36 FIXING AGENTS. 



acid in the concentrated form, but I am not aware of any experiments in 

 this direction. 



54. Picric Acid. Picric acid should always be employed in 

 the form of a strong solution. (That is to say, strong solu- 

 tions must always be employed when it is desired to make 

 sections or other preparations of tissues with the elements in 

 situ, as weak solutions macerate ; but for dissociation prepara- 

 tions, or the fixation of isolated cells, weak solutions may be 

 taken. Flemming finds that the fixation of nuclear figures is 

 equally good with strong or weak solutions.) The saturated 

 solution is the one most employed. Objects should remain in 

 it for from a few seconds to twenty-four hours, according to 

 their size. For Infusoria, one to at most two minutes will 

 suffice ; whilst objects of a thickness of several millimetres 

 require from three to six hours' immersion. 



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 treatment, 

 water should be avoided ; staining should be performed by 

 means of alcoholic solutions, the only exceptions to this rule 

 being in favour of picro-carmine, which, probably on account 

 of the picric acid contained in it, does not appear to exert so 

 injurious an influence as other aqueous stains, and of methyl 

 green, and some few other aqueous stains that are themselves 

 weak hardening agents. It is one of the advantages of picric 

 acid that, by sufficiently prolonged soaking, it can with cer- 

 tainty be entirely removed from any tissue by means of alcohol. 



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

 by soaking, be perfectly stained in any stain. Mayer's 

 cochineal, alcoholic borax-carmine, Kleinenberg's haematoxy- 

 lin, Grenadier's alcoholic carmine, may be recommended. 



The most important property of picric acid is its great 

 penetration. This renders it peculiarly suitable for the prepa- 

 ration of chitinous structures. For such objects, alcohol of 

 70 per cent, to 90 per cent, should be taken for washing out, 

 and staining should be done by means of Mayer's cochim-jil 

 or Kleinenberg's hsematoxylin. 



In very many if not most cases it is advantageous to employ 

 picric acid in the manner suggested by Kleinenberg (see 

 below), that is, in combination with sulphuric acid; or with 

 nitric acid, or hydrochloric acid, as suggested by P. Mavci 



