48 FIXING AND HARDENING AGENTS 



Very small objects may be instantaneously fixed by means of vapour 

 of Iodine. Crystals of iodine may be heated in a test-tube till the 

 vapours are given off ; then on inclining the tube the heavy vapours 

 may be made to flow over the objects arranged on a slide. The slide 

 should then be warmed to about 40° C. for one to three minutes in order 

 to evaporate the iodine from the objects, which may then be mounted 

 or otherwise treated as desired (Overton, Zeit. wiss. Mik., vii, 1890, 

 p. 14). 



ORGANIC ACIDS AND OTHER AGENTS 



89. Acetic Acid. A substance most injurious to the finer 

 elements of the cytoplasm ; in some cases it is indicated for a 

 study of the nuclear elements. Flemming, who has made a special 

 investigation of its action on nuclei, finds [Zellsuhstanz, etc., 

 p. 380) that the best strength is from 0-2 to 1 per cent. Strengths 

 of 5 per cent, and more bring out the nuclear structure clearly at 

 first, but after a time cause them to swell and become pale, which 

 is not the case with the weaker strengths {ibid., p. 103). The 

 strong acid is, however, a valuable fixative of certain objects, 

 which it kills with the utmost rapidity, and leaves fixed in a state 

 of extension. 



The modus operandi of Van Beneden is as follows : — Pour glacial 

 acetic acid in liberal quantity over the organisms, leave them until they 

 are penetrated by it — which should be in five or six minutes, as the 

 strong acid is a highly penetrating reagent — and wash out in frequent 

 changes of alcohol of gradually increasing strength. Some persons begin 

 with 30 per cent, alcohol, but this appears to us rather weak, and we 

 think 70 per cent, or at least 50 per cent, should be preferred. 



Other energetic reagents may be combined with the glacial acetic 

 acid if desired. Dr. Lindsay Johnson (m Hit.) has found that one of 

 the best fixatives for retina is a mixture of equal parts glacial acetic 

 acid and 2 per cent, osmic acid. S. Lo Bianco adds to his " concen- 

 trated " (49 per cent.) acid one-tenth of a 1 per cent, solution of chromic 

 acid. He finds that even this small proportion of chromic acid serves 

 to counteract in a marked degree the softening action of the acetic acid. 



Acetic acid, used alone, is only a fixative for a limited time. If its 

 action be prolonged, 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 combined, and by its 

 swelling action enhancing the penetration of the mixture ; whilst by 

 clarifying tissues it adds to the optical differentiation of their elements. 



The proportions in which it should enter into mixtures in general seem 

 to us 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. §§ 55, 

 90) should only be allowed to act for a very short time. 



90. Acetic Alcohol (Carnoy, La Cellule, iii, 1886, p. 6 ; and 

 ibid., 1887, p. 276 ; v. Beneden et Neyt, Bull. Ac. Sci. Belg., 



