FIXING AND HARDENING AGENTS. 63 



that it kills them. The rationale of the process is that the 

 acid kills the tissues, whilst the alcohol comes in and hardens 

 them sufficiently before they have had time to become de- 

 formed by the action of the acid. Acetic acid, used alone, 

 is only a fixative for a limited time. If its action be pro- 

 longed and not controlled by the action of some other agent, 

 it becomes a swelling agent. Its function in mixtures is, 

 besides that of killing, the valuable one of counteracting the 

 shrinking action of the ingredients with which it is com- 

 bined, and by its swelling action enhancing the penetration 

 of the mixture ; whilst by clarifying tissues it aids in the 

 optical differentiation of their elements. For these reasons 

 it is, in all cases in which its presence is not absolutely 

 counter-indicated (connective tissue, delicate calcareous struc- 

 tures, etc.), a most valuable ingredient, almost a sine qua non, 

 in fixing mixtures. 



The proportions in which it should enter into mixtures in 

 general seem to me to be from 0'5 per cent, to 5 per cent, of 

 the glacial acid ; higher strengths, such as 25 per cent, to 

 100 per cent., being only indicated in cases in which the 

 highest possible penetration is the chief consideration. 



Throughout this work, wherever acetic acid is mentioned, 

 it is the glacial acid that is meant unless the contrary is 

 stated. 



All liquids containing a large proportion of this acid (e. g. 

 83, 84) should only be allowed to act for a very short 

 time. 



83. Acetic Alcohol (CARNOY, La Cellule, t. iii, 1, 1886, p. 6 ; 

 and ibid., 1887, 2, p. 276; v. BENEDEN et NEYT, Bull. Ac. 

 roy. d. sci. de Belg., t. xiv, 1887, p. 218 ; ZACHARIAS, Anat. 

 Anz., iii, Jahrg. 1, 1888, pp. 24 27; v. GEHUCHTEN, ibid., 

 8, p. 227). CARNOY -has given two formulae for this important 

 reagent. The first is 



Glacial acetic acid . . .1 part. 



Absolute alcohol . . .3 parts. 



The second is 



Glacial acetic acid . . .1 part. 



Absolute alcohol . . .6 parts. 



Chloroform 3 



