400 SMITH'S INTERMEDIATE CHEMISTRY 



in the dark. They can be recognized, not only by their appear- 

 ance, but, without a microscope, by the iodine test. When a 

 drop of a potassium iodide solution, rendered brown by the ad- 

 dition of a little free iodine, is placed on the leaf or other part of 

 the plant, the granules of starch become blue while the other parts 

 are not affected. 



Preparation of Starch. If flour, which is made by grinding 

 wheat, and is three-fourths starch, is placed in a muslin bag and 

 kneaded under water, the granules of starch are washed out and 

 render the water milky (p. 5). After a tune the granules settle 

 and the water can be poured off. Starch is manufactured by 

 washing disintegrated potatoes (in Europe) or maize (in America) 

 on sieves, and collecting and drying the white powder deposited 

 in the water used for the washing. 



Starch is not soluble in water. If it be boiled with water, how- 

 ever, the granules swell and break, and the starch becomes finely 

 diffused through the water, forming a clear liquid. With little 

 water, a sort of transparent jelly is produced. When the liquid 

 is poured through a filter, a large part of the starch goes through 

 the paper as if it were truly dissolved. Such a liquid is called a 

 colloidal suspension (p. 109). Imitation solutions like this are 

 constantly met with in using complex organic compounds such as 

 enter into jellies, glues, soaps, and the juices of the bodies of an- 

 imals. Even inorganic substances, of the insoluble class, give 

 such suspensions. A description of their peculiarities must be 

 noticed under soap (p. 440) . 



The colloidal suspension of starch is used in the laundry, for 

 stiffening white goods. Glucose is manufactured from it. 



Since neither cellulose nor starch can be vaporized without 

 decomposition, and since they do not form true solutions in any 

 of the common solvents, we have no means of determining their 

 molecular weights, and are therefore forced to write their formulae 

 in the indeterminate forms (CeHioOs),, and (C^H-wO^x respectively. 



