CHAPTER III. 19 



some more than that of others, thus producing optical differentiation 

 where there was little or none before. Compare the aspect of the 

 epithelium of the tail of a living tadpole, observed in water, with its 

 aspect after the action of a little diluted solution of Flemming. In 

 the living state the protoplasm of its cells has a refractive index 

 little superior to that of water, and consequently so low an index of 

 visibility that hardly any structure can be made out in the object. 

 But as soon as the protoplasm has been sufficiently 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. 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, 

 however, 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. 



The insolubility of these bodies is an advantage in that it ensures 

 that the tissues shall not be robbed of their essential constituents, 

 nor deprived of their desired consistency and optical differentiation, 

 by the reagents subsequently employed. 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 their 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 hsemalum, or, after 

 sectioning, with iron hsematoxylin or tar colours. 



According to FISCHER (Fixirung, Fdrbung, und Bau des Proto- 



