2 PRACTICAL BOTANY. 



When it is desired to study the structure of the 

 protoplasm and of the nucleus, special methods must 

 be employed for hardening them, or rather, for fixing 

 them as nearly as possible in the condition in which 

 they were during life. For this purpose the fluids 

 mentioned below must be used. Care must be taken 

 that the objects are of small size, that the quantity of 

 hardening fluid is very large relatively to the bulk of 

 the object, and that the fluid has ready access to all 

 parts of it. 



The following are the best fluids for this purpose : 



1. Absolute alcohol. 



2. Picric acid (saturated solution in water). 



3. Chromic acid (O'l 0'5 per cent, solution in 



water). 



4. Osmic acid ('1 - 1 per cent, solution in water). 

 These reagents can only be applied to fresh 



material. 



The following is a useful method for preparing sea-weeds : to 

 a quantity of saturated solution of picric acid in sea-water add 

 three or four times its volume of sea- water, and treat the tissue 

 with it for | hr. 2 hrs. : then treat successively with 30, 50, 70, 

 and 90 per cent, alcohol. 



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 reme- 

 died 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 consistency. This 



