58 CHAPTER V. 



Tissues fixed in picric acid can be perfectly stained in 

 any stain. It is seldom necessary to remove the picric acid 

 by washing ont before staining. Paracarmine, Borax- 

 carmine, or Ha3macalcium may be recommended for entire 

 objects. 



The most important property of picric acid is its great 

 penetration. This renders it peculiarly suitable for the 

 preparation of chittnous structures. 



94. Picric Alcohol (GAGE, Proc. Amer. Soc. Micr., 1890, p. 120). 

 Alcohol (95 per cent.), 250 parts ; water, 250 parts ; picric acid, 1 part. 



* 



95. Picro-acetic Acid. BOVERI (Zellenstudien, 1, 1887, p. 11) dilutes 

 a concentrated aqueous solution of picric acid with two volumes of water 

 and adds 1 per cent, of acetic acid. According to my experience, the 

 results are miserable. 



ZIMMEB'S mixture (from DEEGENER, Zool. Jahrb. Abtli. Morpli., xxvii, 

 1909, p. 634). Saturated aqueous solution of picric acid, 10 parts ; abso- 

 lute alcohol, 9; acetic acid, 1. 



96. Picro-sulphuric Acid (KLEINENBERG, Quart. Journ. Mic. Sci., 

 April, 1879, p. 208; MAYER, Mitt. Zool. Stat. Neapel, ii, 1880, p. 2). 

 MAYER takes distilled water, 100 vols. ; sulphuric acid, 2 vols. ; picric 

 acid, as much as will dissolve. 



Liquid of KLEINENBERG is made by diluting the concentrated picro- 

 sulphuric acid prepared as above with three times its volume of water. 



I hold that the concentrated solution is generally preferable. This 

 particularly applies to marine organisms. 



Wash out with successive alcohols, beginning with 70 per cent., never 

 with water. 



Warm alcohol extracts the acid much more quickly than cold, without 

 which weeks may be required to fully remove the acid from chitmous 

 structures. 



This liquid may still be useful for Arthropoda, on account of its great 

 power of penetrating chitin ; and for some embryological purposes. 

 For a fuller account see early editions. 



97. Picro nitric acid (MAYER, Mitth. Zool. Stat. Neapel, 1881 



P- 5).- 



Water 100 vols. 



Nitric acid (of 25 per cent. N 2 O 5 ) . . 5 



Picric acid, as much as will dissolve. 



Properties of this fluid similar to those of picro-sulphuric acid, with 

 the advantage of avoiding the formation of gypsum crystals, and the 

 disadvantage that it is much more difficult to soak out of the tissues. 

 Mayer states that with eggs containing a large amount of yolk material, 

 like those of Palinurus. it gives better results than nitric, picric, or 



