38 FIXING AGENTS. 



56. 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. (One part of picric acid 

 dissolves in about 75 parts of cold water ; in hot water it is 

 very much more soluble.) 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 

 certainty 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 para- 

 carmine, Grenacher's alcoholic borax-carmine, or Mayer's 

 haemacalcium may be recommended. 



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 taken for washing out, 

 and staining should be done by means of Mayer's cochineal 

 or hsemacalcium. 



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 



