GENERAL METHODS. 4! 



-with those of cellulose and of most of the contents of the 

 vegetable cell, these substances are hardly to be used except 

 for stained preparations or such as are intended for observa- 

 tion by polarized light. Besides, the transfer of easily col- 

 lapsible objects to balsam is so complicated that other 

 mounting media are preferable for them. It depends 

 wholly on the nature of the objects to be mounted what 

 mounting medium is best to be used , and it will be neces- 

 sary, in the second and third parts of this book, to re- 

 peatedly indicate what method of preservation is best 

 adapted to the case in hand. 



Since the methods of mounting in Canada balsam, Dam- 

 mar lac and turpentine have been already described in 

 : 14 to 27, only the remaining methods, in which glycer- 

 ine especially plays an important part, need be here brought 

 together. 



54. Glycerine. Pure glycerine in various degrees of dilu- 

 tion, or a mixture of this with an acid, was formerly a much 

 -esteemed and almost universally used medium. In its use, 

 however, especial care must be taken that the glycerine used 

 is not diluted by too long exposure to the air, since in that 

 case a gradual drying up of the preparation takes place, if 

 the subsequently applied cement ring (cf. 62) is not air- 

 tight. Concentrated glycerine often cannot be used, how- 

 ever, on account of its strong clearing and dehydrating 

 power. In such cases a dilute solution of glycerine with a 

 few drops of acetic acid offers great advantages, but must, 

 as already remarked, be very carefully protected against 

 -evaporation. (Cf. especially Dippel, II, 1010.) 



55. Glycerine and Chrome Alum. For preserving prepara- 

 tions of Schizophycece and Floridece in their natural colors, 

 Kirchner uses (I, p. Vll) dilute glycerine, to which is added 

 -enough chromium-potassium sulphate (chrome alum) to give 

 the fluid a clear bluish color. 



56. Glycerine-gelatine. This is of late most used and 

 offers undeniable advantages, in most cases, over the fluid 

 glycerine mixtures. It is conveniently prepared from the 

 recipe recommended by Kaiser, as follows : One part by 



