SPECIAL FIXATIVES 359 



Specimens, after they have been fixed with picric, 

 should not be soaked in water ; but if they are not 

 wanted for histological work, then formalin may be 

 used instead of alcohol. 



Picro-formol Solution. This solution is a splendid fixative, 

 with a great power of penetration, and kills very quickly. 

 Bourn's formula : 



Picric acid, saturated aqueous solution, 75 parts ; formalin, 

 30 or 40 per cent., 25 parts ; acetic acid, 5 parts. 



When fixation is completed wash out the stain with alcohol, 

 and preserve in 70 per cent, alcohol. If the specimens should 

 not be wanted for histological work, then formalin may be 

 used instead of alcohol. 



Chromic Acid. Weak solutions of chromic acid in water 

 are often useful ; 2 per cent., i per cent., and 0-5 per cent, 

 solutions are generally employed, the specimens remaining in 

 the solution for some hours. After chromic acid the speci- 

 mens should be very thoroughly washed in fresh water, being 

 allowed to remain in running water for even twenty-four hours. 

 After washing they are transferred to 30 per cent, alcohol, 

 then to 50 per cent, alcohol, and finally to 70 per cent, alcohol, 

 in which they may be kept. Chromic acid specimens are best 

 kept in the dark. 



The disadvantage of chromic acid is that objects preserved 

 in it are often difficult to stain. 



Chromo-acetie Acid. Chromic acid, 0-2 to 0-25 per cent.; 

 acetic acid, o-i per cent., in water. Proceed as in the case of 

 chromic acid. 



Flemming's Fluid (Strong). One per cent, chromic acid, 

 15 parts ; 2 per cent, osmic acid, 4 parts ; glacial acetic acid, 

 i part. 



This is one of the best fixing agents for preserving the 

 minute structure of cells. Only small pieces of tissue should 

 be put into this fluid, as it does not penetrate well. The 

 specimens should remain in the fluid for several hours, or if 

 they are large for several days. Wash well in running water, as 

 in the case of chromic acid specimens, and bring gradually to 

 70 per cent, alcohol. 



The specimens are generally very much blackened by the 

 osmic acid in the solution, and are often difficult to stain, 

 especially if the washing has not been sufficiently thorough. 



