404 CENTRAL NERVOUS SYSTEM. 



The Gold and Iron Method. 



Material must be very carefully hardened for from four to 

 six months in bichromate of potash (not solution of Mtiller, 

 which does not allow of the same precision of stain). A 1 per 

 cent, solution should be taken at first, and should at first be 

 frequently changed. Only after some weeks should the 

 strength be gradually increased up to 2 or 2 '5 per cent. All 

 the hardening should be done in the dark. Over-hardened 

 material is not available. The surface of section of a properly 

 hardened piece of material should show microscopically no 

 marked difference of colour between the white and the grey 

 matter. If the white substance appear very dark, almost 

 black, and the grey substance whitish grey, good results will 

 probably not be obtained ; a medullary stain will probably 

 result, whereas the method ought to give an axis-cylinder 

 and cell-process stain. 



After hardening, the specimens are washed with water, and 

 put for two or three days into 50 per cent, alcohol, changed 

 as often as necessary, and then into 95 per cent, alcohol, in 

 which they should remain until they show a decidedly green 

 coloration (two to four weeks), the alcohol being changed as 

 often as precipitates occur. Sections are made either under 

 water or by the celloidin process (in the former case they 

 should not be allowed to remain in contact with the water 

 longer than is absolutely necessary, and should be put back 

 into alcohol for two or three days before passing to the next 

 stage) . 



The next stage consists in a bath of from one to two hours' 

 duration in 1 per cent, solution of chloride of gold acidified 

 with 2 per cent, of hydrochloric acid. After this the sections 

 are rinsed with water, and treated for half a minute with a 

 freshly prepared solution of ferricyanide of potassium in 10 

 per cent, potash solution (a lump of ferricyanide not half so 

 big as a pea to 5 c.c. of the potash solution). They are then 

 washed for half a minute more in pure 10 per cent, potash 

 solution, and after that for some time in distilled water. 



The following reducing mixture (which must be freshly 

 made up at the very instant of using it for each section) is 

 now to be prepared : 



