FIXING AND HARDENING AGENTS. 33 



of beer. MAYER finds that addition of common salt will 

 produce the same effect. 



36. Fixation by the Vapours. Osmic acid is frequently 

 employed 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 (ji rec tly 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 recom- 

 mended 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 employ 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 osmic acid 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. lu many cases delicate structures are better pre- 

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



37. Fixation by Solution. Osmic acid is now very seldom 

 used pure in the shape of solutions, as it has been found to 

 give better results when combined with other ingredients, as 

 in the mixture of FLEMMING. When, however, it is employed 

 in pure aqueous solutions it is used in strengths varying from 

 -^j per cent, to 1 per cent. I should say myself that, as a 

 rule, not more than 0*1 per cent., and never more than 0'5 

 per cent., should be used. 



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