VEGETABLE PKODUOTS. 149 



ment for the student to examine the form of grains in dift'cront 

 p]ants with a magnifying power of at least 100 linear. 1)V rub- 

 bing up any cellular substance in water, and washing the mass 

 in a linen cloth in a vessel of water, the starch grains Avill pass 



throuoh the cloth and settle in the water. Tlie starch o-mins in 



... * 



hot water swell up thirty times their volume and spread out, 



forming the paste used for stiffening clothes. At the tempera- 

 ture of 150° it is converted into dextrin^ or British gum, which 

 is soluble in cold water. 



286. The Potato contains little else than starch and water. 

 The bursting of the potato in cooking is owing to the swelling 

 of the starch grains. The starch diminishes as the potato grows 

 old. In combination with nitrogenized compounds it forms our 

 flour and meal. It exists in the cotyledons of the Pea and 

 Bean. Arrowroot is pure starch, l^ajnoca, from the poisonous 

 root of the Janipha manihot, is starch, partially altered by 

 heating. Sulphuric acid, or diastase, converts it into sugar. 

 Other agents do the same. Frost seems to produce the same 

 effect. It is the stored-up food of the plant. Iodine detects the 

 minutest portion, turning it blue. Iodine, diluted a million 

 times, can be detected by starch. The presence of starch has 

 been revealed in many places by Iodine, where it was not sup- 

 posed to exist, as in the latex and fovilla. 



287. There are several substances found in plants closely re- 

 sembling starch, but not formed into grains. Inulin^ in many 

 roots of the Compositae, is a white tasteless powder. Licheniti^ 

 in the Iceland moss, resembles starch in composition and prop- 

 erties. 



288. Gum, 0,2, H,o, 0,0, abounds in certain genera of plants, 

 existing in seeds, and exuding from stems when wounded. It 

 has been supposed by some to be the form which all elabonited 

 sap assumes before assimilation. The gum which exudes from 

 trees, and is perfectly soluble in water, is called Arabiii. It is 

 insoluble in alcohol, and is precipitated from its solution by it. 

 It seems to differ in some respects from dextrin, but in what is 

 not easily pointed out. Some chemists have given character- 

 istics whicli, with us, have not proved true, so far as we have 

 determined. GuriK Arabic is the type of gums. Various species 

 of Acacia afford it. It is produced in Egypt, Nubia, Arabia, 

 and Senegambia. The cherry and plum yield it. Mucilage, a 



What is dextrin ? — 286. Give some of the sources of starch. What effect 

 has sulphuric acid on it? What is the test of starch?— 2S7. What sub- 

 Btances resemble starch? — 2SS. Constitution of Gum? Wlicro found? 

 What is arabin? Its properiie^s? What is the source of Gum Arabic f 



What is mucilage? 



