FIXING AND HARDENING AGENTS, 33 



from forming again and again. It lias, lioweverj been found 

 by Hans Viechow [Arch. mik. Anat., xxiv, 1885, p. 117) that 

 it may be entirely prevented by simply keeping the prepara- 

 tions in the dark. The alcohol becomes yellow as usual (and 

 should be changed as often as this takes place), but no 

 precipitate is formed. If this precaution be taken, previous 

 washing with water may be omitted, or at all events greatly 

 abridged. 



Mayee {Grundzilge, 1st ed., p. 28) proceeds as follows; — 

 The fixed material is merely rinsed in water and brought 

 direct into 70 per cent, alcohol. It is washed therein, pre- 

 ferably in the dark, until after several changes the alcohol 

 remains colourless. It is then either passed through higher 

 alcohols and imbedded in paraffin, the chromous oxide (or 

 whatever chrome compound it may be that is present in the 

 tissues) being removed from the sections after these are 

 made ; or this necessary removal is performed at once. If 

 this be preferred, the material is brought into sulphuric acid 

 diluted with twenty volumes of water, or into nitric acid 

 diluted with ten volumes of water. After at most a few 

 hours therein, it will have become of a light greyish green, 

 and on removal of the acid may be readily stained. If it be 

 preferred to treat the sections, it is sufficient to put them into 

 the usual hydrochloric acid alcohol (four to six drops of HCl 

 to 100 c.c. of 70 per cent, alcohol), in which after a short 

 time they become almost white, and will stain excellently 

 with any of the usual stains. So also Edikgeb {Zeit. u-iss. 

 Mik., i, 1884, p. 126; nitric acid 1 : 20 for five minutes). 

 Unna {Arch. mik. Anat., xxx, 1887, p. 47) holds that the 

 chrome is present in the tissues in the form of chromic 

 chromate, and removes it by treatment with peroxide of 

 hydrogen. Ovekton (Zeit. wiss. Mik., vii, 1890, p. 9) employs 

 a weak solution of sulphurous acid, which converts it into a 

 sulphate. See also the directions for bleaching osmic acid 

 preparations, § 35. 



Tissues that have been fixed in chromic acid maj' be 

 stained in aqueous solutions, as water does not have an 

 injurious effect on them. 



The best stain for chromic material that has not been 

 treated by Mayer's special process, or by a similar one, is 

 hsematoxylin, or, for sections, the basic tar colours. 



3 



