FIXING AND HARDENING AGENTS. 49 



it causes chromatin to swell,, and therefore should not be 

 employed for the study of nuclei. His readers, plus royalistes 

 que le rui, took that to be a reason for abandoning it 

 altogether, and from that time until quite recently it has 

 lain in the cold shadow of neglect for almost all purposes 

 except the hardening of nervous tissue. Erroneously, for, 

 duly corrected with acetic acid, it affords a correct and fine 

 fixation of nuclei; whilst preserving hyaloplasm and its 

 inclusions, secretions, etc., much better than chromic acid 

 (sometimes overmuch) . 



For an elaborate study of the action of chrome salts on 

 nucleus and cytoplasm, see BUKCKHAKUT, La Cellule, xii, 

 2, 1897, p. 335. He finds that the bichromates of sodium, 

 ammonium, magnesium, strontium, and zinc have the same 

 destructive action on nuclei that the bichromate of potassium 

 has ; but that the bichromates of barium, calcium, and 

 copper have not. The practical results of his researches 

 may be summed up as follows : Acetic acid ought always to 

 be added, not only to ensure the correct fixation of nuclei, 

 but also to enhance penetration and the good preservation 

 of cytoplasm. 



The following is recommended by him as a good combina- 

 tion for the fixation both of cytoplasm and nucleus : 



Bichromate of barium, 4 per cent, solution . 60 vols. 

 Bichromate of potassium, 5 per cent, solution . 30 ,, 

 Glacial acetic acid . . . . . 5 ,, 



(Instead of the barium you may take 4 per cent, solution 

 of bichromate of calcium, or 6 per cent, solution of bichro- 

 mate of copper.) 



For the demonstration of the achromatic figure of cell 

 division he recommends 



Chromic acid, 1 per cent, solution . . 60 vols. 



Bichromate of potassium, 5 per cent, solution . 30 ,, 

 Glacial acetic acid . . . . 5 



55. Bichromate of Potash. Perhaps the most important of 

 all known hardening agents, sensu stricto. It hardens 

 slowly, much more so than chromic acid, but it gives an in- 

 comparably better consistency to the tissues, and it has not 

 the same tendency to make them brittle if the reaction be 



4 



