be used, it is well to allow it to 
become in some measure fixed and dried; but where no 
