20 FIXING AGENTS. 



KOLOSSOW (Zeit. f. wis. Mik., v, i, 1888, p. 51) recommends 

 a 0*5 per cent, solution of osmium in 2 or 3 per cent, solution 

 of nitrate or acetate of uranium, as having a greatly enhanced 

 penetrating power. 



29. Osmic Acid and Alcohol (RANVIEB ET VIGNAL. RANVIER, Leg. 

 d'Anat. Gen., " App. term, des muscles de la vie org./ p. 76 ; Vignal, Arch, 

 de Physiol.,' 1884, p. 18J). Equal volumes of 1 per cent, osinic acid and 

 90 per cent, alcohol (freshly mixed). Allow it to act, for medium-sized 

 objects, such as embryos of a few millimetres diameter, for an hour or two. 

 Wash for some hours in 80 per cent, alcohol. Then wash with water and 

 stain for forty-eight hours in picro-carmine or haematoxylin. Viallanes has 

 applied this method to the histology of insects. 



30. Chromic Acid. Chromic anhydride, Cr0 3 , is found in 

 commerce in the form of red crystals that dissolve readily in 

 water, forming chromic acid, H 2 Cr0 4 . These crystals are very 

 deliquescent, and it is therefore well to keep the acid in stock 

 in the shape of a 1 per cent, solution. Care must be taken 

 not to allow the crystals to be contaminated by organic matter, 

 in the presence of which the anhydride is readily reduced into 

 sesquioxide. 



Chromic acid is employed in solution either in water or in 

 alcohol. 



The most usual strengths in which it is employed in aqueous 

 solution are from O'l to TO per cent, for a period of immer- 

 sion of a few hours (structure of cells and ova) . For nerve- 

 tissues weaker solutions are taken, -^th to |-th per cent, for a 

 few hours. Stronger solutions, such as 5 per cent., should 

 only be allowed to act for a few seconds. 



The object should be washed out with water before passing 

 into alcohol or staining fluids. Long washing in water is 

 necessary to prepare them for staining, except an anilin stain 

 be used. It is possible to wash out in alcohol, and this may 

 be useful in special cases, but in general I think the practice 

 is not to be recommended. It is well to wash for many hours 

 in running water. 



Tissues that have been fixed in chromic acid may be stained 

 in aqueous solutions if desired, as water does not appear to 

 have an injurious effect on them ; the acid appears to enter 

 into some chemical combination with the elements of the 

 tissues, forming with them a compound that is not affected 

 either physically or chemically by water. The best stain to 



