MINERAL ACIDS AND THEIR SALTS 29 



mixture of 1 part commercial peroxide with 10 to 25 parts 70 

 per cent, alcohol. The commercial peroxide, slightly aciduated 

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

 alcohol must be made fresh for use. According to Bristol 

 {Ainer. Natural., xxvii, 1893, p. 176) the peroxide acts best in the 

 sun. BiNET {Journ. de VAnat. et de la Physiol., xxx, 1894, p. 449) 

 has successfully used permanganate of potash. Mann {Methods, 

 etc., p. 83) takes a solution of 0-25 per cent., and treats the browned 

 tissues with 1 part of saturated solution of sulphurous acid to 9 

 of normal salt solution. . . . Monckeberg and Bethe {Arch. 

 Mik. Anat., liv, 1899, p. 135) have succeeded in satisfactorily 

 restoring the staining susceptibility of osmium material by 

 means of sulphurous acid (obtained by adding hydrochloric acid 

 to bisulphite of sodium, 2 to 4 drops of the acid added to 10 c.c. 

 of a 2 per cent, solution of the salt). 



FoL {Lehrb., p. 174) recommends a weak aqueous solution of ferri- 

 cyanide of potassium. 



Lee finds that sulphate of iron solution used in Benda's haematoxylin 

 stain has a marked bleaching effect, and so also, though in a less degree, 

 the iron alum of Heidenhain's process. 



Altmann {Die Elementarorganismen, pp. 33 and 35) puts sections 

 overnight into gold chloride of 2 per cent., and reduces in formic acid 

 in the sun, and removes the gold by iodised alcohol. 



But perhaps the best plan is the chlorine method of Mayer, or 

 his magnesium peroxide, for both of which see " Bleaching." 



The same stains recommended for objects fixed by the vapours 

 will be found useful here. For sections, of course, in both cases 

 safranin and other anilin stains may be employed with advantage, 

 as may haematoxylin. 



In general, osmic acid, especially when used in the form of vapour, 

 fixes protoplasm very faithfully, nuclei badly. It is pre-eminently a 

 fixative of the hyaloplasm or enchylema of cells. The penetrating 

 power of the solution is very low, so that if any but very small pieces 

 of tissue be taken the outer layers become over-fixed before the reagent 

 has penetrated to the deeper layers. Over-fixed cells have a certain 

 homogeneous, glassy, or colloid look, and are unfit for study, and 

 attention should be confined to cells four or five layers deeper down, 

 which will generally be found to present the required intensity of 

 fixation. In these the fixation is admirable, with no shrinkage and 

 next to no swelling of anything. See also § 31. 



41. The Osmium Tetroxide Reaction. Mann believed that 

 during the osmic reaction on fatty substances the OSO4 was 

 reduced to osmium tetra-hydroxide Os(OH)4. Other observers 

 have assumed the reaction to be the reduction of the OSO4 to 

 some lower oxide. The matter has recently been reviewed by 

 Professor J. R. Partington and Mr. D. B. Huntingford who, 

 find that the reduced substance is a hydrated form of OsOg 

 possibly OsOg, oH^O, or OsOg, 6H2O. In all probability, 



