32 CHAP PER IV. 



NICOLAS (Intern. Monatsschr., 1891, p. 3) adds A- per cent, of osmic 

 acid to nitric acid of 3 per cent. I have employed a similar mixture 

 and not had good results, though I find the mixture keeps perfectly. 



BUSCH (Neurol. Central])., xvii, 1898, No. 10, p. 476 ; Zeit. wiss. Mik., 

 xv, p. 373) finds that the penetration of osmic acid is enhanced by 

 combining it with iodate of sodium, which by hindering its too rapid 

 decomposition in the tissues ensures a more energetic action in the 

 deeper layers. He adds 3 per cent, of sodium iodate to a 1 per cent, 

 solution of osmic acid. 



UNNA (Monatschr. prald. Deim., xxvi, 1898, p. 602) adds 1 per cent, 

 of alum to a 1 per cent, solution. For some mixtures of KOLOSSOW, 

 see 5th ed., or Zcit. wiss. Mllcr., v, 1888, p. 51, and ix, 1892, p. 39. 



37. Chromic Acid. Chromic anhydride, Cr0 3 , is found in 

 commerce in the form of red crystals that dissolve readily in 

 water, forming chromic acid, H 2 Cr0 4 . These crystals are 

 very deliquescent, and it is therefore well to keep the acid 

 in stock in the shape of a 1 per cent, solution. Care must 

 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 PRITC HARD; PERENYI) have ro- 

 commended 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 ;H|IKM>IIS 

 solution are from O'l 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 % 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 tin- alcohol docs not prevent it 



