CHROMIC ACID. 28 



Chromic acid is employed in solution either in water or in 

 alcohol. 



The most useful strengths in which it is employed in aqueous 

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

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

 weaker solutions are taken, -^ih to th per cent, for a few 

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

 be allowed to act for a few seconds. 



The object 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. 



Tissues that have been fixed in chromic acid may be stained 

 in aqueous solutions if desired, as water does not appear to 

 have an injurious effect on them; the acid appears to enter 

 into some chemical combination with the elements of the 

 tissues, forming with them a compound that is not affected 

 either physically or chemically by water. The best stain to 

 follow chromic acid is haematoxylin, or, for sections, some 

 anilin stain. But the previous washing out with water must 

 be very thorough if good results are to be insured ; it may 

 take days. 



Chromic acid is not a very penetrating reagent, and for this 

 reason, as well as for others, is seldem used pure, but plays an 

 important part in the mixtures described below, of which the 

 chief is certainly the mixture of Flemming. A chief objection 

 to the use of chromic acid is that it precipitates certain of the 

 liquid albuminoids of tissues in the form of filaments or net- 

 works, which are often of great regularity, and simulate struc- 

 tural elements of the tissues. This objection applies to all 

 mixtures into which chromic acid enters. 



Action of light on alcohol containing chromic objects. When objects 

 that have been treated by chromic acid or a chromate are put into alcohol 

 for hardening or preservation, it is found that after a short time a fine pre- 

 cipitate is thrown down on the surface of the preparations, thus forming a 

 certain 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 been 

 found by Hans Virchow (Arch.f. mik. Anat., Bd. xxiv, 1885, p. 117) that 



