FIXING AND HARDENING AGENTS. 31 



Mamn {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. . . . 

 MoNCKEBEEG and Bethe (^Arch. Mile. 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 fevri- 

 cyanide of potassium. 



I find the sulphate of iron solution used in Benda's ha3matoxylin 

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

 degree, the iron alum of Heidenhain's process. 



Altmann {Die Elemeniarorganisnien, pp. 33 and 36) 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 em- 

 ployed with advantage, as may hsematoxylin. 



In general osiuic 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 poiver of the solution is very low, so 

 that if any but very small pieces of tissue be taken the outer 

 layei'S 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. 



36. Osmic Mixtures.— Ranvier bt Vignal (Ranvier, iep. 

 d'Anat. Gen., 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 

 find stain for forty-eight hom-s in picro-carmine or hsematoxylin. 



