26 FIXING AGENTS. 



37. Nitric Acid (ALTMANN, Arcli. Anat. u. Phys., 1881, p. 219). (Of 

 general histological application, but most specially referring to embryology.) 



Altmann employs dilute nitric acid, containing from 3 to 3^ per cent, pure 

 acid. Such a solution has a sp. gr. of about 1*02 ; an araeometer may con- 

 veniently be used to determine the concentration of the solution. Stronger 

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



His (Ibid., 1877, p. 115) recommended a 10 per cent, solution. Altmann 

 tried it, but found he could not demonstrate the nuclear figures. He con- 

 siders that the strong solutions coagulate the soluble albuminoids of the 

 tissues too strongly, which is a hindrance to the optical differentiation of 

 structure. Flemming writes to Altmann that he employs solutions of 40 to 

 50 per cent, for the ova of invertebrates. This of course has the advan- 

 tage of a very rapid fixing action. 



The embryos, or other objects, are to be put fresh into the solution ; it is 

 useful, though not necessary, to employ a liquid cooled to zero ; the cold 

 stops all molecular processes, and the acid has time to fully complete the 

 fixing process. 



The objects must not be left too long in the liquid ; for blastoderms and 

 small embryos a quarter to half an hour is enough, for larger ones two to 

 four hours. Only small pieces of tissues other than embryonic should be 

 employed. 



I understand the author to say that he then washes out the acid, and com- 

 pletes the hardening in strong alcohol. He points out that the process does 

 not afford a true hardening such as is obtained by the use of chromic acid ; 

 but then he considers that by the use of paraffin imbedding such strictly so- 

 called hardening is superfluous. I consider myself that it does not harden 

 enough to faithfully preserve delicate structures. 



I have used this reagent extensively, and have at length abandoned it, for 

 general purposes, in favour of " Flemming." It may still be useful for 

 certain ova. It has the valuable property of hardening yolk without making 

 it brittle. 



Before imbedding, the objects are stained according to Altmann's instruc- 

 tions, in toto, slowly, with dilute hsematoxylin. By moderately staining 

 (either before or after the haematoxylin) with eosin, good double stains of 

 nuclear figures are obtained, the chromatin structures taking up the blue 

 colour. But any other staining process may be used. 



38. Silver Nitrate. Silver nitrate is frequently used in the study of 

 epithelia, not alone for the purpose of demonstrating the outlines of cells by 

 staining the intercellular cement-substance, but also with the totally distinct 

 object of rapidly fixing the cells. Solutions of from ^ to 2 per cent, are em- 

 ployed and allowed to act for merely a few seconds. Solutions of only :J to 

 1000 strength may be allowed to act for an hour. Wash out with water. 

 Stain as desired. Weak solutions, rapidly applied, do not hinder subsequent 

 staining ; strong solutions do. 



For the purpose of fixation, as well as for that of staining, nitrate of silver 

 is at present a reagent too uncertain in its action to be generally recom- 

 raendable. 



