FIXING AGENTS. 



For ova of Teleostea. Fix for twelve hours, wash with water for twelve 

 hours, then remove the chorion, and put the ova into 70 per cent, alcohol. 



41. Picro-chromic Acid (Fol Lehrb., p. 100). 



Picric acid, sol. sat. in water. ... 10 vols. 

 1 per cent, chromic acid solution . . . 25 



Water 65 



At the instant of using, you may add O'OOS of osniie acid, which makes 

 the action more energetic. Wash with water (hot, nearly boiling water is 

 best), and then with alcohol. Fol says, "This reagent hardens tissues 

 admirably, without hindering staining in any way ; but it is not very pene- 

 trating and fixes slowly." 



42. Chromic Acid and Platinum Chloride (MerJcel's solution ; 

 from Mitth. Zool. Stat. Neapel, 1881, p. 11). 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, as it does not harden very rapidly. After 

 washing out with alcohol of 50 per cent, to 70 per cent., 

 objects stain excellently, notwithstanding the admixture of 

 chromic acid. This is a very delicate and admirable fixative. 

 If objects that have been fixed by osmium be put into it for 

 some hours, blackening is effectually prevented. 



Salts. 



43. The Chromates are useful as hardening rather than fixing agents. 

 They have a very mild and even action on tissues, but are not at all pene- 

 trating and act very slowly. For hardening they are very valuable. They 

 may still be found useful for fixing certain tissues, some of those of Mollusca, 

 for example. It must be borne in mind that (as pointed out by Flemming) 

 they do not preserve the true structure of nuclei, and must absolutely be 

 avoided in all cases in which it is desired to demonstrate that structure. 



(1) Potassium Bichromate. Used in solutions of from 1 per cent, to 

 2 per cent., or sometimes more, for most classes of objects. It is less of a 

 hindrance to future staining than chromic acid. Wash out with water or 

 alcohol. Altmann strongly recommends the use of a 2 per cent, solution, 

 containing a little free chromic acid, and cooled to zero, followed by washing 

 out in strong alcohol. The cooling of the liquid servos to stop instantly all 

 molecular processes ; and the slowly-acting mixture has time to complete the 

 fixing. 



(2) Ammonium Chromate. Appears to be generally used in 5 per 

 cent, solution, for twenty-four hours. Wash out in water and slain in picro- 

 carmine. This salt is a still more delicate reagent than the preceding. 



For the mixtures of bichromate with sulphates, see the chapter on 

 Hardening Agents, Liquid of Milller, and Liquid of Erlicki. 



