MAKING PREPARATIONS 



The following are the best fluids for this purpose :- 



1. Absolute alcohol or methylated spirit. 



2. Picric acid (saturated solution in water). 



3. Chromic acid (o'l 0*5 per cent, solution in water). 



4. Osmic acid (*i r per cent, solution in water). 



These reagents are only to be applied to fresh material. 



When absolute alcohol is used, the object may be kept 

 in it for an indefinite period. Such treatment generally makes 

 the object brittle ; this may be remedied when the object is to 

 be mounted in glycerine by placing it, for at least twenty-four 

 hours before it is to be cut, in a mixture of glycerine and 

 absolute alcohol in equal parts, leaving it exposed to the air so 

 that the alcohol may gradually evaporate. The glycerine 

 slowly saturates the object and restores its toughness. This 

 can only be done when the sections are to be mounted in 

 glycerine. 



When picric or chromic acid is used, the object should 

 be immersed in it until each part of it is thoroughly permeated 

 by the reagent ; the length of time required for this varies with 

 different material, and in the case of chromic acid, with the 

 strength of the solution used, from a few minutes to twenty-four 

 hours or more. The objects must then be washed thoroughly 

 with water: they are then to be placed in dilute methylated 

 spirit (50 per cent.), subsequently in stronger spirit (70 per 

 cent.), and finally in absolute alcohol or strong methylated 

 spirit, which must be changed so long as any colour is 

 still extracted from the objects. They may be preserved in 

 this for future use. 



When osmic acid is used, the fixing effect is produced much 

 more rapidly ; in the case of simple structures, such as unicel- 

 lular or filamentous Algae, a few minutes (5 15) generally 

 suffices ; in the case of more complex structures, such as ovules, 

 sporangia, growing points, &c., the object may be left in the 

 acid till it looks black on the exterior : it must be then well 

 washed with dilute alcohol (50 per cent.), and left in it for some 

 time, and be then removed to 70 per cent. The sections are 

 best mounted in dilute glycerine. In some cases osmic acid 



