24 CHAPTER III. 



penetration it gives only very bad results. Osmic acid is 

 hopelessly deficient in power of penetration, and no admixture 

 with other substances has been successful in curing 1 this 

 defect. So that whenever a fair degree of penetration is 

 required, some other reagent must be resorted to. Picric 

 acid is one of the most penetrating fixatives known, but its 

 hardening power is very slight, so that in order to produce 

 the best results it ought to be combined with some more 

 energetically hardening reagent. Corrosive sublimate is 

 very penetrating and hardens very energetically, but in no 

 form in which I have tried it does it give the fine optical 

 differentiations that are obtained by means of the osmic 

 mixtures. Bichromate of potash is an admirable preserva- 

 tive of protoplasm, but is not very penetrating, and does not 

 properly preserve the chromatin of nuclei, causing it to 

 swell. This defect may be overcome by combining it with 

 sufficient acetic acid ; but the defect of want of penetration 

 will still remain. 



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

 as a general rule, in order to get the best results, all fixatives 

 should have an acid reaction. Consequently, if their chief 

 ingredients 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 facilitating penetration and producing differentiation, 

 as explained above, and also for ensuring the fixation of 

 nuclein (if the other ingredients are not adequate thereto, as, 

 for example,, in the case of bichromate of potash) ; but this 

 is probably not all. FISCHER (in the work quoted '2'ia, 

 pp. 10, '27, and other places) holds that its function in 

 these mixtures is chiefly that of a neutralise?" 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 precipitating power, that is in his view the fix- 

 ing power of a reagent, varies according to the reaction, 

 acid or alkaline, of the things to be fixed; and a weakly 

 acid reaction is the one most Favourable for ensuring pre- 

 cipitation. .Many things that are quite unprecipitable by 

 certain reagents whilst in alkaline or neutral solution are 



