20 FIXING AGENTS. 



fixation by osmium vapours, and for making up solution of 

 Flemming (or solution of Hermann) which is the form in 

 which osmium is most generally employed. A small quantity 

 of osmic acid may also be made up in 1 per cent, solution in 

 distilled water, and kept carefully protected from dust for use 

 in special cases. Those who have to do a great deal of fixing 

 by means of the vapours may also keep a supply of the solid 

 oxide for this purpose. 



COEI (Zeit. f. wiss. Mik., vi, 4, 1890, p. 442) finds that 

 solutions in distilled water keep perfectly if there be added to 

 them enough permanganate of potassium to give a very 

 slight rosy tint to the liquid. From time to time, as the 

 solution becomes colourless, further small quantities of the 

 salt should be added, so as to keep up the rosy tint. 



27. Fixation by the Vapours. Osmic acid is frequently em- 

 ployed in the form of vapour, and its employment in this form 

 is indicated in most of the cases in which it is possible to 

 expose the tissues directly to the action of the vapour. The 

 tissues are pinned out on a cork which must fit well into a 

 wide-mouthed bottle in which is contained a little solid osmic 

 acid (or a small quantity of 1 per cent, solution will do). 

 Very small objects, such as isolated cells, are simply placed on 

 -a slide, which is inverted over the mouth of the bottle. They 

 remain there until they begin to turn brown (isolated cells will 

 generally be found to be sufficiently fixed in thirty seconds, 

 whilst in order to fix the deeper layers of relatively thick 

 objects, such as retina, an exposure of several hours may be 

 desirable). It is well to wash the objects with water before 

 staining, but a very slight washing will suffice. For staining, 

 methyl-green may be recommended for objects destined for 

 study in an aqueous medium, and, for permanent preparations, 

 alum-carmine, picro-carmine, or haematoxylin. 

 . In researches on nuclei, it is possible and may be useful to 

 jemploy the vapours of a freshly-prepared mixture of osmic 

 and formic or acetic acid (Gilson, La, Cellule, i, 1885, p. 96). 



The reasons for preferring the process of fixation by vapour of osmium, 

 where practicable, are that osmium is more highly penetrating when em- 

 ployed in this shape than when employed in solution, and produces a more 

 equal fixation, and that the arduous washing out required by the solutions is 

 "here done away with. In many cases delicate structures are better pre- 

 served, all possibility of deformation through osmosis being here eliminated. 



