BO rHAPTKn IV. 



than 0-1 per cent., and ucrrr more than 0'5 per cent.,, 

 should he u>ed. 



On account of it- feeble penetrating power the objects to 

 IK- fixed should In- n* small as possible. 



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

 irem-rally with advantage be added to the solutions just 

 before using. 



The solutions should be kept protected from the light 

 during the immersion of tissues. (This precaution is not 

 necessary if Flemming's or Hermann's solution be used). If 

 the immersion is to be a long one the tissues must be placed 

 with the solution in well-closed vessels. The objects 7iiay 

 be deemed to be fixed as soon as they have become brown 

 throughout. 



After-Treatment.- -The excess of osmic acid must be well 

 washed out before proceeding to any further steps in 

 preparation ; water should be used for washing. Notwith- 

 standing the greatest care in soaking', it frequently happens 

 that some of the acid remains in the tissues, and causes 

 them to blacken in time, and in any case hinders staining. 

 To obviate this blackening it has been advised to wash them 

 out in ammonia-carmine or picro-carmine (not very effectual) , 

 or to soak them for twenty-four hours in a solution of bichro- 

 mate of potash (Midler's solution or Erlicki's will do), or in 

 ()") per cent, solution of chromic acid, or in Merkel's 

 solution. The treatment with bichromate solutions has the 

 great advantage of highly facilitating staining with carmine 

 or hasmatoxylin. Max Schultze recommended washing, and 

 mounting permanently in acetate of potash. Fol, treatment 

 with a weak solution of carbonate of ammonia. But the best 

 plan of all is to properly bleach the preparations (see 

 ' Bleaching"). This may be done by means of peroxide of 

 liy<lro(jen. OVERTON (Zeit. wlss. Mil:, vii, 1890, p. 10) finds 

 that it is completed in a few minutes in a mixture of 1 part 

 commercial peroxide with 10 to 25 parts 70 per cent. 

 alcohol. (The commercial peroxide, slightly acidulated with 

 HC1, will keep well in the dark; but the mixture with 

 alcohol must be made fresh for use.) According to BRISTOL 

 (Amer. Natural, xxvii, 189:5, p. 176) the peroxide acts best 

 in the sun. BINKT (Journ. <le VAnat. et de la Physiol., xxx, 

 |. ; p. 449) has successfully used permanganate of potash, 



