300 REAGENTS AND PROCESSES 



use of eau de Javelle in differentiating cutinized and suberized 

 membranes. 



Eosin. — An aqueous solution of eosin is an excellent stain for 

 protoplasmic cell-contents and cellulose walls. The solution 

 should be quite dilute. For the use of eosin in double staining 

 see under Cyanin and Gram's Method. See also in the next 

 chapter under Aleurone Grains. 



Fehling's Solution. — Prepare three separate solutions: (i) 

 17.5 gm. of copper sulphate in 500 c.c. of water; (2) 86.5 gm. 

 of sodium-potassium tartrate in 500 c.c. water; (3) 60 gm. of 

 caustic soda in 500 c.c. of water. To prepare for use, mix i 

 volume of each of these with 2 volumes of water. The solutions 

 keep well separately, but the mixture becomes changed after a 

 time, and for this reason the solutions should not be mixed until 

 needed. 



Sections may be treated with this solution on the glass slip. 

 Two small drops of distilled water are placed on the slip with 

 I small drop of each of the three solutions; then sections, not 

 too thin, of the material which is to be tested for glucose are 

 placed in the mixture on the slide. It is best to cut the sections 

 without wetting the razor, and the sections should not be placed 

 in water, but should be transferred directly to the mixture on the 

 slide. The sections should be covered with a coverglass and the 

 slide carefully heated over the flame of an alcohol lamp, or a 

 very small flame from a Bunsen burner, until bubbles rise in the 

 solution. If glucose is present, the sections will appear reddish 

 from very small crystals of cuprous oxide which have been reduced 

 from the solution. If it is not desired to observe the crystals of 

 cuprous oxide within the cells, but simply to demonstrate the 

 presence of grape-sugar, small pieces of the tissues to be tested 

 may be placed in a test-tube containing a few cubic centimeters 

 of the solution, which is then heated to boiling; if grape-sugar is 

 present in considerable quantity, a copious precipitate will after 

 a time settle to the bottom of the tube. See under Copper Acetate. 

 This is particularly suitable for demonstrating the presence of grape- 

 sugar in those cells which contained it in the uninjured tissues. 



