MINERAL ACIDS AND THEIR SALTS 37 



added just before use, fixes for twenty-four hours, and washes out 

 the sections before staining with water containing a httle calcium 

 acetate. 



53. Nitric Acid (Altmann, ^rcft.^naf. P/i«/s., 1881, p. 219). Altmann 

 employs for fixing embryos dilute nitric acid, containing from 3 to 3| 

 per cent, pure acid. Such a solution has a sp. gr. of about 1 02. Stronger 

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

 extensive trial Lee found Alt.mann's solution to be a second-rate 

 reagent, giving a weak and thin fixation. 



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 Invertebrates. 



Tellyesniczky (Arch. mik. Anat., In, 2, 1898, p. 222) thinks that " for 

 general cell-fixing " the proper strength is 2 to 2J per cent., as stronger 

 grades act too energetically on the superficial layers. 



Mayer has had good results with 5 per cent, solution. 



Nitric acid has the valuable property of hardening yolk without making 

 it brittle. 



Pure water should in no case be used for washing out ; the prepara- 

 tions should be brought direct into alcohol. Some persons take absolute, 

 but 70 per cent, is more generally indicated. Rabl has employed a 

 1 or 2 per cent, solution of alimi. 



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

 sometimes 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 

 hardens in bichromate of potash. 



Fol's Mixture (verbally communicated to Lee). Three vols, of nitric 

 acid, with 97 vols, of 70 per cent, alcohol. 



54. Chromic Acid and Platinum Chloride (Merkel's Macula lutea 

 des Menschen, Leipzig, 1870, p. 19). Equal volumes of 1-400 solution 

 of chromic acid and 1-400 solution of platinum chloride. Objects 

 should remain in it for several hours or even days. After washing out 

 with alcohol of 50 to 70 per cent., objects stain excellently. If objects 

 that have been fixed by osmic acid be put into it for some hours, 

 blackening is said to be effectually prevented. 



This is an excellent hardening medium for delicate objects. Merkel 

 allowed from three to four days for the action of the fluid for the 

 retina ; for Annelids Eisig employs an immersion of three to five 

 hours, and transfers to 70 per cent, alcohol ; for small leeches Whitman 

 finds one hour sufficient, and transfers to 50 per cent, alcohol. 



A similar mixture, with the addition of 0-25 to 0-1 per cent, of acetic 

 acid, is recommended by Brass for Protozoa ; and Lavdowsky has 

 used for nuclei a mixture of 10 parts of 1 per cent, chromic acid, 5 of 1 

 per cent, platiniun chloride, and 100 of 5 cent, acetic acid. 



Whitman recommends for the hardening of pelagic fish ova a stronger 

 mixture (due, we believe, to Eisig), viz.— 



0-25 per cent, solution of platiniun chloride . 1 vol. 

 1 per cent, solution of chromic acid . . 1 „ 



The ova to remain in it one or two days (Whitman, Methods in Micro. 

 Anat., p. 153). 



SALTS 



55. Chromates. The chromates are amongst the oldest and 

 best tried of hardening agents. The bichromate of potash 



