CHAPTER IV. 41 



51. Bichromate of Potash. Perhaps the most important of all 

 known hardening agents, sensu stricto. It hardens slowly, much 

 more so than chromic acid, but it gives an incomparably better 

 consistency to the tissues. They may remain almost indefinitely 

 exposed to its action without much hurt. 



The strength of the solutions employed is from 2 to 5 per cent. 

 As with chromic acid, it is extremely important to begin with weak 

 solutions and proceed gradually to stronger ones. About three 

 weeks will be necessary for hardening a sheep's eye in solutions 

 gradually raised from 2 to 4 per cent. Spinal cord requires from 

 three to six weeks, a brain at least as many months. 



After hardening, the objects should be well soaked out in water 

 before being put into alcohol, or be treated as directed for chromic 

 acid, 38. They had better be kept in the dark when in alcohol. 

 See 38. (Bomi and OPPEL [Taschenbuch, 3 Auf., 1896, p. 22] 

 fix in the dark.) // you wish to have a good stain with carmine you 

 should not put the objects into alcohol at all, even for a second, until 

 they have been stained, 



You may stain either with carmine or hsematoxylin, as well as 

 with tar colours. 



Bichromate objects have an ugly yellow colour which cannot be 

 removed by mere soaking in water. It is said that it can be removed 

 by washing for a few minutes in a 1 per cent, solution of chloral hydrate. 



Prof. GILSON writes me that alcoholic solution of sulphurous anhydride 

 (S0. 2 ) is very convenient for the rapid decoloration of bichromate 

 objects. A few drops suffice. See also 38, and " Bleaching." 



To facilitate staining with hsematoxylin, WOLFF (Zeit. wiss. Mik., xv, 

 3, 1899, p. 311) first stains in Boehmer's hsematoxylin for twenty-four 

 hours, and then for a few minutes in the same hsematoxylin to which 

 lias been added 1 drop per watch-glassful of 5 per cent, solution of 

 oxalic acid. 



The simple aqueous solution of bichromate is hardly to be recom- 

 mended as a fixing agent, because not only does it not preserve 

 nuclei properly, but also because it penetrates very slowly. The 

 first of these defects may be overcome entirely, the second to some 

 extent by addition of acetic acid ; whence the liquid of TELLYES- 

 NICZKY, next . 



58. Acetic Bichromate (TELLYESNICZKY, Arch. mik. Anat., Hi, 

 1889, p. 242) : 



Bichromate ...... 3 grms. 



Glacial acetic acid ..... 5 c.c. 



Water ....... 100 ,, 



Smaller objects to remain in the fluid for one or two days, larger 



