FIXING AND HARDENING AGENTS. 37 



be taken not to allow the crystals to be contaminated by 

 organic matter, in the presence of which the anhydride is 

 readily reduced into sesquioxide. 



Chromic acid is generally employed in aqueous solution. 

 Some observers (KLEIN ; URBAN PRITCHARD ; PERENYI) have 

 recommended alcoholic solutions ; but this would appear to 

 be an irrational practice. 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, - 5 ^y to - per cent, for a few 

 hours. Stronger solutions, such as 5 per cent., should only 

 be allowed to act for a few seconds. 



The objects should be washed out with water before 

 passing into alcohol or staining fluids. Long washing in 

 water is necessary to prepare them for staining, except an 

 anilin stain be used. It is possible to wash out in alcohol, 

 and this may be useful in special cases, but in general I 

 think the practice is not to be recommended. It is well to 

 wash for many hours in running water. 



Mayer notes her eon (Grundzuge) that the operation of 

 washing may be done away with in the following manner : 

 The fixed material is merely rinsed in water and brought 

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

 preferably in the dark (see 41), 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 HC1 to 100 c.c. of 70 per cent, alcohol), 



