28 PRACTICAL HISTOLOGY. 



either by itself, or to complete the hardening processes begun 

 with other fluids. Moreover, it is almost universally used for 

 the preservation of tissues already hardened. There are three kinds 

 of alcohol used for histological purposes, viz., absolute alcohol, recti- 

 fied spirit, and methylated spirit. 



(a.) Absolute Alcohol is alcohol without water, but that sold 

 usually contains 96 per cent, of pure alcohol, and is sufficiently 

 strong for microscopical purposes. If an absolutely water-free 

 alcohol is desired, place well-dried potassic carbonate in the alcohol. 

 This rapidly absorbs the moisture. Or powdered and heated cnpric 

 sulphate a white powder is added. If water is present, it absorbs 

 it, and becomes blue again. Lowne uses slips of gelatine. 



(b.) Rectified Spirit contains 84 per cent, of spirit and 16 per 

 cent, of water. 



(c.) Methylated Spirit contains a. little wood naphtha, and is 

 nearly as strong as the ordinary absolute alcohol, and may be made 

 stronger by placing some well-dried carbonate of potash in it. The 

 carbonate of potash absorbs any water present in the alcohol. Others 

 use cupric sulphate well heated in a metallic capsule, until it 

 becomes a white powder. When it becomes blue in the spirit it 

 must be replaced by new CuSO 4 . It can be heated and used again. 



Alcohol is used of various strengths for hardening purposes. 



Seventy-five per Cent. Alcohol. To every 75 cc. of absolute 

 alcohol add 25 cc. of distilled water. 



Seventy per Cent. Alcohol. To every 70 cc. of absolute alcohol 

 add 30 cc. of distilled water. 



Fifty per Cent. Alcohol. Take equal volumes of absolute 

 alcohol and distilled water. 



To Harden in Alcohol Alone. The tissues are placed for 12-24 

 hours in the weaker (50 per cent.) alcohol, and passed through the 

 stronger alcohols (in each a day or thereby), and finally kept in 95 

 per cent, alcohol until they are required. 



For certain special purposes the tissues, e.g., glands and struc- 

 tures for the preservation of mitotic figures, are "fixed," and at the 

 same time hardened by being placed at once, and while as fresh as 

 possible, in absolute or 96 per cent, alcohol. 



B. Chromium and its Compounds. 



1. Chromic Acid (Stock Solution). It is well to prepare a 

 strong solution 10 per cent. and to keep this as a stock to be 

 diluted when required. Dissolve 10 grams of fresh chromic acid 

 in 90 cc. of distilled water. 



2. Half per Cent. -Solution of Chromic Acid. To 50 cc. of the 

 10 per cent, solution add 950 cc. of distilled water, or dissolve i 

 gram of chromic acid in 200 cc. of water. Similar solutions 



