OSMIC ACID. 21 



28. Fixation by Solutions. When employed in aqueous solu- 

 tions osmic acid is used in strengths varying from -fa per cent, 

 to 2 per cent. Solutions of per cent, to 1 per cent, have- 

 been very largely used, but the tendency of modern practice 

 seems to be towards weaker solutions and longer immersion. 

 For Infusoria \ per cent, for a few seconds ; for Porifera -^ 

 to rfo per cent, for some hours ; for Mollusca 1 to 2 per cent, 

 for twenty-four hours ; for epithelia -fa to \ per cent, for an 

 hour or two ; for meroblastic ova -^ per cent, for twenty-four 

 hours ; for medullated nerve-fibre ^ to 1 per cent, for from 

 twenty minutes to two hours ; for tactile corpuscles -J- to 1 per 

 cent, for twenty-four hours ; for retina J to 2 per cent, for 

 from ten minutes to twenty-four hours ; for nuclei T V to 2 per 

 cent, for two or three hours. Such figures as these will serve 

 to give a general idea of the practice, whilst more precise in- 

 structions will be given when dealing with the tissues in detail. 

 (The durations here quoted appear to me exaggerated, except 

 for very voluminous specimens.) 



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

 quently 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. 



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

 per cent, solution of osmium 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 g. osmium. This mixture is said to keep 

 indefinitely in a cool place. 



The principle of combining osmium solutions with alcohol is due to 

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

 de la vie org.," p. 76 ; Vignal, Arch, de Physiol, 1884, p. 181). They take 

 equal volumes of 1 per cent, osinic 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-carmine or hsematoxylin. Viallanes has applied 

 this method to the histology of insects. 



29. After-treatment. The osmium must be well washed out 



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