30 CHAPTRR IV. 



more than O'l per cent.j and never more than 0'5 per cent., 

 should be used. 



On account of its feeble penetrating power the objects to 

 be fixed shonld be as small as possible. 



A little acetic or formic acid (O'o to 1 per cent.) may 

 generally 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). It 

 the immersion is to be a long one the tis.sue.s must be placed 

 with the solution in well-closed vessels. The objects may 

 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 m 

 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 tilachen in time, and in any case hinders staining. 

 To obviate this blackening it has been advised to wash them 

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

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

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

 0"5 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 hsematoxylin. 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 

 hydrogen. Ovebton {Zeit. wiss. Mih., vii, 1890, p. 10) finds 

 that it is completed in a few minutes in a mixture of 1 pai't 

 commercial peroxide with 10 to 25 parts 70 per cent, 

 alcohol. (The commercial peroxide, slightly acidulated with 

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

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

 [Amrr. Natural, xxvii, 1893, p. 176) the peroxide acts best 

 in the sun. Binkt {.Toiirn. de I'Anat. ct de la Physiol., xxx, 

 1S9-1, p. 419) has successfully used permanganate of potash. 



