HANVIER'S LEMON-JUICE METHOD. 147 



of muscle) . They are then rapidly washed in water, brought 

 for about twenty minutes into 1 per cent, gold chloride solu- 

 tion, washed again in water, and brought into a bottle con- 

 taining 50 c.c. of distilled water and two drops of acetic acid. 

 They are exposed to the light, and the reduction is complete 

 in twenty-four or forty- eight hours. The preparations thus 

 obtained are good for immediate study, but are not permanent 

 on account of their over-blackening with time, the reduction 

 of the gold being incomplete. In order to obtain perfectly 

 reduced, and therefore permanent, preparations, the reduction 

 should be done in the dark in a few cubic centimetres of dilute 

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

 complete in twenty-four hours. 



227. 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 1*5000 (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. 



228. 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 

 employing a solution of gold chloride of a strength of 1 to 

 2000, acidulated with HC1 (1 drop to 75 c.c.), and perform- 

 ing 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 



