PRESERVATION. 199 



iib. d. Bau d. quergestr. Muskelf., 1885; or Zeit. f. wiss Mik., 

 iii, 1, 1886, p. 93). Others take equal volumes of absolute 

 alcohol and water. SCHIEPFERDECKEE (Zeit. f. iviss. Mik., v, 

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

 alcohol of 82 per cent. (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). The question may now be con- 

 sidered to be finally settled by experiments, specially directed 

 to the clearing up of this point, made by BUSSE (Zeit. f. wiss. 

 Mik., ix, 1, 1892, p. 49). Busse finds 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 

 the mass moist whilst cutting, as it dries by evaporation very 

 quickly. 



Lastly, the mass may be frozen. After preliminary hard- 

 ening by alcohol it is soaked for a few hours in water in 

 order to get rid of the greater part of the alcohol (the alco- 

 hol 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 FLOBMAN (Zeit. /. 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. 



302. 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, I, 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. 



