ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 153 



Wlien tlie grouping is finally inspected, it is permanently fixed to the 

 gelatin layer by holding the slide on a level, under the mouth, and 

 breathing on it very carefully a few times. This is perfectly reliable and 

 more expeditious than breathing through rubber or glass tubes for the 

 same purpose. 



For the purpose of mounting, it is well to have a quantity of cells 

 finished on slides and kept on hand. The slides are centered on the 

 turntable, and shallow cells of black shellac are built up to suit the 

 diameter of the cover-glass to be mounted thereon. This cell is filled 

 with a drop of Canada balsam pressed out of a metal tube. The cover- 

 glass containing the arranged diatoms is now freely immersed in filtered 

 spirits of turpentine, and also flushed with it, so as to expel all air from 

 the diatoms and to clean off all motes or particles that may have lodged 

 upon it during or after preparation of the same. The cover-glass is then 

 set upon its edge to drain off superfluous turijentine, and while it drains 

 gently soften the shellac cell over a spirit-lamp, pick up the cover-glass 

 and gently lay it centrally over the cell, and press firmly into contact 

 with the cell ; the slide is then set aside with the cell-side down, and 

 supported on a level, to obviate as much as possible the floating out of 

 place of any of the forms, which are sometimes displaced while drying. 



The procedure described above is essentially that followed by the 

 leading preparers, with more or less slight variations as to finish of cells 

 and media used in mounting. 



For the arrangement of type-plates of diatoms, the guide-lines and 

 squares ruled on the cover-glass carrier serve to allow the forms to 

 be adjusted in lines and properly spaced with the same ease as in 

 symmetrical grouping. When such beautiful results are produced by 

 simple and inexpensive means, it does not seem to be worth while to 

 attempt this class of work with compound Microscopes, with mechanical 

 fingers and ruled guides set in the eye-piece. 



Xylol-dammar.* — M. Martinotti advocates the use of dammar dis- 

 solved in xylol as a mounting medium to be preferred to balsam in 

 certain cases. He prepares his solution in the following way : — 



Forty grm. of dammar and 40 grm. of xylol are mixed together 

 in a stoppered bottle, and allowed to stand for three or four days at the 

 ordinary temperature. The solution is then filtered. The filtrate, 

 which will amount to about 70 grm., is then evaporated in a water- 

 bath down to about 45 grm. 



The object of this concentration is to obtain a solution of the resin 

 in the smallest quantity of xylol possible, just enough in fact to merely 

 dissolve the resin. This concentrated solution becomes yellow, but 

 retains its limpidity. The next step is to dilute this solution with oil 

 of turpentine, by which means the yellowish colour is made to almost 

 disappear. 



Kaiser's Gelatin for arranging microscopical preparations in 

 series.j — Dr. A. Poll arranges objects on the slide with Kaiser's gelatin 

 in the following manner : — With a fine brush, just as many daubs are 

 made with the melted gelatin as there are preparations to be mounted, 

 the preparations are then transferred on the brush to the places where 

 the thin layers of gelatin are, slight pressure being used in order to 

 make them stick. Should the preparations not lie in the desired 



* Malpighia, ii. (1888) p. 270. f Ibid., pp. 107-9. 



