44 FIXING AGENTS. 



and mounted in balsam, but is frequently excellent and sometimes indis- 

 pensable for objects that are to be studied in as fresh a state as possible in 

 aqueous media. Objects fixed in it stain instantaneously and perfectly with 

 methyl green. Osmic acid may be added to the liquid to increase the fixing 

 action. For cytological researches this is a most invaluable medium. 



65. Acetate of Uranium (ScHENK, Mitth. a. d. Embryol. Inst. Wien, 

 1882, p. 95 ; cf. GILSON, La Cellule, 1, 1885, p. 141). This reagent is very 

 similar in its properties to picric acid. It has a mild fixing action, and a high 

 degree of penetration, which may make it useful for Arthropoda. It may be 

 combined with methyl green, which it does not precipitate. 



66. Iodine. Iodine possesses considerable hardening properties, and a 

 very high degree of penetration ; and, in point of fact, iodised serum, which 

 is generally employed as an "indifferent liquid," that is, one which is 

 supposed to exert no action whatever on tissues, is, in reality, a feeble 

 hardening agent, and forms a most admirable fixing agent for delicate 

 tissues. It is so classed by Kanvier (see Iodised Serum, Chap. XIX). 

 KENT (Manual of the Infusoria, 1881, p. 114; Journ. Roy. Mic. Soc. (N.S.), 

 iii, 1883, p. 730), has found it to act in a manner almost identical with osmic 

 acid, and in some instances even more efficiently (for fixing Infusoria). His 

 instructions are as follows : " 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 the solution of LUGOL, of which the formula is as 

 follows : 



Water . . . 100 parts. 



Iodide of potassium 6 



Iodine ........ 4 



Iodine certainly kills cells very rapidly, without deforming them. Per- 

 sonally I have found it very useful for the examination of spermatozoa. 

 Unfortunately I am not acquainted with any nuclear stain that will work 

 well with it. 



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 2 or 3 minutes in order to evaporate the iodine from the 

 objects, which may then be mounted or otherwise treated as desired (OvEE- 

 TON, Zeit.f. wiss. Mik., vii, 1, 1890, p. 14). 



