54 CHAPTEft V. 



Strengths of 5 per cent, and more bring out the nuclein 

 structures 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 me rather 

 weak, and I 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 (in. litt.) 

 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 the "concentrated"* 

 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 in the optical differentiation of their 

 elements. 



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, 



* The acid referred to as " concentrated " by Lo BIANCO in his 

 Metodi (Miff}). Zot 1. 8 fat. Neapel, xi, 3, p. 435) is an acid of approximately 

 1!) porcent. (sp. gr. 



