APPENDIX. 103 



that the heat be about 50 C. ; this temperature will not do 

 for all objects. The next thing is to finish the slide neatly. 

 Paper covers are sold for this purpose ; but the best plan for 

 durability is simply to finish with a ring of Berlin black var- 

 nish ; or even this may be omitted in some cases ; the advan- 

 tage of this plan is that the slides can be kept much cleaner 

 &c. The white paper label can then be affixed with the 

 English and scientific names, also what fluid &c. it is mounted 

 in, and the date of the preparation. It can then be stored, if 

 money is an object, in the cheap rack-boxes sold by Mr. 

 Wheeler and other opticians at prices varying from sixpence 

 upwards (these boxes are not covered with cloth) ; the boxes 

 may then be numbered and placed on shelves in the same 

 manner as books', whereby the objects are kept in a horizontal 

 position. 



Fluids had, until lately, greatly gone down in the esti- 

 mation of microscopists as vehicles in which to mount various 

 specimens ; but glycerine and the more recent, if not so 

 useful, substance silicate of potassa, which appears to be a 

 very favourable semifluid for mounting certain structures, 

 have again caused them to be used ; and there are now var- 

 nishes and cements, such as the india-rubber and shellac 

 cement, which, with care, will hermetically seal fluid prepara- 

 tions for many years. We will treat of the process. Take 

 a slide, centre it on the turn-table, charge a earners hair brush 

 with the india-rubber cement, place the table in action and a 

 ring or circular cell of the cement is formed, varying in depth 

 according to the thickness of the fluid and the quantity used ; 

 turn a number of these cells and put aside to dry, to be used 

 as wanted. With a pipette take from the alcoholic and cam- 

 phor preservative fluid a sufficient quantity to fill the cell, 

 soak the object in proof spirit for an hour or so to exhaust 

 the air (it is better if the object has been kept in alcohol, see 

 fig. 109) ; or it may be done in much less time under the air- 

 pump ; it is then placed in the centre of the cell, the thin glass 

 cover placed gently over it, so as to exclude all air-bubbles ; 

 soak up all the surplus fluid with blotting-paper, centre the 

 slide again on the turn-table, and seal the cell with a ring of 



