132 PREPARATION AND MOUNTING 
We will now suppose the cell employed has been made 
by placing a glass ring upon the slide with marine-glue or 
gold-size, and is quite dry. Around the edge of the cleaned 
thin glass which is to cover it, I trace with a camel-hair 
pencil a ring of gold-size, and also around the edge of the 
cell to which it is to adhere. Dr. Carpenter objects to this, 
as rendering the later applications of the gold-size liable to 
“run in.” All danger of this, however, is completely ob- 
viated by leaving the slide and cover for awhile until the 
cement becomes partially fixed, but still adhesive enough 
to perform its function (Chapter III.). With many slides 
this is not accomplished in less than twenty-four hours, even 
if left two or three days no injury whatever ensues; but 
with other kinds an hour is too long to leave the exposed 
cement, so that the operator must use his own discretion. 
It is not always necessary to size the edge of the cover, 
since perfect adhesion may in many cases be secured with- 
out it, and it is always best to use the least quantity of 
cement that will answer, as it will then be less likely to run 
in. The liquid required may be drawn up by the mouth 
into the pointed tube mentioned in Chapter II., and then 
transferred to the cell. In the various books of instruction, 
the object is now to be placed in the cell; this, however, I 
think a great mistake, as another process is absolutely 
necessary before we advance so far. The cell, full of liquid, 
must be placed ander the receiver of an air-pump, and the 
air withdrawn. Almost immediately it will be perceived 
that the bottom and sides of the cell are covered with minute 
bubbles, which are formed by the air that is held in sus- 
pension by the liquid. The slide may now be removed, and 
the bubbles may require the aid of a needle or other point 
to displace them, so obstinately do they adhere to the 
surface of the glass. This process may then be repeated, 
and one cause, at least, of the appearance of bubbles in cells 
of liquid will be removed. The object to be mounted should 
also be soaked in one or two changes of the preservative 
liquid employed, and, during the soaking, be placed under 
