40 FIXING AGENTS. 



cold, with which weeks may be required to fully remove the 

 acid from chitinous structures. I call attention here to what 

 was said as to washing out under the head of picric acid, viz. 

 that washing out must never be done with water. This is a 

 most important point, and one that is not sufficiently attended 

 to. You may stain as directed above for picric acid. You 

 may, of course, stain sections with alcoholic solutions of 

 safranin or the like. 



The advantages of picro-sulphuric acid as a fixing agent are, that it kills 

 tissues very rapidly, that it has great penetrating power, that it can be 

 totally soaked out of the structures with alcohol (it is much more easily 

 removed from the tissues than pure picric acid), leaving them in a good con- 

 dition for staining, and, in the case of marine organisms, that it effectually 

 removes the different salts of sea-water that are present in them. 



It has many disadvantages. For vertebrata it should be used with caution, 

 on account of the swelling caused by sulphuric acid in connective tissue. 

 For structures that contain much lime it is not to be recommended, for it 

 dissolves the lime and throws it down as crystals of gypsum in the tissues. 

 (For such structures the picro-nitric or picro-hydrochloric acid is to be pre- 

 ferred.) In numberless cases it produces swellings and maceration. For 

 the preservation of delicate, watery organisms, such as Medusae, it is an 

 abomination. For cytological researches it should be avoided, as its action 

 on both cytoplasm and nuclei is frequently most injurious. On the whole, 

 I find that for such objects as Arthropoda it is valuable on account of its 

 great penetrating power, the possibility of removing the acid entirely by 

 washing, and the facility thereby given for staining in toto. But for 

 general work, I consider that it is one of the most overrated reagents that 

 ever came into favour through the prestige of authority. 



58. Picro-nitric Acid (MAYER, Mitth. Zool Stat. Neapel, 1881, p. 5 ; 

 Journ. Eoy. Mik. Soc. (N.S.), ii, 1882, p. 868). Prepared in the same way 

 as picro-sulphuric acid except that instead of 2 vols. sulphuric acid you take 

 5 vols. pure nitric acid (of 25 per cent. N 2 5 ) Mayer now dissolves the 

 picric acid in the nitric acid water, so that the formula runs : 



Water 100 vols. 



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



Picric acid, as much as will dissolve. 

 The fluid is used undiluted. 



The properties of this fluid are very 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 recommends it strongly, and 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." 



59. Picro-hydrochloric Acid (MAYER, ibid.). Prepared in the same 

 way as picro-sulphuric acid, except that instead of 2 vols. of sulphuric acid 



