CHAPTER II. -REAGENTS AND PROCESSES 



The different kinds of cell-walls and cell-contents may be demonstrated by the 

 use of reagents which, in some cases, impart characteristic colors to walls and con- 

 tents ; in other cases act as selective solvents, dissolving some of the walls and con- 

 tents, leaving others undissolved; or the reagents may produce precipitates the 

 nature of which furnishes good evidence regarding the character of the substance 

 which has united with the reagent to produce the precipitate. 



These reagents, together with their uses, will now be given in alphabetical order. 



Acetic acid dissolves most ethereal oils, while most fatty oils are insoluble in it; 

 dissolves calcium carbonate with evolution of COz, while calcium oxalate is un- 

 affected by it, and it therefore serves to distinguish between these two salts of cal- 

 cium; solvent of crystals of hesperidin which have been deposited from the cell- 

 sap of oranges, etc., when these have lain for some time in alcohol; when various 

 lichens are treated with it, crystals of calycin in acicular form are deposited after 

 the lichens thus treated have been powdered and dried; one per cent, solution dis- 

 solves globoids in aleurone grains, while any crystals of calcium oxalate present 

 are unaffected by it; when pieces of potatoes, carrots, etc., are macerated in it, 

 the separate cells become isolated. Used in the preparation of various fixatives. 



Albumen. The white of egg is used with an equal amount of glycerine and a 

 trace of salicylate of soda for fixing microtome sections to the glass slide, the sodium 

 salicylate acting partly as an antiseptic. 



Alcannin. This is a coloring matter, obtained from the roots of Alcanna tinc- 

 toria. A tincture of alcannin to be used as a reagent is prepared by Guignard as 

 follows: 10 Gm. of alcannin are pulverized and added to 30 mils of absolute alcohol; 

 the solution is filtered and allowed to evaporate in a drying oven ; the residue is then 

 dissolved in 5 mils of glacial acetic acid, and this solution is mixed with 50 mils of 50 

 per cent, alcohol. After twenty-four hours the solution is filtered and is ready for 

 use. The solution prepared in this way is said not to be subject to precipitation on 

 long standing. When sections are being treated with this reagent under a cover- 

 glass, evaporation should be guarded against by the addition of drops of 50 per 

 cent, alcohol as needed. A quicker way of preparing an alcannin solution is to 

 extract the coloring matter from roots of Alcanna in absolute alcohol, and then to 

 add an equal bulk of distilled water and filter the solution. 



(i) Suberized and cutinized walls, when treated with a solution of alcannin for 

 some hours, take on the color of the alcannin. (2) Alcanna tincture mixed with I 

 per cent, glacial acetic or formic acid is used to fix and stain sections of elaioplasts 

 from fresh material. (3) Where sections containing fatty oils are treated with 

 tincture of alcannin, the oil is colored red. Sections containing ethereal oils and 

 resins behave in the same manner, in that the ethereal oils and resins are stained 

 red. 



Alcohol. The commercial alcohol obtained in this country is about 95 per cent, 

 alcohol. In making alcohols from this of different strengths it answers all practical 

 purposes to proceed as if the commercial 95 per cent, alcohol were absolute that is, 

 very nearly 100 per cent. Thus, if 50 per cent, alcohol is desired, 50 mils commer- 

 cial alcohol and 50 mils distilled water will give sufficiently accurate results for all 

 histological work. If absolute alcohol is desired, it may be prepared by pouring 

 the commercial alcohol over unslaked lime, and then distilling from this over a 

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