FIXING AND HARDENING AGENTS. 67 



Picric acid should always be washed out with alcohol, as 

 water is hurtful to tissues that have been prepared in it. 

 For the same reason during all remaining stages of treat- 

 ment, water should be avoided ; staining should be performed 

 by means of alcoholic solutions, the only exceptions to this 

 rule being in favour of methyl green, and some few other 

 aqueous stains that are themselves weak hardening agents, 

 such as haemalum, carmalum, etc. 



It has been found by JELINEK (Zeit. f. wiss. Mik., xi, 2, 

 1894, p. 242) that the extraction of picric acid is greatly 

 quickened by the addition of a base to the wash-alcohol. 

 He recommends carbonate of lithia. A few drops of a 

 saturated solution of the salt in water are added to the 

 alcohol ; a slight precipitate is formed. The objects are put 

 into the turbid alcohol, which becomes clear and yellow in 

 proportion as the picrin is extracted. Furthur quantities of 

 carbonate are added from time to time until the colour has 

 been entirely extracted from the tissues. 



Tissues fixed in picric acid can, after removal of the acid 

 by soaking, be perfectly stained in any stain. It is not 

 generally necessary to remove the picric acid by washing 

 out before staining. Mayer's paracarmine, Grenadier's alco- 

 holic borax-carmine, or Mayer's haemacalcium may be recom- 

 mended for entire objects. 



The most important property of picric acid is its great 

 penetration. This renders it peculiarly suitable for the pre- 

 paration of chitinous structures. For such objects alcohol 

 of 70 per cent, to 90 per cent, should be taker, for washing 

 out, and staining should be done by means of Mayer's 

 cochineal or haemacalcium. 



89a. Picro-acetic Acid. Saturated solution of picric acid in 

 1 per cent, acetic acid has given me better preparations than 

 those obtained by any other picric liquid, except picro-aceto- 

 osmic. 



BOVEEI (Zellenstndien, 1, 1887, p. 11) dilutes a concentrated aqueous 

 solution with two volumes of water and adds 1 per cent, of acetic acid. For 

 most purposes this dilution is evidently a great mistake, for the concen- 

 trated solution is itself none too strong, and would apparently do much 

 better if it were much stronger. 



90. Picro-sulphuris Acid (KLEINENBEBG, Quart. Journ. Mie. Sci., 

 April, 1879, p. 208 ; MAYER, Journ. Roy. Mic. Soc. (N. S.), ii (1882), 



