CH. VII] 



MOUNTING AND LABELING 



167 



little common salt (water 100 cc, common salt T "jjths gram) is employed. This is 

 the so-called normal salt or saline solution. For the ciliated cells from frogs and 

 other amphibia, nothing has been found so good as human spittle. Whatever is 

 used, the object is put on the middle of the slide and a drop of the mounting 

 medium added, and then the cover-glass. The cover is best put 

 on with fine forceps, as shown in Fig. 136. After the cover is in 

 place, if the preparation is to be studied for some time, it is better 

 to avoid currents and evaporation by painting a ring of castor oil 

 around the cover in such a way that part of the ring will be on the 

 slide and part on the cover ( Fig. 165. ) 



Fig. 135. Needle Holder (Queen & Co.). By means of the 

 screw clamp or chuck at one end the needle may be quickly 

 changed. 



Fig. 136. To show the method of putting a cover-glass upon a 

 microscopic preparation. The cover is grasped by one edge, the 

 opposite edge is then brought down to the slide, and the cover 

 gradually lowered upon the object. 



Fig. 136. 



2 245. Permanent Mounting. — For making permanent microscopical prepara- 

 tions, there are three great methods. Special methods of procedure are necessary 

 to mount objects successfully in each of these ways. The best mounting medium 

 and the best method of mounting in a given case can only be determined by ex- 

 periment. In most cases some previous observer has already made the necessary 

 experiments and furnished the desired information. 



The three methods are the following : (A) Dry or in air (| 246); (B) In some 

 medium miscible with water, as glycerin or glycerin jelly (§ 250); (C) In some 

 resinous medium like Canada Balsam [\ 255). 



\ 246. Mounting Dry or in Air. — The object should be thoroughly dry. If 

 an}' moisture remains it is liable to cloud the cover-glass, and the specimen may 

 deteriorate. As the specimen must be sealed, it is necessary to prepare a cell 

 slightly deeper than the object is thick. This is to support the cover-glass, and 

 also to prevent the running in by capillarity of the sealing mixture. 



'i 246a. Order of Procedure in Mounting Objects Dry or in Air. 



1. A cell of some kind is prepared. It should be slightly deeper than the 

 object is thick (I 248). 



2. The object is thoroughly dried (dessicated) either in dry air or by the aid 

 of gentle heat. 



3. If practicable the object is mounted on the cover-glass ; if not it is placed 

 in the bottom of the cell. 



4. The slide is warmed till the cement forming the cell wall is somewhat 

 sticky, or a very thin coat of fresh cement is added ; the cover is warmed and put 



