THE PRESERVATION OF MICROSCOPIC SPECIMENS. 281 



preservation reached an extent and importance, which make it 

 the duty of every microscopist to become acquainted with the 

 processes of making such preparations. Since it is in the first 

 place necessary to select the proper medium for the preservation 

 of the objects, the methods which are employed may be described 

 with reference to these media. We specially note the following : 



(1.) Air as a medium for dry substances for instance, scales of 

 butterflies' wings, diatoms, crystals, and sections of various hard 

 substances. After we have ascertained that these objects produce 

 a satisfactory image in air, they are covered with a thin cover- 

 glass, which is cemented upon the slide preferably with thick fluid 

 gum or Canada balsam. The whole slide is then covered with 

 paper, which has a hole cut out over the middle of the cover-glass 

 and projects somewhat beyond the edges of the slide. 



(2.) Canada balsam, for hard formations of every kind diatoms, 

 fossil woods, polished bones and teeth, &c. Among the resinous 

 substances which have been proposed as media for microscopic 

 objects, none is so important or so generally employed as Canada 

 balsam. For vegetable objects its use, however, is limited, since 

 it renders the majority of cell -membranes too transparent to 

 produce a distinct image ; on the other hand, this quality con- 

 siderably enhances its importance for animal preparations. The 

 object to be mounted is thoroughly dried by warming or by being 

 placed over sulphuric acid or chloride of calcium, and then soaked 

 in oil of turpentine. A drop of liquid Canada balsam is put upon 

 the slide ; the preparation is then spread out on it and covered by 

 a second drop. To remove the air-bubbles, the slide may be 

 slightly heated (no boiling should occur), and this is continued, if 

 necessary, after the application of the thin cover-glass, by placing 

 it, for instance, near a stove ; a more rapid hardening of the balsam 

 thus takes place. 



As, however, the balsam always remains somewhat viscous, it is 

 advisable after the lapse of a few days to coat the edges of the 

 cover-glass with a rapid-drying varnish which forms a firm non- 

 adhesive ring. Solid Canada balsam may be liquefied with chloro- 

 form or oil of turpentine. Its solidification in air may be con- 

 siderably retarded if kept in wide-necked glass bottles with ground 

 stoppers. Any surplus of the balsam which oozes out at the edges 

 of the cover-glass may be scraped off after it has hardened, and 

 the glass then cleaned with oil of turpentine. 



