16 FIXING AND HARDENING 



coagulated by the reagent the refractive indices of some of its elements 

 will have been raised to above that of balsam, the chromatin of the 

 nuclei will be brought out, and other structures be revealed where none 

 was visible before. 



29. The Action of Fixing Agents consists in coagulating and 

 rendering insoluble certain of the constituents of tissues. 



According to Fischer [Fixirung, Fdrbung, und Bau des Proto- 

 jplasmas, 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 precipitatio7if or jns, 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 coiDious descriptions of the precipitation forms of 

 the chief organic compounds found in tissues, and of the precipita- 

 tion 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 incom- 

 plete 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 coagulation brought about by them is in part accom- 

 panied 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 /i in diameter, while corrosive sublimate of 7 per cent, 

 causes granules of 0-4 to 1 /x in size ; one might be led away, as 

 was Fischer, to consider that Altmann's fluid used on cells there- 

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



