FIXING AND HARDENING AGENTS. 35 



have succeeded in satisfactorily restoring the staining sus- 

 ceptibility of osmium material by means of sulphurous acid 

 (obtained by adding hydrochloric acid to bisulphide of 

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

 cent, solution of the salt). But perhaps the most convenient 

 method is the original chlorate of potash method of MAYER, 

 for which see under " Bleaching." 



FOL (Lefor6.,p. 174) recommends a weak aqueous solution of ferricyanide 

 of potassium. MAYER (Grundzilge, p. 27) notes hereon that he has had 

 tolerable results with it, though not with the ferrocyanide, and objects that 

 the ferricyanide only acts in aqueous solution, not in alcoholic. He objects 

 to peroxide of hydrogen the instability of its solution, and adds that the 

 peroxide of sodium has other disadvantages. 



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. 



39. Characters of the Fixation with Osmic Acid. In general 

 osmic acid, especially when used in the form of vapour, fixes 

 protoplasm faithfully, nuclei badly, and there are other 

 drawbacks over and above those before mentioned ( 27). 

 The penetrating power of the solutions is very low, so that if 

 any but very small pieces of tissue be taken the outer layers 

 become over-fixed before the action of the reagent has pene- 

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

 homogeneous, glassy, or colloid look, owing to all their con- 

 stituents having been raised by coagulation to so high an 

 index of refraction that little or no detail is visible in them. 

 They stain very badly, or not at all. Such cells are known 

 as " osmicated cells, oxmirte Zellen." There is no remedy 

 for this state of things if once it has occurred. For this 

 reason it is important to avoid using stronger solutions than 

 is necessary. The danger of osmication is lessened by using 

 the osmic acid in conjunction with certain other reagents, 

 such as chromic acid. But it is not thereby entirely 

 removed ; FLEMMING'S mixture, especially the strong formula, 

 will readily osmicate superficial cells if care be not taken. 

 For ordinary histological work osmication of superficial layers 

 is not of much consequence. But for cytological work care 

 should be taken not to draw conclusions as to the structure 

 of cells from osmicated specimens, and attention should be 

 confined to cells four or five layers deeper down, which will 



