CARBOHYDRATES AND RELATED BODIES. 43 



is insoluble in water at the ordinary temperature, but with 

 elevation of temperature in presence of an excess of water the 

 protective layer breaks and allows the granulose to form a 

 more or less perfect solution of so-called soluble starch. 



On the technical scale starch may be obtained from a variety 

 of substances. The common sources are potatoes, corn, rice 

 and arrowroot. The manufacture is largely a mechanical 

 operation, which may be illustrated as follows : 



Ex. Grate a potato to a pulp by means of an ordinary tin grater, mix 

 the pulp with water and squeeze through a piece of coarse unbleached 

 muslin, collecting the strained liquids in a large beaker. Allow the mix- 

 ture to settle a half hour or longer and pour off the water, which contains 

 some soluble albuminous substances, some cellular floating matter, but very 

 little starch. Most of this will be found in the bottom of the beaker. 

 Add some fresh water, stir up and allow to settle. Now pour the water 

 off again and repeat these operations until the starch appears perfectly 

 clean and white. Transfer this starch to a clean shallow dish and allow 

 what is not intended for immediate use to dry spontaneously in an atmos- 

 phere free from dust. The dried product will consist of minute glistening 

 particles resembling small crystals. Save this starch for tests given below. 



Ex. Examine starch from several sources under the microscope, em- 

 ploying a power of about 300 diameters. Clean a glass slide thoroughly, 

 place in the center of it a small drop of water, and stir into this by means 

 of a needle, or glass rod, a minute quantity of starch. Now drop on a 

 clean cover glass in such a manner as to exclude air bubbles, and place 

 under the microscope for observation. 



Ex. Repeat the last experiment, using an aqueous solution of iodine 

 instead of water. The starch granules will now appear blue. For the 

 detection of starch in mixtures the use of iodine is often indispensable. 



Some idea of the size of the starch cells may be obtained 

 from this table which gives the mean diameter in fractions of 

 a millimeter. For starch granules which are oval instead of 

 circular, the averages of the longer and shorter diameters is 

 given : 



Potato 0.06 -o.io 



Common arrowroot o.oi -0.07 



Corn 0.007-0.02 



Wheat 0.002-0.05 



Rice 0.005-0.008 



Pea 0.016-0.028 



Bean 0.035 



Barley 0.013-0.040 



Rye 0.002-0.038 



