128 The Microscope. 



closet, or covered with black paper, as this reduction goes on 

 much more slowly in darkness than in light. These remarks as 

 to the purity of the water employed will also apply to the pur- 

 ity of reagents with which osmic acid may be mixed. 



This reagent, alone or in combination, is of the highest 

 value in fixing all delicate protoplasmic structures and cell 

 figures. It stains black all fatty substances, and so is of especial 

 service in studying the secretory action of gland cells, and the 

 nervous system. By its use medullated nerve-fibers can be 

 traced with exceeding ease. Unless carefully used and but for 

 a short time, osmic acid interferes with the after staining, some- 

 times blackening the entire object treated until it is worthless. 

 This may usually be arrested by immediately tranferring the 

 specimens from the acid to picrocarmime, or a mixture of picro 

 and borax carmines. When it is desirable to black the object 

 preparatory to other treatment, it may be done in the following 

 manner as suggested by Dr. Mayer. The objects are placed in 

 alcohol (seventy per cent, and ninety per cent.) and chlorate 

 of potash in crystals added in quantities slightly exceeding the 

 bulk of the specimens operated on. A few drops of hydro- 

 chloric acid or nitric acid are added, and the liberated chlorine 

 gas soon bleaches the object which is then transferred to pure 

 alcohol. 



Chromic Acid. — This valuable histological reagent occurs 

 in commerce as red needle-like crystals, which to be sufficiently 

 pure for biological uses, should not contain more than a trace of 

 sulphuric acid. These crystals are very deliquescent, and when 

 exposed to the air quickly absorb water, and undergo various 

 chemical changes. For this reason they should be kept tightly 

 sealed, or as some prefer, a ten per cent, aqueous solution may 

 be made and diluted as required at time of using. In all cases 

 the solution used for killing or hardening should be made of a 

 known percentage of strength, and not judged by the color, a 

 careless procedure which has no excuse. 



For merely killing tissues, preparatory to hardening with 

 other reagents, a very weak solution, one-tenth to three-tenths 

 per cent, is employed for a short time, one-half to three hours 

 sufficing, according to the size and nature of the object. For 

 hardening, a one-half to one per cent, may be employed, and as 



