FIXATION BY SOLUTIONS. 19 



cent, for twenty-four hours ; for retina J to 2 per cent, for 

 from ten minutes to twenty-four hours ; for nuclei ^ to 2 per 

 cent, for two or three hours. Such figures as these will serve 

 to give a general idea of the practice, whilst more precise in- 

 structions will be given when dealing with the tissues in detail. 

 (The durations here quoted appear to me exaggerated, except 

 for very voluminous specimens.) 



The osmium must be well washed out before proceeding to 

 any further steps in preparation ; water should be used for 

 washing. Notwithstanding the greatest care in soaking, it 

 frequently happens that some of the acid remains in the 

 tissues, and causes them to over-blacken in time. To obviate 

 this it is necessary to wash them out in ammonia-carmine or 

 picro-carmine, or to soak them for twenty-four hours in a solu- 

 tion of bichromate of potash (Muller's solution or Erlicki's will 

 do), or in 0'5 per cent, solution of chromic acid, or in MerkePs 

 solution, or in a weak solution of f errocyanide of potassium or 

 cyanide of potassium, or to bleach them (see BLEACHING, No. 

 542). The treatment with bichromate solutions has the great 

 advantage of highly facilitating staining with carmine or 

 haematoxylin. Max Schultze recommended washing, and 

 mounting permanently in acetate of potash (see 359), but I 

 believe the virtues attributed to this method are illusory. Fol 

 has lately recommended treatment with a weak solution of car- 

 bonate of ammonia. 



The same stains recommended for objects fixed by vapour 

 will be found useful here, with the addition of ammonia car- 

 mine, which is really very useful for strongly fixed specimens. 

 For sections, of course in both cases, safranin and the other 

 nuclear anilin stains may be employed with advantage. 



Osmic acid stains all fatty structures black ; it must there- 

 fore be avoided for tissues in which much fat is present ; or 

 the fat may afterwards be dissolved out with turpentine 

 (Flemming, see the chapter on Connective Tissues in Part II) . 

 All solutions of osmic acid must be kept protected from the 

 light even during the immersion of tissues. If the immersion 

 is to be a long one the tissues must be placed with the solution 

 in well-closed vessels, as osmium is very volatile. 



A little acetic or formic acid (0'5 to 1 per cent.) may fre- 

 quently with advantage be added to the solutions just before 

 using. 



