34 CHAPTEll IV. 



A little acetic or formic acid (0'5 to 1 per cent.) may 

 generally with advantage be added to the solutions just 

 before using. 



If solutions made with pure water be used, they must be 

 kept protected from the light during the immersion of tissues. 

 This precaution is not necessary if Flemming's or Hermann's 

 solution be used. If the immersion is to be a long one the 

 tissues must be placed with the solution in well- closed vessels, 

 as osmium is very volatile. 



38. After-treatment. The excess of osmic acid must be well 

 washed out before proceeding to any further steps in prepara- 

 tion ; 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, and in any case hinders staining. 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. The treatment with bichromate 

 solutions has the great advantage of highly facilitating 

 staining with carmine or hsematoxyiin. Max Schultze 

 recommended washing, and mounting permanently in acetate 

 of potash but I believe the virtues attributed to this method 

 are illusory. Fol has 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 may be done (as recommended by FOL, BRASS, and 

 OVERTON) by means of peroxide of hydrogen, which regenerates 

 the osmium to osmic acid. OVERTON (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.) CARAZZI'S peroxide of sodium may be found con- 

 venient for this purpose. BINET (Journ. de I'Aiiat. et de la 

 PhysioL, xxx, 1894, p. 449) has successfully used perman- 

 ganate of potash. MONCKEBERG and BETHE (Arch. mik. 

 Anat., liv, 1899, p. 135; Zeit. f. wiss. Mik., xvi, 2, p. 244) 



