10 PRESERVING AND HARDENING 



conveniently be mentioned here, as it is used for the same pur- 

 pose as the true stains, i.e., for the sake of rendering certain 

 parts of the cells especially distinct. Acetic acid, of which a 

 1 per cent, solution is employed, causes the protoplasm of cells 

 to swell up and become transparent, and brings the nuclei into 

 special prominence. It is used with fresh specimens. 



VI. ON PRESERVING AND HARDENING. 



The reagents in common use for killing and preserving small 

 animals are valuable also from their power of "fixing" the 

 tissues, i.e., of coagulating the protoplasm of the cells. The 

 objects to be attained are to effect this coagulation quickly, 

 before the tissues can undergo any alteration, and thoroughly, 

 i.e., throughout the whole thickness of the object to be hardened. 

 They are as follows : 



1. Alcohol. Specimens may be placed at once in 70 per 

 cent, alcohol ; and thence transferred after a couple of days to 

 90 per cent., in which they may be left till required. 



2. Osmic Acid. For this purpose a 1 per cent, solution in 

 water is used : it acts almost instantaneously, and so allows no 

 change to occur in the tissues ; it has also the merit of staining 

 the tissues as well as hardening them. It can, however, only 

 be employed when the specimens are very small, as it hardens 

 the surface layers so rapidly that it is unable to penetrate 

 beyond a very slight depth. A few minutes' immersion is 

 usually sufficient. The vapour must not be inhaled. 



3. Corrosive Sublimate. This is by far the best genei-al 

 reagent for killing and fixing small animals. A saturated solu- 

 tion in water is employed, in which the object is placed for 

 half an hour or more. 



4. Chromic Acid. A 0-25 to 0-5 per cent, solution of chromic 

 acid in water is a useful hardening reagent ; it acts much more 

 slowly than osmic acid, but penetrates to greater depths. 

 Specimens should usually be left in the solution for one or 

 more days, and then thoroughly washed for some days in 

 increasing strengths of alcohol. These processes should be 

 carried out as far as possible in a dark drawer or cupboard. 



5. A Mixture of chromic acid with a few drops of osmic acid 



