132 PREPARATION AND MOUNTING 



We will now suppose the cell employed hns been made 

 by placing a glass ring upon tiie siicle 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-fours 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 tie 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 under the receiver of an air-pump, and the 

 air withdrawn. Almost immediately it will be perceived 

 that the bottom and sides of tne cell are covered with minute 

 bubbles, which are formed by the air that is held in sus- 

 pension by the liquid. The slide may row 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 



