498 AXIS-CYLINDER 



able for the silver impregnation up to thirty, forty or fifty days. 

 In cold weather good results can seldom be obtained under a 

 month ; when this is the case, the material may continue to 

 give good impregnations for two, three or even four months. 

 The only way to make sure is to pass, at intervals, trial portions 

 of the tissue into the silver nitrate solution — -in summer frequently, 

 in winter every eight or ten days — and observe whether and 

 when the reaction has been obtained. 



It is a good practice to inject the organs (see § 978) with the 

 hardening fluid, generally 2-5 per cent, potassium bichromate, 

 to which, according to Golgi, 5 to 6 per cent, of gelatin may be 

 added, in which case, however, the fluid must be injected after 

 warming it to body temperature. Stoving at a temperature of 

 20° to 25° C. is useful for abridging the hardening, but there is a 

 risk of over-hardening ; and Golgi thinks that the results are 

 never quite so delicate as after hardening at room temperature. 



(6) Impregnation. As soon as the pieces of tissue have attained 

 the proper degree of hardening, they are brought into a large 

 quantity of silver nitrate solution, the usual strength of which is 

 0-75 per cent., but 0-50 per cent, may be used for material which 

 has not been quite enough hardened, and 1 per cent, for material 

 that has been slightly over-hardened. 



The moment the pieces are put into the silver bath an abundant 

 precipitate is formed. This, of course, weakens the bath pro 

 tanto. It is, therefore, advisable first to wash them well in a 

 weaker silver solution until, on being put into a fresh quantity 

 of it, no further precipitate is formed. Used solutions will do 

 for this purpose. The final silver bath needs, generally, no further 

 attention ; but it should be changed for a fresh one if it becomes 

 yellowish, as it sometimes does, particularly in the case of tissues 

 which have taken up a great deal of bichromate. 



It is not necessary to keep the material in the dark during the 

 impregnation ; in winter it is well to keep it in a warmed room. 

 The time generally necessary for the impregnation is from twenty- 

 four to forty-eight hours ; but tissues may remain in the bath 

 without hurt for days, weeks or months. 



(c) Preservation. As soon as a trial has shown that a sufficiently 

 satisfactory impregnation has been obtained, the pieces are 

 brought into 80 to 90 per cent, alcohol. The alcohol is changed 

 two, three or more times, until it remains transparent, even 

 after specimens have been two or three days in it ; for, in view 

 of good preservation, it is necessary that the excess of silver 

 nitrate should be washed out from them thoroughly. 



Sections are now made (see § 1039). These are to be washed 

 thoroughly in three or four changes of absolute alcohol and 

 cleared, first in creosote, in which they should remain only a few 

 minutes, then in oil of turpentine, in which they are usually left 



