FIXING AND HAIIDIONING. 21 



property of combining with certain colouring matters, and 

 thus affording important stains which could not otherwise be 

 obtained ; or in other words, of acting as mordants. 



But it is sometimes a disadvantage, inasmuch as these 

 same compounds which render possible the production of 

 some stains are hindrances to the production of others. 

 Tissues that have been fixed with osniic or chromic acid or 

 its salts are in general not easily to be stained with carmine 

 or similar colouring matters, unless the metals have been 

 previously removed by special chemical treatment ; though 

 they may generally be stained with hasmalum, or, after 

 sectioning, with iron haamatoxylin or tar colours. 



According to Fischer (^Fixiruiig, Fdrbuiuj, nnd Bait des 

 Protoplasma-', Jena, Gr. 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. ]3ut all the liquid ones, in so far 

 as they are fixed at all, are visibly precipitated in special 

 precipitation forms, which vary according to the precipitant. 

 Each fixing agent gives its own characteristic /ia;((iio?i image, 

 which may be more or less lifelike, but can never be 

 absolutely so. Fischer gives copious descriptions of the pre- 

 cipitation 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 fixa- 

 tion by precipitation that the most energetic fixing agents 

 should always be found amongst the most energetic precipi- 

 tants. 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. cif. 

 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 



