MINERAL ACIDS AND THEIR SALTS 31 



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 pre- 

 parations 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 (Grundziige, 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 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. See also Edinger {Zeit. iviss. 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 chromium chromate, and removes it by treatment with peroxide 

 of hydrogen. Overton (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. 



Tissues that have been fixed in chromic acid may 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. 



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

 reason, as well as for others, is now seldom used pure ior fixing. 



For prolonged hardening it is generally employed in strengths 

 of 1^ to 1^ 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 grm. 

 for a piece of tissue of 1 cm. 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 2 litres of solution, and change it for 



