36 CHAPTER IV. 



generally be found to present the desired intensity of 

 fixation. 



Osmic acid stains certain fatty bodies black; it should therefore be 

 avoided for tissues in which much fat is present ; or if not, the preparations 

 should be subsequently very thoroughly bleached, or the blackened fat 

 may be afterwards dissolved out by means of oil of turpentine. See Fat. 



According to ALTMANN, STAEKE, and HANDWEECK, only free oleic acid 

 and olein are directly blackened by osmic acid, stearin and palmitin, and 

 stearic and palmitic acid are only browned by it, with an after-blackening 

 which is produced by subsequent treatment with alcohol. Neither reaction 

 occurs with the fatty bodies in the solid state, and can only be obtained when 

 they are either in a state of fusion or solution (from the paper of HAND- 

 WEECK in Zeit.f. wiss. Mik., xv, 2, 1898, p. 177). 



39a. Osmic Mixtures. The chief osmic mixtures are those 

 of FLEMMING and of HERMANN, for which see 46, 47, and 

 50. The following, however, may be mentioned here. 



KANVIEE ET VIGNAL (RANVIEB, Leg. d'Anat. Gen., " App. term, des 

 muscles de la vie org.," p. 76 ; VIGNAL, Arch, de Physiol., 1884, p. 181) 

 take equal volumes of 1 per cent, osmic acid and 90 per cent, alcohol (freshly 

 mixed). They wash out in 80 per cent, alcohol, then wash with water and 

 stain for forty-eight hours in picro-carrnine or hsematoxylin. Viallanes has 

 applied this method to the histology of insects. 



KOLOSSOW (Zeit. f. wiss. Mik., v, 1, 1888, p. 51) has recommended a 0'5 

 per cent, solution of osmic acid in 2 or 3 per cent, solution of nitrate or 

 acetate of uranium, as having a greatly enhanced penetrating power. 



He has more lately (op. cit., ix, 1, 1892, p. 39) recommended for the same 

 reason a mixture of 50 c.c. absolute alcohol, 50 c.c. distilled water, 2 c.c. 

 concentrated nitric acid, and 1 to 2 grin, osmic acid. This mixture is said to 

 keep indefinitely in a cool place. 



BUSCH (Neurol. Centralb., xvii, 1898, No. 10, p. 476 ; Zeit. f. wiss. Mik., 

 xv, 3, p. 373) holds that the penetration of osmic acid is enhanced by com- 

 bining it with iodate of sodium, which by hindering its too rapid decompo- 

 sition in the tissues ensures a more energetic action in the deeper layers. 

 He adds.3 per cent, of sodium iodate to a l,per cent, solution of osmic acid. 



This mixture appears to me rational, whilst the above-mentioned mixtures 

 with alcohol do not. Alcohol is a reducing agent, and therefore surely in- 

 compatible with so easily reducible a substance as osmic acid, which should 

 rather be combined with oxidising agents. Now sodium iodate is a power- 

 ful oxidising agent, and so far seems quite indicated. 



40. 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 



