FIXING AND HARDENING AGENTS. 53 



of 5 to y^j per cent. From specimens I have seen I should say it is 

 useless. 



80. Perchloride of Iron (FoL, Zeit. wiss. Zool, xxxviii, 1883, p. 491, 

 and Lehrb. d. vergl. mile. Anat., p. 102). Fol recommends 1 vol. of 

 Tinct. Ferri Perclilor. B.P. diluted with 5 to 10 vols. of 70 per cent, 

 alcohol. 



The tincture diluted with 3 to 4 vols. of either alcohol or water has 

 been recommended for fixing medullated nerve by PLAINER (Zeit. wiss. 

 Mik., vi, 1889, p. 187). 



81. Iron Alum. STRONG (Journ. comp. Neur., xiii, 1903, p. 296) fixes 

 (and decalcifies) heads of young Acanthias in 9 parts of 5 per cent, 

 solution of iron alum with 1 of formol, for about two weeks. 



82. Chloride of Zinc is sometimes used for hardening brain (see 

 Part II). G-ILSON (La Cellule, vi, 1890, p. 122) has used it as a fixative 

 for the silk glands of Lepidoptera, as follows : 



Glacial acetic acid 5 c.c. 



Nitric acid of 46 (or 80 per cent, nearly) . 5 



Alcohol of 80 per cent 100 



Distilled water 300 



Dry chloride of zinc ..... 20 grms. 



83. Iodine. KENT (Manual of the Infusoria, 1881, p. 114) uses it for 

 fixing Infusoria. Prepare a saturated solution of potassic iodide in 

 distilled water, saturate this solution with iodine, filter, and dilute to a 

 brown-sherry colour. A very small portion only of the fluid is to be 

 added to that containing the Infusoria. 



Or you may use LUGOL'S solution : 



Water 100 parts. 



Iodide of potassium . . . 6 

 Iodine 4 



Or for small marine animals, a solution of iodine in sea- water. 



Personally I have found it very useful for the examination of sperma- 

 tozoa. 



Very small objects may be instantaneously fixed by means of vapour 

 of Iodine. Crystals of iodine may be heated in a test-tube till the 

 vapours are given off ; then on inclining the tube the heavy vapours 

 may be made to flow over the objects arranged on a slide. The slide 

 should then be warmed to about 40 C. for one to three minutes in 

 order to evaporate the iodine from the objects, which may then be 

 mounted or otherwise treated as desired (OvERTON, Zeit. wiss. Mik., vii, 

 1890, p. 14). 



Organic Acid*, and other Agents. 



84. Acetic Acid. Flemming, who lias made a special 

 investigation of its action on nuclei, finds (Zellsiibstanz, etc., 

 p. 380) that the best strength is from 0'2 to 1 per cent. 



