56 FIXING AND HARDENING AGENTS 



Objects may be left for twenty-four hours in this alcohol ; not 

 more, unless there be no reason for avoiding maceration, which 

 will generally occur after that time. You may conveniently stain 

 with picro-carmine, alum-carmine, or methyl green. 



This reagent is a very mild fixative. Its hardening action is so 

 slight that it is not at all indicated for the fixing of objects that 

 are intended to be sectioned. Its chief use is for temporary and 

 dissociation preparations. 



HO. Pyridin. Pyridin has been recommended as a hardening agent 

 (by A. DE Souza). It hardens, dehydrates, and clears at the same time. 

 It is said to harden quickly, and to give particularly good results with 

 brain. See Comptes Rendus de Biologic, 8 ser., t. iv, 1887, p. 622. 



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

 water, dissolves many albumens and fats. It causes considerable 

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

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

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

 small brains in a week. 



111. Acetone is said to harden very rapidly. It precipitates lipins, 

 and may yet prove an important reagent. Scholz (Zeit. zviss. Mikr., 

 xxii, 1905, p. 415) fixes small objects in warm acetone for half 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 LiNTWAREW (ibid., ccvi, 1911, p. 36) for erythrocytes, in which it 

 preserves the haemoglobin. 



112. Lucidol, see last edition. 



113. 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 forinaldehyde. 



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 oi formol. 



Solutions of formaldehyde sometimes decompose partially or 

 entirely, with formation of a white deposit of paraformaldehyde. 

 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 a.cid reaction, due to the 

 presence of formic or other acid. W. R. G. Atkins has investigated 



