CHAPTER V. 55 



Tissues fixed in picric acid can be perfectly stained in any stain. 

 It is seldom necessary to remove the picric acid by washing out 

 before staining. Paracarmine, Boraxcarmine, or Haemacalcium 

 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 chitinous 

 structures. 



94. Picric Alcohol (GAGE, Proc. Amer. Soe. 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. 



ZIMMER'S mixture (from DEEGENER, Zool. Jahrb., Abth. Morph., xxvii, 

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

 absolute 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 chitinous 

 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. 6)- 



Water 100 vols. 



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



Picric acid, as much as will dissolve. 



Properties . of this fluid similar to those of pier o -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 Palinums, it gives better results than 

 nitric, picric, or picro-sulphuric acid. I myself consider it distinctly 

 superior to picro-sulphuric for most things. See Hill's fluid, 586. 



