PJXING AND HARDENING AGENTS. 39 



A mixture of Boebbl's (from Caitlleet and Mesnil, Arch. Protis- 

 tenlc, vi, 1905, p. 281), consists of 2 parts of osmic acid, 2 of platinum 

 chloride, 3 of chromic acid, 20 of acetic acid, and 350 of water, 



46. Kawitz {Zeit. wiss. Mihr., xxv, 1909, p. 386) takes 4 parts of 

 Kahlbaum's Phospho-Tungstic acid, 5 of alcohol, and 1 of acetic acid, 

 added just before use, fixes for 24 hours, and washes out the sections 

 before staining with water containing a little calcium acetate. 



47. Nitric Acid (Altmann, Arch. Anat. Phys , 1881, p. 219)— 

 Altmann employs for fixing embryos dilute nitric acid, containing from 

 3 to Sj per cent, pure acid. Such a solution has a sp. gr. of about 102. 

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

 results. After extensive trial I find Altmann's solution to be a second- 

 rate reagent, giving a weak and thin fixation. 



His (26ii.,1877, p. 115) recommendedalO per cent, solution. Plemming 

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

 Invei'tebrates. 



Telltesniczky {Arch. mih. Anat., lii, 2, 1898, p. 222) thinks that " for 

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

 stronger grades act too energetically on the superficial layers. 



Maybe has had good results with 5 per cent, sohition. 



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 pre- 

 pai-ations should be brought direct into alcohol. Some persons take 

 absolute, but I should say 70 per cent, is more generally indicated. 

 Rabl has employed a 1 or 2 per cent, solution of alum. 



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 en- 

 cephalon. 



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 me). — Three vols, of nitric 

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



48. Chromo-nitric Acid (Peeenti's formula, Zool. Anzeig-, v, 1882, 

 p. 459) : 



4 parts 10 per cent, nitric acid. 

 3 parts alcohol. 



3 parts 0"5 per cent, chromic acid. 

 Fix for 4 to 5 hours and pass into alcohol of 70 per cent. 

 This mixture has been criticised (see previous editions) as irrational, 

 the alcohol reducing the chromic acid and itself becoming etherised by 

 the nitric acid. Some workers reject it, especially for ova, for which it 

 is specially intended. But others speak highly of it. I myself have 

 used it extensively for preparing objects for dissection and museum 



