330 CHEMISTRY. 



451. Properties of Starch. Starch is not soluble in cold 

 water. But while cold water produces no effect at all upon 

 it, a very peculiar effect is produced upon it by boiling water. 

 The hot water is absorbed by the granules, swelling them up 

 and uniting them together, so that the mixture becomes at 

 first mucilaginous, and at length is as thick as jelly. If this 

 jelly be boiled with water for some time the starch is ren- 

 dered soluble. It is this swollen starch that is so much used 

 in giving stiffness and smoothness to linen and cotton cloth- 

 ing, and in thickening the colors used in printing cloth. The 

 swelling of beans, pease, rice, etc., when they are cooked, 

 is owing to this absorption of the hot water by the starch 

 granules, which compose so large a part of these vegetables. 



452. Iodide of Starch. You remember that in 64 we 

 told you how to test for ozone by means of iodide of potas- 

 sium and starch paper. Ozone, however, is not necessary to 

 the production of the blue color ; any thing will do which 

 can set the iodine free nitric acid, for example. Instead 

 of taking potassium iodide to get the blue iodide of starch, 

 you may take an alcoholic solution of free iodine. A very 

 weak solution of starch will give you a beautiful blue color. 

 Warm this solution and the color will disappear, let it cool 

 and it will return; this is because the iodine separates 

 from the starch while hot and returns on cooling. 



453. Gums. Gum is a generic term for various substan- 

 ces which are alike in both constitution and in properties. 

 They exist in certain plants, and sometimes so abundantly 

 that they exude from the bark as a thick liquid, and harden 

 on exposure to the air. We have familiar examples of this 

 in our peach and cherry trees. The most widely known of 

 the gums is gum-arabic, which exudes from several of the 

 species of acacia in Africa. Most of the gums dissolve 

 readily in water, forming a mucilage. The mucilage of 

 gum-arabic is quite adhesive, and therefore is much used 



