MODIFICATIONS OF GOLGl's METHOD. 373 



the silver nitrate, and formation of a brown precipitate of bi- 

 chromate of silver (K 2 Cr 2 7 + 2AgN0 3 =Ag 2 Cr 2 7 + 2KNO H ; 

 the precipitate is brown by reflected light, but appears black 

 by transmitted light). The problem is to preserve this pre- 

 cipitate in the tissues free from chemical or molecular change. 

 And the problem is not an easy one ; without special precau- 

 tions the preparations will not resist the processes necessary 

 for imbedding, will not always resist those necessary for 

 merely mounting in balsam, and even then may easily " go 

 bad" after they have been mounted for a short time. 



708. SEHRWALD, in an elaborate paper (Zeit. f. wiss. Mik., 

 vi, 4, 1889, p. 443), develops the theory that the deterioration 

 of the preparations is due to solution of the precipitate in the 

 reagents employed in the ulterior processes of preparation 

 in water, in chlorides (which may be present in alcohol), in 

 the xylol, in the paraffin, in the very balsam itself. He made 

 elaborate attempts (fruitless, but instructive) to turn the diffi- 

 culty by transforming the soluble chromic salt of silver into 

 some insoluble compound. He tried to replace the acid of 

 the salt ; he tried to replace the metal of the salt ; he tried to 

 make a sulphide of it ; he tried to reduce it to the metallic 

 state with photographic developing solutions (and had some 

 success with hydroquinone, but does not recommend the pro- 

 cess on account of its giving rise to deceptive precipitates 

 that simulate organised elements). None of these experi- 

 ments gave the desired result ; and the upshot of the paper is 

 that it is better not to attempt to modify the state of the pre- 

 cipitate, but rather so to modify the preparation liquids as to 

 reduce their solvent action on the precipitate. This may be 

 done by supersaturating them with bichromate of silver before 

 using. The powdered salt should be added to each of them 

 in excess, including the clearing media, the medium used for 

 fixing the sections to the slide, the paraffin itself, the solvent 

 used for removing it, and the balsam, and made to dissolve 

 by the aid of heat. By this means thin sections may be pre- 

 pared and mounted in balsam without injury to the stain. 

 Sehrwald does not say how long they will keep. 



One of the annoyances of Golgi's process is that it fre- 

 quently gives rise to the formation at the periphery of the 

 preparations of voluminous precipitates that are destructive 



