APPENDIX TO PART I. 



MOUNTING MEDIA. 



Before this section is closed the most important 

 substances used in mounting botanical specimens 

 should be noticed. In many cases the student 

 must learn by trial the best medium for a prepara- 

 tion. Still there are a number of substances 

 which are known to be so well adapted for the 

 preservation of very different objects that they 

 should be tried first. 



GLYCERINE is excellent for nearly all botanical 

 preparations. The Florideae and Diatoms, how- 

 ever, usually require preservation in other media ; 

 the former since their cell-walls often swell greatly 

 in this fluid, especially if they have not been pre- 

 viously anhydrated with absolute alcohol ; the 

 latter because their structure does not appear in 

 it with the required degree of distinctness. Bac- 

 teria, also, become so transparent in glycerine, 

 and in particular if they have not been stained, as 

 to be almost indistinguishable. 



[FARRANT'S SOLUTION, which is employed some- 

 what in animal histology, is a good substitute for 

 glycerine in mounting many vegetable tissues. 

 Frey's formula is : Equal parts of gum arabic, 

 glycerine, and a saturated solution of arsenious 

 acid. If the slide is allowed to lie a day or two 



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