CHAPTER V. 51 



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. See also under Goodrich's lodine-Bouin method. 



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. Mile., vii, 1890, 

 p. 14). 



Organic Acids, and other Agents. 



84. Acetic Acid. A substance most injurious to the finer elements 

 of the cytoplasm ; in some cases it is indicated for a study of the 

 nuclear elements. Flemming, who has made a special investigation 

 of its action on nuclei, finds (Zellsubstanz, etc., p. 380) that the best 

 strength is from O2 to 1 per cent. Strengths of 5 per cent, and more 

 bring out the nuclein structures clearly at first, but after a time 

 cause them to swell and become pale, which is not the case with the 

 weaker strengths (ibid., p. 103). The strong acid is, however, a 

 valuable fixative of certain objects, which it kills with the utmost 

 rapidity, and leaves fixed in a state of extension. 



The modus operandi of VAN BENEDEN is as follows : Pour glacial 

 acetic acid in liberal quantity over the organisms, leave them until they 

 are penetrated by it which should be in five or six minutes, as the 

 strong acid is a highly penetrating reagent and wash out in frequent 

 changes of alcohol of gradually increasing strength. Some persons begin 

 with 30 per cent, alcohol, but this appears to me rather weak, and I 

 think 70 per cent, or at least 50 per cent, should be preferred. 



Other energetic reagents may be combined with the glacial acetic 

 acid if desired. Dr. LINDSAY JOHNSON (in litt.) has found that one of 

 the best fixatives for retina is a mixture of equal parts glacial acetic 

 acid and 2 per cent, osmic acid. S. Lo BIANCO adds to his "concen- 

 trated " (49 per cent.) acid one-tenth of a 1 per cent, solution of 

 chromic acid. He finds that even this small proportion of chromic 



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