TESTS FOR SUGARS AND OIL 247 



nectar of flowers it attracts insects. To test for glucose: 

 Make a thick section of a bit of the edible part of a 

 pear and place it in a bath of Fehling's solution. After 

 a few moments boil the liquid containing the section for 

 one or two minutes. It will turn to an orange color, 

 showing a deposit of an oxide of copper and perhaps 

 a little copper in the metallic form. A thin section 

 treated in like manner may be examined under the micro- 

 scope, and the fine particles, precipitated from the solution 

 by the sugar of the pear, may be clearly seen. (Fehling's 

 solution is made by taking one part each of these three 

 solutions and two parts of water: (1) Copper sulfate, 9 

 grams in 250 c.c. water; (2) sodium hydroxide, 30 grams 

 in 250 c.c. water; (3) rochelle salts, 43 grams in 250 c.c. 

 water.) To test for cane sugar: (1) Make a thin section 

 of sugar beet and let it stand a few minutes in a strong 

 solution of copper sulfate. Then carefully rinse off all 

 the salt. (2) Heat in a very strong solution of potassium 

 hydroxide. There will be seen a blue coloration in the 

 section, gradually washing out into the liquid. 



To test for oil: Mount a thin section of the endosperm 

 of castor-oil seed in water and examine with high power. 

 Small drops of oil will be quite abundant. Treat the 

 mount with alcanin (henna root in alcohol). The drops 

 of oil will stain red. This is the standard test for fats 

 and oils. 



To examine gum-resin : Mount a little of the "milky" 

 juice of the leaf stem of the garden poinsettia (Euphorbia 

 pulcherrima) . It is of a creamy consistency. Examina- 

 tion under the microscope shows that it is not white, as 

 it seems to the naked eye. The particles are yellowish 

 or colorless and are insoluble. These particles are gum 

 resin. They have been emulsified by the plant, making 

 the juice appear white. 



398. CONTENTS NOT IN SOLUTION. Starch is the most 



