CHAPTER IV. 33 



observers (KLEIN ; URBAN PRITCHARD ; PERENYI) have recom- 

 mended alcoholic solutions ; but this is evidently irrational. For 

 in the presence of alcohol chromic acid has a great tendency to 

 become reduced to chromous oxide or sesquioxide, neither of which 

 appears to have any fixing power. 



The most useful strengths in which it is employed in aqueous 

 solution are from 0-1 to 1-0 per cent, for a period of immersion of a 

 few hours (structure of cells and ova). For nerve tissues weaker 

 solutions are taken, -^ to J per cent, for a few hours. Stronger 

 solutions, such as 5 per cent., should only be allowed to act for a few 

 seconds. 



Washing out. The general practice is to wash out very thoroughly 

 with water (by preference running water, for many hours) before 

 bringing into alcohol or any staining liquid. For if the objects are 

 put direct into alcohol it is found that after a short time a fine 

 precipitate is thrown down on the surface of the preparations, thus 

 forming an obstacle to the further penetration of the alcohol. 

 Previous washing by water does not prevent the formation of this 

 precipitate, and changing the alcohol does not prevent it from 

 forming again and again. It has, however, been found by HANS 

 VIRCHOW (Arch. Mik. Anat., xxiv, 1885, p. 117) that it may be 

 entirely prevented by simply keeping the preparations 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. 



MAYER (Grundzuge, 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, preferably 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 (4- to 6 drops of 

 HC1 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 

 M. 3 



