FIXING AND HAUDRNINCx AGENTS. 29 



has the same effect, e.g. ten drops of 5 per cent, solution of 

 sublimate added to 100 c.c. of 1 per cent, solution of osmic 

 acid. 



For the so-called " regeneration " of reduced solutions, 

 see previous editions. 



Osmic acid is found in commerce in the solid form in sealed tubes. 

 The assigned weights should be cliecked, as they may vary greatly 

 (this does not apply to the tubes sent out by Griibler and HoUborn, or 

 Kahlbaum). 



Fixation hy the Vapours. — This 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 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 thei'e until they begin to 

 turn brown (isolated cells will generally be found to be 

 suiBciently 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, 

 picrocarmine, or hsematoxylin. 



In researches on nuclei, it may be useful to employ the 

 vapours of a freshly prepared mixture of osmic and formic 

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



An apparatus for conveniently employing the vapours is described 

 by Andrews {Zeit. wiss. Mile, xiv, 1898, p. 448). 



The reasons for preferring fixation by the vapour ai'e that osmic acid 

 is more highly penetrating in vapour than in solution ; that the 

 arduous washing out required by the solutions is done away with ; and 

 that all possibility of deformation through osmosis is eliminated. 



Fixation by Solutions. — Osmic acid is now very seldom 

 used pure in the shape of solutions. When, however, it is 

 so employed it is used in strengths varying from -^i^ per 

 cent, to 1 per cent. I should say myself that, as a rule, not 



