116 METALLIC STAINS. 



formic acid (1 part acid to 4 of water). The reduction is 

 complete in twenty-four hours. 



210. Viallanes' Osmic-acid Method (Hist. et. dev. des Insectes, 

 1883, p. 42). The tissues are treated with osmic acid (1 per 

 cent, solution) until they begin to turn brown, then with 

 one-fourth formic acid for ten minutes ; they are then put 

 into solution of chloride of gold of T5000 (or even much 

 weaker) for twenty-four hours in the dark, then reduced 

 in the light in one-fourth formic acid. According to my 

 experience, this is a very excellent method, both the fixation 

 by osmic acid, and the great dilution of the gold solution, 

 being features likely to be of advantage in many cases. 



211. Other Methods. The numerous other methods that have 

 been proposed differ from the foregoing partly in respect of 

 the solutions used for impregnation, but chiefly in respect of 

 details imagined for the purpose of facilitating the reduction 

 of the gold, and rendering it as complete as possible. 



Thus BASTIAN modified Cohnheim's original method by em- 

 ploying a solution of gold chloride of a strength of 1 to 2000, 

 acidulated with HC1 (1 drop to 75 c.c.), and performing the 

 reduction in a mixture of equal parts of formic acid and water, 

 kept warm ; heat being an agent that furthers reduction. 



HENOCQUE (Arch, de VAnat. et de la Physiol., 1870, p. Ill) 

 impregnates in a 0*5 per cent, solution of gold chloride, 

 washes in water for twelve to twenty-four hours, and reduces, 

 with the aid of heat, in a nearly saturated solution of tartaric 

 acid. The tartaric acid solution must be contained in a well- 

 stoppered bottle. The best temperature for reduction is 40 

 to 50 C. Reduction is effected very rapidly, sometimes in a 

 quarter of an hour. 



This process has been described as the method of CHRSCHT- 

 SCHONOWIC (Arch.f. mik. Anat., vii, 1872, p. 383). 



HOYER (Arch. mik. Anat., ix, 1873, p. 222) proceeds as 

 follows: (For corneal nerves.) The double chloride of gold 

 and potassium has the following advantages over the simple 

 gold chloride. It is more easy to be obtained of unvarying 

 composition, it is more perfectly neutral, and its solutions are 

 more perfectly stable. It is used in solutions of the same 

 strength as chloride of gold, viz. 0*5 per cent. CorneaB 

 must be very thoroughly imbibed with the solution. Small 



