FIXING FLUIDS. 403 



lower surface. Tubular organs should be cut open before being put in the 

 fluid, and their contents together with blood upon the surface of the block 

 may be washed away with salt solution. Membranes may be kept flat and 

 smooth by being tied across the end of a short tube or a detached bottle 

 neck. After being used once the fixing fluids should be thrown away, ex- 

 cept alcohol, which can be put to other uses. The following mixtures are 

 those most frequently used. 



Zenker's Fluid is kept in stock as glacial acetic acid and the following 

 solution, in preparing which the water is heated and the ingredients are 

 stirred with a glass rod. (Metal instruments should not be put in Zenker's 

 fluid.) 



Bichromate of potassium 25 gr. 



Sodium sulphate 10 gr. 



Mercuric chloride (corrosive sublimate) 50 gr. 



Water 1000 cc. 



Shortly before using, Zenker's fluid is to be completed by adding 5 cc. 

 of glacial acetic acid to 100 cc. of the solution. The blocks of tissue placed 

 in it should be from 4 to 6 mm. thick; after remaining in the fluid from 10 

 to 24 hours they are to be placed in running water (or in water frequently 

 changed) for the same length of time. Then they are transferred to 80 per 

 cent, alcohol. 



The transfer of tissues from water to alcohol or vice versa is one of the 

 commonest procedures. The abrupt change from water to strong alcohol 

 causes violent diffusion currents which may distort the tissues; therefore 

 graded percentages of alcohol are used, 50 per cent., 70 per cent., 80 per 

 cent., 95 per cent., and absolute alcohol being always at hand. (Sometimes 

 90 per cent, also is used.) These may be prepared from the commercial 

 95 per cent, alcohol by adding water in the following proportions: 



Ninety per cent., 475 cc. of 95 per cent, and 25 cc. of distilled water. 

 Eighty per cent., 42; " ' 75 " 



Seventy per cent., 370" " " " 130" 



Fifty per cent., 265 " " " " 235 " 



Tissues may generally be transferred between water and 50 per cent, al- 

 cohol without injury. In passing from 50 per cent, to absolute they may be 

 placed successively in 70 per cent., 80 per cent., ancf95 per cent., remaining 

 in each only long enough to become saturated. Stains may be rated accord- 

 ing to the alcohol they contain; the transition from 80 per cent, to an aque- 

 ous stain should be graded as from 80 per cent, to water. It is a general 

 principle that all these transfers should be gradual for the best results. 

 Nevertheless abrupt transitions are often made, and ordinarily the tissue 

 preserved in Zenker's fluid and washed in water is next immersed in 80 

 per cent, alcohol. 



