INTRODUCTION. 7 



five, or ten parts of water, depending upon the particular case 

 in which it is to be used. 



24. For use as a macerating agent in separating cells, a strong 

 .solution is preferable, and is more efficient when it is slightl}- 

 warmed. For dissolving or rendering transparent most of the 

 contents of cells, more dilute solutions are better. Owing to the 

 prompt effect produced, on the cell-wall, and upon the contents 

 of cells, especially of young ones, a moderately strong solution 

 of potassa is the most useful clearing agent that we have. After 

 a mass of tissue, for instance an embryo, has been acted on by 

 a solution of potassa until it has become translucent, it is to be 

 cautiously subjected to the action of an acid, preferably acetic 

 or hydrochloric, and then washed. A second treatment, or even 

 a third, may be necessary to make the object sufficiently clear. 

 Sometimes, however, the potassa renders the tissues too nearly 

 transparent, in which case they may be slightly clouded by a 

 little alum-water. This process of clearing tissues was first 

 used by Hanstein in the examination of the tissues at points of 

 growth, and it is of very wide applicability. 



25. Some structures are darkened at first by the use of 

 potassa, but cautious treatment afterwards with a dilute acid 

 and a second application of potassa will generally produce a 

 good degree of transparency. 



26. Potassa is a solvent for many of the substances which 

 incrust the cell-wall, but in most cases the solutions must be 

 used warm ; in a few instances heated even to boiling. The 

 cell-wall, washed alter such treatment, will give the cellulose 

 reactions (see 145). Suberin can thus be removed from the 

 cell-walls of cork, forming with the potassa yellowish drops. 



27. As the aqueous solution of potassa causes considerable 

 swelling of the cell-wall, it is desirable to have also at hand 

 an alcoholic solution. This is best made by mixing 95 per 

 cent alcohol with a strong aqueous solution of potassa until a 

 cloudiness appears. The mixture is then to be shaken fre- 

 quently, and, after a day or so, the clear liquid above is to be 

 carefully poured off. This solution may be diluted with alcohol 

 if necessary. 1 



28. Solutions of caustic soda can replace potassa in most 

 of the foregoing reactions. The special cases in which these 

 alkalies are employed for the identification of certain contents 

 of cells will be described later. 



1 Russow's Potash-alcohol. 



