104 Journal of Comparative Neurology. 



the growth of hyphomycetes, as camphor, saHcyHc acid, etc. 

 The most important point and at the same time the most diffi- 

 cult to determine in order to obtain good results with this meth- 

 od is the length of time during which the pieces must be kept 

 in the bichromate solution before one passes on to the second 

 part of the process, the reaction with the silver nitrate. 



"The proper duration of immersion for the pieces to ob- 

 tain that degree or particular kind of hardening which is best 

 fitted to secure, when they are laid in the silver solution, a fine 

 and diffused action upon the various elements of the nervous 

 system varies according to various conditions. These are the 

 strength of the fluid, the condition of the pieces, the quantity 

 of fluid, temperature and, consequently, the time of the year. 



" The differences arising from the strength and quantity of 

 the fluid may be eliminated by paying strict attention to the 

 strength of the fluid, by using covered vessels and preserving 

 the same ratio between the number of pieces and quantity of 

 fluid. 



" The influence of temperature upon the results of the re- 

 action is more important, indeed practically all the uncertainties 

 of the method depend upon this. For example, to mention 

 extremes, good results (which, with the progressive changes, of 

 which I shall speak later, continue to appear and extend) can be 

 obtained in the warm season after an immersion of i 5 to 20 days 

 and seldom after 30 to 40 or 50 days ; on the other hand in the 

 cold season good results are scarcely obtainable after an immer- 

 sion in bichromate of less than i to i}4 months. The reaction 

 (with the progressive accompanying changes) may then con- 

 tinue to manifest itself for 2, 3 or 4 months, provided, of course, 

 the pieces are preserved according to the rules given above. It 

 is almost superfluous to say that during the gradual change from 

 the warm to the cold season and vice versa corresponding 

 changes in the appearance of the reaction take place. It is not 

 easy to remedy these temperature changes, especially because 

 these changes of environment are united with the other causes 

 of uncertainty mentioned and so act that observations made 

 upon one series of pieces, never agree closely with those made 



