FIXING AND HARDENING. 23 



them. A still better effect is obtained if to the osmic acid 

 tliiM-e be added not only acetic acid, but also chromic acid. 

 For osmic acid has the property of blackening tissues, thus 

 rendering them opaque. Chromic acid counteracts in a 

 considerable degree this blackening action. It also helps 

 probably to bring out the chomatin of nuclei, which is 

 insufficiently fixed by the other two ingredients, and probably 

 also helps in other ways to bring about optical differentiation; 

 so that in the result a much clearer picture is obtained. 



I take it that it has been established by experience that, 

 as a general rule, in order to get the most complete fixation, 

 fixatives should have an acid reaction. Consequently, if 

 they have not naturally an acid reaction, they should be 

 acidified, e.g. osmic acid should be acidified with acetic acid 

 or the like. As a matter of fact, it will be found that 

 acetic acid is very largely employed in mixtures. It is 

 generally held that it acts in them as an agent for facilita- 

 ting penetration and producing differentiation, as explained 

 above, and also for ensuring the fixation of nuclein (if the 

 other ingredients are not adequate thereto) ; but this is 

 probably not all. FISCHER (in the work quoted above, pp. 10, 

 27, and other places) holds that its function in these mixtures 

 is chiefly that of a neutraliser or acidifier (Ansaurer) for 

 ensuring that the other ingredients shall have an acid, or at 

 least a neutral medium to do their work in. For the pre- 

 cipitating power, that is, in his view, the fixing power of a 

 reagent, varies according to the reaction, acid or alkaline, of 

 the things to be fixed ; and a feebly acid reaction is the one 

 most favourable for ensuring precipitation. Many things 

 that are quite unprecipitable by certain reagents whilst in 

 alkaline or neutral solution are immediately precipitated by 

 them if the solution is rendered acid. " Many kinds of cell 

 contents," he says (op. cit., p. 10) " indeed the majority, 

 have an alkaline reaction, and are thereby quite inaccessible 

 to the precipitating action of certain agents, such as osmic 

 acid, or bichromate ; and the action of certain other fixatives, 

 such as platinum chloride and chromic acid, is more or less 

 hindered by the presence of free alkalies. For neither the 

 chromic acid (of solution of Flemming) nor the platinum 

 chloride (of solution of Hermann) would be adequate to act 

 as acidifiers to the osmic acid of the mixtures. They cannot 



