CUTTING. 163 



alcohol and water. SCHIEFFERDECKER (Zeit. f. wiss. Mik., v, 

 4, 1888) recommends alcohol of 50 to 60 per cent. ; THQMA 

 alcohol of 82 percent. (0.842 sp. gr., see Journ. Roy. Mic. Soc., 

 1883, p. 305) ; DUVAL, alcohol of 36 (= 90 per cent., Journ. 

 de Microgr., 1888, p. 197). 



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. The sections are brought into warm water. 

 If the mass have frozen too hard, cut with a knife warmed 

 with warm water. 



A paper has been lately 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. No doubt the author is right in claiming for this pro- 

 cess a superior degree of hardening of the mass ; but I doubt whether it is 

 possible to carry the hardening much beyond the point attained by the 

 chloroform or alcohol method without incurring a degree of shrinkage that 

 must be destructive of the scientific value of the preparations. 



295. Preservation. The hardened blocks of collodion may 

 be preserved till wanted in weak alcohol (70 per cent.). They 

 may also be preserved dry by dipping them into melted 

 paraffin (APATHY, Zeit.f. wiss. Mik. } v, 1, 1888, p. 45). 



Reference numbers may be written with a soft lead pencil 

 on the bottom of the paper trays, or with a yellow oil pencil 

 on the bottom of the watch-glasses in which the objects are 

 imbedded. On removal of the paper from the collodion after 

 hardening, the numbers will be found impressed on the 

 collodion. 



296. Cutting. If the object has not been stained in the 

 mass, it will form so transparent a mass with the collodion 

 that the arrangement of the object and sections in the right 

 position may be rendered very difficult. It is, therefore, well 

 to stain the collodion lightly, just enough to make its outlines 

 visible in the sections. This may be done by adding picric 

 acid or carmine dissolved in alcohol to the collodion used for 

 imbedding, or to the bergamot oil used for clearing. 



