100 COLLODION AND OTHER IMBEDDING METHODS. 



Good chloroform is a necessity. 



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 160), 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 con- 

 sistency of trie mass at least equal to that obtained by alcohol 

 hardening. 



The more commonly employed hardening method is the alcohol 

 method. The objects are thrown into alcohol and left there until 

 they have attained the right consistency (one day to several weeks). 

 The bottle or other vessel containing the alcohol ought not to be 

 tightly closed, but should be left at least partly open. 



The strength of the alcohol is a point on which the practice of 

 different writers differs greatly. BUSSE (Zeit. f. wiss. Mikr. } ix, 

 1, 1892, p. 49) has found, as I also have done, that alcohol of about 

 85 per cent, is the best, both as regards the cutting consistency and the 

 transparency of the mass. (Care must be taken to keep masses 

 hardened in this grade of alcohol moist while cutting, as they dry 

 by evaporation very quickly.) 



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

 AP!THY (Microtechnik, p. 185) mentions " glycerin-alcohol," but 

 without giving details. BLUM (Anat. Anz., xi, 1896, p. 724) mentions 

 " weak spirit with formol added to it," saying that formol hardens 

 celloidin. 



Lastly, the mass may 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. 



FLORMAN (Zeit. wiss. Mik., vi, 1889, p. 184) recommends that the 

 definitive hardening should be done without the aid of alcohol or chloro- 

 form, 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. I doubt 

 whether it is possible in this way to carry the hardening much beyond 

 the point attained by the chloroform or alcohol method without incurring 

 a very undesirable degree of shrinkage. 



164. Preservation. The hardened blocks of collodion may be 

 preserved till wanted in weak alcohol (70 per cent.), or dry, by dipping 



