iflXlNG AND HARDENING AGKNTS. 63 



This substance is strongly alkaline, and, either pure or diluted with 

 water, dissolves many albumens and fats. It causes considerable shrink- 

 age of nuclei (not so much of cytoplasm). It is now in much use in 

 certain neuro-fibril stains, see Bielschowskt and Ramon. It is 

 soluble in water and in alcohol. Pure, it will harden and dehydrate 

 small brains in a week. 



107. Acetone is said to harden very rapidly. Scholz 

 [Zeit. wiss. Mikr., xxii, 1905, p. 415) fixes small objects in 

 warm acetone for lialf an hour to an hour and brings them 

 direct, or through alcohol and ether, into celloidin. 



Similarly Fuss [Arch. path. Anat., clxxxv, 1906, p. 5), 

 using it cold, and Lintwaeew {ihid., ccvi, 1911, p. 36) for 

 erythrocytes, in which it preserves the haemoglobin. 



108. Formaldehyde, Formic Aldehyde, Methyl Aldehyde 

 (Formol, Formalin, Formalose). — Formaldehyde is the chemical 

 name of the gaseous compound HCOH, obtained by the 

 oxidation of methyl-alcohol. " Formol," " Formalin," and 

 " Formalose " are commercial names for the saturated (40 per 

 cent.) solution of this in distilled water. This quickly loses 

 in strength through contact with air, and laboratory solutions 

 rarely contain more than 38 per cent, of formaldehyde. 



Much confusion has been caused by indiscriminate use of 

 the terms "formaldehyde" and "formol." The proper way is 

 evidently either to state the strengths of solutions in terms 

 of formaldehyde, and say so ; or to say " formol — or formalin 

 — with so many volumes of water." The majority of writers 

 seem to state in terms of formul. 



Solutions of formaldehyde sometimes decompose partially 

 or entirely, with formation of a white deposit of paraformal- 

 dehyde. Fish says that to avoid this the solution should be 

 kept in darkened bottles in the cool, or, according to some, 

 it suffices to add glycerin to them. 



The solutions almost always have an acid reaction, due to 

 the presence of formic acid ; but that is, as a rule, rather an 

 advantage. But some observers hold that neutral or feebly 

 alkaline solutions fix better than acid ones. Solutions may 

 be neutralised by the usual methods : it will generally suffice 

 to make them up with spring water. 



It was said above that formaldehyde possesses certain 

 hardening and preserving qualities. It hardens gelatine, 



