108 CHAPTER IX. 



sary 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 consistency 

 of the 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 prac- 

 tice of different writers differs greatly. BUSSE (Zeit. f. wis. 

 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. APATHY (Micro technOc, p. 185) mentions " gly- 

 cerin-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. ivits. 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 care- 

 fully 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 incur- 

 ring a very undesirable degree of shrinkage. 



164. Preservation. --The hardened blocks of collodion may 

 he preserved till wanted in weak alcohol (70 per cent.), or 



