46 CHAPTER IV. 



the fixation is certainly not in all cases superior. See 

 further under '' Cytological Methods." 



51. Nitric Acid (ALTMANN, Arch. Anat. u. Phys., 1881, p. 219). 

 Altmann employs for fixing dilute nitric acid, containing from 3 to 3^ per 

 cent, pure acid. Such a solution has a sp. gr. of about 1'02; an areometer 

 may conveniently be used to determine the concentration of the solution. 

 Stronger solutions have been used, but do not give such good final results. 

 After extensive trial I find ALTMANN'S solution to be a good reagent, but 

 rather weak. 



His (ibid., 1877, p. 115) recommended a 10 per cent, solution. Flemming 

 at one time employed solutions of 40 to 50 per cent, for the ova of Inverte- 

 brates. This of course has the advantage of a very rapid fixing action. 



The action of nitric acid as a fixative has been lately investigated by 

 TELLYESNICZKY (Arch. mik. Anat., Hi, 2, 1898, p. 222). He thinks that 

 "for general cell-fixing" the proper strength is 2 per cent, to 2 per cent., 

 as stronger grades act too energetically on the superficial layers. His re- 

 sults, or the interpretation of them, are therefore not quite concordant with 

 mine. 



Nitric acid has the valuable property of hardening yolk without making 

 it brittle. 



Pure water should in no case be used for washing out after nitric acid ; 

 the preparations should be brought direct into alcohol, as recommended by 

 Altmann. Some persons take absolute alcohol, but I should say 70 per cent, 

 is more generally indicated. Rabl has employed for washing out a 1 or 2 

 per cent, solution of alum. 



For prolonged hardening, strengths of from 3 to 10 per cent, are some- 

 times employed. A strength of 12 per cent., allowed to act for two or three 

 weeks, is said to afford very tough preparations of the encephalon. 



BENDA (Verh. Anat. Ges., 1888; Ergeb. d. Anat., i, 1891, p. 7) fixes for 

 twenty-four to forty-eight hours in 10 per cent, nitric acid, and then brings 

 the preparations direct into a cold saturated solution of bichromate of potash 

 diluted with three vols. of water. After a few hours this solution is changed 

 for a stronger one, and the strength is gradually increased in such a manner 

 as to arrive at a concentration of one vol. of the saturated solution to one of 

 water in two or three days (or, for encephalon and spinal cord only in four- 

 teen days). This process, which is of general applicability (except in so far 

 as epidermis, especially that of embryos, may be loosened by the nitric acid), 

 is said to furnish very tough preparations. 



Fol's Mixture. Three vols. of nitric acid, with 97 vols. of 70 per cent, 

 alcohol ( vcrli;illy communicated to me by Prof. Fol). 



52. Chronic-nitric Acid (PEKENYI'S formula, Zool. An::eig., 

 v, L882, p. l-V.t) : 



\ parts 10 per cent, nitric acid. 



:'> parts alcohol. 



" parts 0'5 per cent, chromic acid. 



