COLLODION AND OTHElt IMBEDDING METHODS. 127 



The above processes are excellent, but I regard them as 

 primitive forms of the chloroform method. I now almost 

 always harden in vapour of chloroform. All that is necessary 

 is to put the liquid mass (after having removed bubbles as 

 directed in 154) with its recipient into a desiccator on the 

 bottom of which a few drops of chloroform have been poured. 

 The action is very rapid, and the final consistency of the 

 mass at least equal to that obtained by the best alcohol 

 hardening. We shall revert to this subject, 165. 



The more commonly employed hardening method is the 

 alcohol method. The objects are thrown into alcohol and 

 left there until they they have attained the right consistency 

 (one day to several weeks) . The bottle or other vessel con- 

 taining the alcohol ought not to be iiqhtly closed, but should be 

 left at least partly open. 



The strength of the alcohol is a point on which the prac- 

 tice of different writers differs greatly. The question may 

 now be considered to be finally settled by experiments 

 specially directed to the clearing up of this point, made by 

 BHSSE (Zeit. f. iviss. Mifo, ix, 1, 1892, p. 49), and which I 

 have repeated and confirmed. BUSSE finds that alcohol of 

 about 85 per cent, is the best, both as regards the cutting con- 

 sistency and the transparency of the mass. Care must be 

 taken to keep masses hardened in this grade of alcohol 

 moist whilst cutting, as they dry by evaporation very quickly. 

 Some workers use lower grades, 70 to 80 per cent., or 

 even lower. APATHY (Mikrotechnik, p. 185) mentions 

 " glycerin-alcohol," but without giving details. BLUM (Anat. 

 Anz.j xi, 1896, p. 724) mentions "weak spirit with formol 

 added to it/' saying that formol hardens celloidin. 



Lastly, the mass ma)- be frozen. After preliminary hardening by alcohol 

 it is soaked for a few hours in water, in order to get rid of the greater part 

 of the alcohol (the alcohol should not be removed entirely, or the mass may 

 freeze too hard). It is then dipped for a few moments into gum mucilage 

 in order to make it adhere to the freezing plate, and is frozen. If the mass 

 have frozen too hard, cut with a knife warmed with warm water. 



A paper has been written by FLOEMAN (Zeit. f. wiss. Mik., vi, 2, 1889, 

 p. 184) to recommend that the definitive hardening should be done without 

 the aid of alcohol or chloroform, by simply cutting out the blocks, turning 

 them over, and carefully continuing the evaporation process in the way 

 described above. I described this process myself in the first edition of 

 this work. No doubt the author is -right in claiming for it a superior 



