32 PRACTICAL HISTOLOGY. 



They are hardened for twenty-four hours in this mixture, and after- 

 wards washed for twenty-four hours in water, and the hardening 

 completed successively in 70, 80, and 90 per cent, alcohol each for 

 12-24 hours. Sections can be stained with hsematoxylin. 



8. Osmic Acid. ^lake a i per cent, solution by dissolving i 

 gram in 100 cc. of distilled water. This substance is rather ex- 

 pensive, and is sold in sealed glass tubes. Carefully clean the 

 surface of the tube, snip off one end of it, and place it with the 

 requisite quantity of water in a glass-stoppered bottle, which has 

 been carefully cleaned, so that it does not contain a trace of 

 organic matter. Organic matter decomposes it very rapidly. It takes 

 several hours to dissolve. Some prefer to use normal saline solution 

 instead of water to dissolve it. Its vapour is very irritating to the 

 eyes and mucous membranes generally. It should be preserved in 

 yellow-coloured bottles and kept in the dark. If a yellow bottle is 

 not available, cover a bottle with brown or black paper. 



9. Osmic Acid Vapour. This is most useful for thin membranes 

 and glands. Tissues stain readily after its use. 



10. Chromo-Aceto-Osmic Acid (Flemming's Fluid). To 45 cc. 

 of i per cent, chromic acid add 12 cc. of 2 per cent, osmic acid, 

 and then 3 cc. of glacial acetic acid. This can be kept for a long 

 time, and need not be kept in the dark. It is specially useful for 

 fixing the figures in cell-division or mitosis, and for many other 

 purposes. It " fixes " tissues in from a few hours to twenty-four 

 hours or longer ; but the pieces must be small, 2-3 mm. thick, as it 

 does not penetrate deeply. This is Flemming's " strong formula." 



A weaker fluid is sometimes used, and is prepared as follows : 



Osmic acid (i per cent.) . 10 c.c. 



Glacial acetic acid ( i per cent.) . . .10,, 



Chromic acid (i per cent. ) . . . 25 ,, 



Water . . . . 55 ,, 



They must be thoroughly washed by being kept in running 

 water for twelve hours, and then hardened in the various strengths 

 of alcohol, 70, 80, and 90 per cent., each for twenty-four hours. 

 Sections should be stained as soon as possible after they are made, 

 as on keeping they do not stain so well. They may be stained witli 

 safranin, haematoxylin, or methyl-violet, but safranin is the best 

 (p. 75), it stains the chromatin of the cells a bright red. Tissues 

 hardened in it are not well adapted for teasing. 



11. Fol's Solution. This is a modification of 10. It contains 

 less osmic acid, and is used more generally. To 2 cc. of i per cent, 

 osmic acid add 25 cc. of i per cent, chromic acid, five parts of 2 per 

 cent, glacial acetic acid, and 68 cc. water. 



Precautions with Osmic Acid or Solutions containing it. If a 



