MICROSCOPICAL TECHNIC 421 



It is quite clear that the action of the same fixative differs, depend- 

 ing upon the variation of the electrolyte content of various animals 

 and plants. 



From what has been said, it may be concluded that alkaline solu- 

 tions are not to be considered fixatives, since they produce only 

 swelling. The salts of the light metals are not included among fixa- 

 tives; they usually form reversible gels; but, on the contrary, certain 

 heavy metals as well as acids, especially the acid mixtures, are of 

 great importance. Many have an oxidizing action, as a result of 

 which the organic substances lose their ability to swell. 1 



Acids are obviously employed with the object of changing sols 

 into gels, and since a chemical change must simultaneously occur, it 

 follows that all acids are not available for this purpose and that they 

 must always be employed in high concentration. Hydrochloric acid 

 and sulphuric acid (the latter, at least, never unmixed) are not em- 

 ployed as fixatives, but we do frequently employ nitric acid in 2 to 

 10 per cent concentration. Its use is not general since it frequently 

 alters the stains. For organs with epidermal coverings, nitric acid 

 is unsuitable since it raises the epithelium in blisters from the tissues 

 supporting it. 



Chromic acid (introduced by HANNOVER in 1840) is the oldest and 

 most used fixative and hardening agent for cell protoplasm and nucleus. 

 It is used in concentrations of from 0.33 per cent up to 1 per cent. 

 It is employed preferably in the dark, since daylight causes a sort of 

 tanning of the periphery so that the chromic acid penetrates very 

 slowly and amorphous deposits form very easily in the preparation. 



Objects fixed in chromic acid or its salts become green in time 

 (reduction to chromic oxid) and are poorly stained. Many methods 

 for regenerating the staining capacity of such specimens have been 

 proposed (L. EDINGER and MAYER, B. GRAWITZ). 



Osmic acid (0.5 to 2 per cent) is especially recommended for the 

 fixation of protoplasm and nucleus. It is soluble in fat and conse- 

 quently can penetrate the living cell. It does not penetrate far, 

 however, and on this account is available only for small or thin 

 objects. With the fixation there is a blackening, especially of the 

 fats and some other substances (reduction to colloidal osmium). 

 Opinions on the use of osmic acid are very divergent. Though 

 praised by some, A. FISCHER, as the result of his studies of non- 

 biological material, considers it a weak, unsatisfactory precipitat- 

 ing agent, since it precipitates only acid reacting structures. 



1 To fix tissue practically, the exact directions as they are found in the books 

 must be followed; they are employed just as a cooking recipe would be. Only a 

 few generalizations can be given here. 



