METALLIC STAINS (IMPREGNATION METHODS). 253 



gold is effected either by the action of daylight in acidulated 

 water, or in the dark in dilute formic acid (one part of the 

 acid to four parts of water). 



The object of boiling the mixture of gold chloride and 

 formic acid is this, that " by boiling in the presence of the 

 acid the gold acquires a great tendency to reduction, and 

 for this reason its selective action on nervous tissues is 

 enhanced/' 



353. KANVIER'S Lemon-juice Method (Trait<\ p. 831). 

 RAXVIER finds that of all acids lemon juice is the least hurtful 

 to nerve-endings. He therefore soaks pieces of tissue in 

 fresh lemon juice, filtered through flannel, until they become 

 transparent (five or ten minutes in the case of muscle). 

 They are then rapidly washed in water, brought for about 

 twenty minutes into 1 per cent, gold chloride solution, 

 washed again in water, and brought into a bottle containing 

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

 are exposed to the light for twenty-four to forty-eight hours. 

 The preparations thus obtained are good for immediate study,, 

 but are not permanent, the reduction of the gold being in- 

 complete. In order to obtain perfectly reduced, and there- 

 fore permanent, preparations, the reduction should be done 

 in the dark in a few cubic centimetres of dilu e formic acid 

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

 twenty-four hours. 



354. VIALLANE'S 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 25 per cent, 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 25 per cent, formic acid. According 

 to my experience this is an excellent method. 



355. 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 



