OF MICROSCOPIC OBJECTS. 87 



sold before they could possibly be thoroughly dry. As to the air- 

 bubbles, I shall have something to say presently. 



We will now suppose the cell employed is 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 

 Ground the edge of the cell to which it is to adhere. Dr. Carpen- 

 ter objects to tfris, as rendering the later applications of the gold- 

 size liable to " run in." All danger of this, however, is complete- 

 ly done away with by leaving the slide and cover for awhile until 

 the cement becomes partially " fixed," but still adhesive enough 

 to perform its function {Chapter II.). 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 opera- 

 tor must use his own discretion. The liquid required may be 

 drawn up by the mouth into the pointed tube mentioned in Chap- 

 ter I., and then transferred to the cell. In the various books of 

 instruction, the object is now to be placed in the cell ; this, how- 

 ever, 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 with- 

 drawn. 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 suspension 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 be 

 then repeated, and one cause, at least, of the appearance of bub- 

 bles in cells of liquid will be got rid of. 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 the 

 air-pump and exhausted. It may then be transferred to the cell, 

 which will probably cause the liquid to overflow a little. The 

 cover with the gold-size applied to the edge must then be care- 

 fully laid upon the cell, and slightly pressed down, so that all air- 

 bubbles may be displaced. The two portions of gold-size will 

 now be found to adhere wherever the liquid does not remain, 



