FIXING AND HARDENING. 21 



enhanced optical differentiation, hardening' will be super- 

 induced, and the one stage will run into the other. 



27. The Action of Fixing Agents consists in coagulating' 

 and rendering insoluble certain of the constituents of tissues. 

 This is effected sometimes without any chemical action being- 

 involved, as when alcohol is employed, which acts by simple 

 withdrawal of the water of the tissues. But in the majority 

 of cases the fixing agents enter into chemical combination 

 with certain of the elements of the tissues. The compounds 

 thus formed are sometimes unstable and soluble, so that they 

 are removable by washing, as is the case with several of 

 those formed by picric acid. It is found in practice, how- 

 ever, that those formed by chromic acid and its salts, and 

 the salts of the heavy metals, as mercury, iron, platinum, 

 gold, and silver, are mostly insoluble ; but unfortunately we 

 have no exact chemical knowledge of them. 



The insolubility of these bodies is an advantage from the 

 point of view explained in the last section, in that it ensures 

 that the tissues shall not be robbed of their essential con- 

 stituents, nor deprived of their desired consistency and 

 optical differentiation, by the reagents subsequently em- 

 ployed. It is also sometimes an advantage in that certain 

 of the compounds in question have the 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 osmic 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 (see 

 40, and BLEACHING) ; though they may generally be stained 

 with hasmalum, or, after sectioning, with iron hsematoxylin 

 or tar colours. 



27a. Fixation Images and Precipitates. - - According to 

 FISCHER (FLmrung, Fdrbuny, und Bau des Protoplasmas, Jena, 

 G. Fischer, 1899, 'pp. x, 362) the coagulation which con- 

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



