52 FIXING AND HARDENING AGENTS 



added from time to time until the colour has been entirely 

 extracted. 



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 Hsemacalcium 

 may be recommended for entire objects. 



The most important property of picric acid is its great pene- 

 tration. This renders it peculiarly suitable for the preparation of 

 chitinous structures. 



99. Picric Alcohol (Gage, Proc. Amer. Soc. Micr., 1890, p. 120). 

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



100. 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 Lee's 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. 



101. Picro-sulphuric Acid (Kleinenberg, Quart. Journ. Mic. Sic, 

 April, 1879, p. 208 ; Mayer, Mitth. 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. 



Lee holds 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. 



102. Picro-nitric Acid (Mayer, Mitth. Zool. Stat. Neapel, 1881, 



P- 5)- 



Water 100 vols. 



Nitric acid (of 25 per cent. N0O5) . . 5 „ 



Picric acid, as much as will dissolve. 

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

 with the advantages 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 picro-sulphuric acid. Lee considers 

 it distinctly superior to picro-sulphuric for most things. See Hill's 

 fluid, § 803, which gives superior results. 



103. Picro-hydrochloric Acid (Mayer, ifeid.). 



Water 100 vols. 



Hydrochloric acid (of 25 per cent. HCl) . 8 „ 



Picric acid, as much as will dissolve. 



