38 CHAPTER IV. 



in which after a short time they become almost white, and 

 will stain excellently with any of the usual stains. Unna 

 (Arch. f. mik. Anat., xxx, 1887, p. 47, see Journ. R. Mic. 

 8oc., 1887, p. 1060) holds that the chrome is present in the 

 tissues in the form of chromic chromate, and removes it by 

 treatment with peroxide of hydrogen. Overton (Zeit. f. 

 wiss. Mik., vii, 1890, p. 9, employs a weak solution of sul- 

 phurous acid, which converts it into a sulphate. See also 

 the directions for bleaching osmic acid preparations, 38. 



Tissues that have been fixed in chromic acid are usually 

 stained in aqueous solutions, as it is held that water does 

 not have an injurious effect on them ; the acid entering 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. But there is reason 

 to doubt whether the hereby alleged insolubility of the 

 elements is as thoroughgoing as is generally believed ; see, 

 for instance, the paper of TULLYESNICZKY, Arch.f. mik. Anat., 

 lii, 1898, p. 221. 



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, 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 now seldom used pure 

 for fixing, but plays an important part in the mixtures 

 described below, of which the chief is certainly the mixture 

 of Flemming. - 



For prolonged hardening chromic acid is generally employed 

 in strengths of 4- per cent, to | per cent., the immersion 

 lasting a few days or a few weeks, according to the size and 

 nature of the object. Mucous membrane, for instance, will 

 harden satisfactorily in a few days ; brain will require some 

 six weeks. 



Large quantities of the solution must be taken (at least 

 200 grammes for a piece of tissue of 1 centimetre cube 

 Ranvier) . 



In order to obtain the best results you should not employ 

 portions of tissue of more than an inch cube. For a human 

 spinal cord you should take two litres of solution, and change 



