22 FIXING AGENTS. 



before proceeding to any further steps in preparation ; water 

 should be used for washing. Notwithstanding the greatest 

 care in soaking, it frequently happens that some of the acid 

 remains in the tissues, and causes them to over-blacken in 

 time. To obviate this it is necessary to wash them out in 

 ammonia- carmine or picro-carmine, or to soak them for twenty- 

 four hours in a solution of bichromate of potash (Miiller's 

 solution or Erlicki's will do), or in 0'5 per cent, solution of 

 chromic acid, or in Merkel's solution, or in a weak solution of 

 ferrocyanide of potassium or cyanide of potassium. The 

 treatment with bichromate solutions has the great advantage 

 of highly facilitating staining with carmine or haematoxylin. 

 Max Schultze recommended washing, and mounting perma- 

 nently in acetate of potash; but I believe the virtues attributed 

 to this method are illusory. Fol has lately recommended 

 treatment with a weak solution of carbonate of ammonia. 

 But the best plan of all is to properly bleach the preparations 

 (see " Bleaching"). This is perhaps most coveniently done 

 (as recommended by FOL, BEASS, and OVEETON) by means of 

 peroxide of hydrogen, which regenerates the osmium to osmic 

 acid. OVEETON (Zeit. f. wiss. Mik., vii, 1, 1890) finds that 

 bleaching is completed in a few minutes in a mixture of 1 part 

 commercial peroxide of hydrogen with 10 to 25 parts 70 per 

 cent, alcohol. (The commercial peroxide, slightly acidulated 

 with HC1, will keep well in the dark ; but the mixture with 

 alcohol must be made fresh for use.) 



The same stains recommended for objects fixed by vapour will be found 

 useful here, with the addition of ammonia-carmine, which is really very use- 

 ful for strongly fixed specimens. For sections, of course in both cases, 

 saf ranin and the other nuclear anilin stains may be employed with advantage. 



Osmic acid stains all fatty structures black ; it must therefore be avoided 

 .for tissues in which much fat is present ; or if not, the preparations must be 

 subsequently very thoroughly bleached. 



30. 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 CrO 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. 



