38 FIXING AGENTS. 



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 saf ranin 

 or the like. 



The advantages of picro-sulphuric acid as a fixing agent 

 are, that it kills tissues very rapidly, that it has great pene- 

 trating power, that it can be totally soaked out of the struc- 

 tures with alcohol (it is much more easily removed from the 

 tissues than pure picric acid) , leaving them in a good condi- 

 tion 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 some disadvantages. For vertebrata it should be 

 used with caution, on account of the swelling caused by sul- 

 phuric acid in connective tissue. In parasitic Crustacea it 

 also produces swelling and maceration, and should be avoided 

 (as was found by Fraisse, Entoniscus Cavalini, n. sp., Arbeiten 

 Zool. Zoot. Inst. Wurzburg, 1877-78, iv, p. 383). Notwith- 

 standing this, it is, however, according to Emery, very suitable 

 for fishes, and for embryos of vertebrates generally, provided 

 they are not allowed to remain in it more than three or four 

 hours. 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 preferred. 



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

 Journ. Roy. Mic. 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.j0 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." 



