20 FIXING AND HARDENING. 



plasmas, Jena, G. Fischer, 1899), the coagulation which constitutes 

 fixation is, in the case of the liquid and semi-liquid constituents of 

 tissues, always a phenomenon of precipitation. The more solid 

 constituents (such as fibrils that are visible during life, nucleoli, and 

 the like) he admits may be acted on by fixing reagents without the 

 formation of any visible precipitates. But all the liquid ones, in so 

 far as they are fixed at all, are visibly precipitated in special precipi- 

 tation forms, which vary according to the precipitant. Each fixing 

 agent gives its own characteristic fixation image, which may be more 

 or less lifelike, but can never be absolutely so. Fischer gives copious 

 descriptions of the precipitation forms of the chief organic compounds 

 found in tissues, and of the precipitation powers of the chief fixing 

 agents, which the reader will do well to study. 



It seems to be a consequence of Fischer's theory of fixation by 

 precipitation that the most energetic fixing agents should always be 

 found amongst the most energetic precipitants. But on the showing 

 of his experiments this is not so. For instance, it is allowed on all 

 hands that osmic acid is a most energetic fixative. But Fischer 

 finds (op. cit., pp. 12 14, 27) that it is a very incomplete and weak 

 precipitant. Or, to take a contrary instance, he finds that picric 

 acid is an energetic precipitant of the majority of cell constituents ; 

 but surely every cytologist must admit that it is not a highly energetic 

 fixative ! 



It would seem to follow, from these instances and from other 

 similar ones, that Fischer's tables of precipitating power cannot be 

 taken as a measure of the fixing power of the reagents. And further, 

 the study of the fixation images of tissues afforded by osmic acid, 

 formaldehyde, and other reagents, seems to show that the coagula- 

 tion brought about by them is in part accompanied by the formation 

 of visible precipitates, but in part not so, and that they may do their 

 work to a larger extent than he seems to admit through a homogeneous 

 coagulation. Fischer, studying the effects of certain fixatives on 

 albumose, states that mixing 10 per cent, slightly acid deutero- 

 albumose with Altmann's bichromate-osmic fluid causes a precipitate 

 of granules of from 1 to 3 ^ in diameter, while corrosive sublimate 

 of 7 per cent, causes granules of 0-4 to 1 ju. in size ; one might be led 

 away, as was Fischer, to consider that Altmann's fluid used on cells 

 therefore causes artifacts to appear. As a matter of fact corrosive 

 sublimate is much more dangerous than Altmann's fluid, in this 

 respect, while Altmann's fluid merely preserves cell granules which 

 are visible intra vitam. The ground protoplasm after corrosive is 

 more granular and coarsely reticulate than after Altmann ; this 



