CLEARING 293 



according to the size of the pieces of material to be fixed. The 

 material should then be thoroughly washed out in water and 

 dehydrated by slow degrees in ascending grades of alcohol 

 (see page 260). A concentrated aqueous solution of chromic 

 acid may be used as a macerating fluid to cause the separation 

 of tissues into their separate cells. To this end rather thin bits 

 of the tissue to be macerated should be placed in the chromic 

 acid for about half a minute, and then carefully washed in water. 

 This operation may be carried on with sections under the cover- 

 glass. Silicious skeletons of diatoms, incrustations on the 

 epidermis of Equisetum, etc., may be prepared by allowing the 

 material to lie in concentrated sulphuric acid until it becomes 

 black, and then, after transferring to a 20 per cent, solution of 

 chromic acid for some minutes, washing thoroughly in water. 

 In the case of Equisetum and the like the tissues should be 

 scraped away from the inside down to the epidermis before treat- 

 ment with the acids. Chromic acid is useful in the recognition 

 of tannins, since sections containing tannins, when treated with 

 a I per cent, solution of chromic acid, yield a brownish 

 precipitate. 



Clearing. — For clearing media see Carbolic Acid, Cedar Oil, 

 Chloral Hydrate, Canada Balsam, Clove Oil, Eau de Javelle, 

 Glycerine, Origanum Oil, Turpentine, Xylol. A very successful 

 method of clearing whole leaves is to boil them, if fresh, in 95 

 per cent, alcohol to extract the chlorophyll, place them in 5 per 

 cent, hydrochloric acid for about ten hours, and then leave them 

 until quite transparent in a saturated solution of chloral hydrate. 

 In the case of leaves that have been dried, or preserved in formalin 

 or alcohol, and have in consequence become discolored and 

 difficult to clear, place them in a 10 per cent, solution of hydro- 

 chloric acid overnight and then keep them for three to seven days 

 in a saturated aqueous solution of chloral hydrate. From this 

 they are to be taken and thoroughly washed in water and brought 

 into a 5 per cent, solution of potassium hydrate, and there kept 

 until they are clear and transparent; then they are to be thoroughly 

 washed and transferred to dilute glycerine, or, if they are to he 



