32 CHAPTER IV. 



my osmium in the shape of a 2 per cent, solution of osmic 

 acid in 1 per cent, aqueous chromic acid solution. This 

 solution serves for fixation by osmium vapours, and for 

 making up solution of Flemming, which is the form in which 

 osmium is most generally employed. A small quantity of 

 osmic acid may also be made up in 1 per cent, solution in 

 distilled water, and kept carefully protected from dust for 

 use in special cases. Those who have to do a great deal of 

 fixing by means of the vapours may also keep a supply of 

 the solid oxide for this purpose. 



GRUBLEE AND HOLLBOEN now send out osmic acid in tubes 

 containing one tenth of a gramme. 



COEI (Zeit. f. wiss. Mik. } vi, 4, 1890, p. 442) finds that 

 solutions in distilled water keep perfectly if there be added 

 to them enough permanganate of potassium to give a very 

 slight rosy tint to the liquid. From time to time, as the 

 solution becomes colourless, further small quantities of the 

 salt should be added, so as to keep up the rosy tint. 



35. Regeneration of Reduced Solutions. BRISTOL (Amer. 

 Nat., xxvii, 1893, p. 175; Journ. Roy. Mic. Soc., 1893, p. 

 564; Pef. Handbook Ned. Sci., Supp., p. 442) says that 

 reduced solutions may be regenerated by oxidising them by 

 means of peroxide of hydrogen. The reaction is stated to 

 be identical with that which takes place in the bleaching of 

 osmium-blackened tissues by peroxide. It is admitted that 

 the tetroxide of osmium, Os0 4 , is reduced by contact with 

 organic matter into the deutoxide, Os0 3 . Then Os -r 

 2H 3 3 = Os0 4 + 2H 3 O. 



According to Bristol, for regenerating 100 c.c. of 1 per 

 cent, solution of osmic acid (erratim 10 per cent, in Journ. 

 Roy. Mic. /Soc.), ten to twenty drops of fresh peroxide solu- 

 tion should be added. 



KOLOSSOW (Zeit. f. wiss. Mik., ix, 1, 1892, p. 40) says that 

 half -reduced solutions, so long as they have not lost their 

 characteristic odour, may be clarified by the addition of a 

 little powdered potash -alum. 



But this is evidently only a process of clarification, not 

 of regeneration ; the alum acting mechanically by carrying 

 down the suspended matter, as isinglass does in the "fining " 



