66 CHAPTER V. 



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

 very high degree of penetration. 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 osrnic 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 LTJGOL'S solution, 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 two or three minutes in order to evaporate the iodine 

 from the objects, which may then be mounted or otherwise treated as desired 

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



Iodine may be used in combination with alcohol for hardening, and 

 render service through its great penetrating power. See the method of 

 BETZ, post, Part II. 



89. Picric Acid. Picric acid in pure aqueous solution should 

 always be employed in the form of a strong solution. (That is 

 to say, strong solutions must always be employed when it is 

 desired to make sections or other preparations of tissues 

 with the elements in situ, as weak solutions macerate ; but 

 for dissociation preparations or the fixation of isolated cells, 

 weak solutions may be taken. Flemming finds that the 

 fixation of nuclear figures is equally good with strong or 

 weak solutions.) The saturated solution is the one most 

 employed. (One part of picric acid dissolves in about 86 

 parts of water at 15 C. ;* in hot water it is very much more 

 soluble.) Objects should remain in it for from a few 

 seconds to twenty-four hours, according to their size. For 

 Infusoria one to at most two minutes will suffice, whilst 

 objects of a thickness of several millimetres require from 

 three to six hours' immersion. 



* Benedikt and Kneoht, Chemistry of the Coal-tar Colours, p. 214. 



