78 REVIEW OF THE GOLGI METHOD. 



rules for the hardening other than those which must usually be 

 followed to obtain a good uniform hardening, yet it is this part of 

 the process which requires the most care. This is the more so 

 because the time necessary to harden the pieces to the degree 

 required for the action of the second reagent varies very consider- 

 ably according to different circumstances and especially according 

 to the temperature. 



" For the first immersion of the pieces, I use either a simple 

 two per cent, solution of potassium bichromate or the usual for- 

 mula of Miiller. (The reagents should be pure.) There must be 

 an abundant quantity of fluid in proportion to the quantity of 

 pieces to be hardened. 



" The part of the brain or spinal cord to be treated is cut into 

 tolerably small pieces (about i to i|- ccm.). It is important that 

 the pieces be fresh ; the fresher the pieces, the better the results. 

 It is well to use, preferably, the brains of animals just killed, yet 

 satisfactory results can also be obtained twenty-four to forty-eight 

 hours after death. It is hardly necessary to say that the pieces 

 must be cut regularly and in definite directions (according to the 

 part to be studied) so as to permit orientation as to the part and 

 the location of the elements in the future study. 



" T'hat the hardening may proceed with some rapidity and be 

 uniform, it is well to successively increase the concentration of the 

 fluid, raising the quantity of bichromate from 2 per cent, to 2^, 3, 

 4, and 5 per cent. 



" Whether the fluid is increased in strength in hardening the 

 pieces, or remains the same strength, it is always necessary to 

 change it from time to time to avoid the formation of moulds, 

 which, as is well known, develop abundantly in bichromate solu- 

 tion when the pieces are to some extent neglected. For the same 

 reason it is advantageous to place in the vessels with the pieces a 

 small quantity of some substance which will prevent the growth of 

 hyphomycetes, as camphor, salicylic acid, etc. The most import- 

 ant point, and at the same time the most difiicult to determine in 

 order to obtain good results with this method, is the length of 

 time during which the pieces must be kept in the bichromate solu- 

 tion before one passes on to the second part of the process — the 

 reaction with the silver nitrate. 



