HYDROCARBONACEOUS SUBSTANCES. 53 



contain ingredients which interfere with the reaction, and may even 

 dissipate the blue color after it has been produced. These substances 

 must be removed before the application of the test, or the iodine must 

 be added in excess to allow for action on the starch. With these pre- 

 cautions it forms a valuable test. 



Starch has the property of being changed, under certain conditions, 

 into two other substances. 



1. If subjected to torrefaction, that is, a dry heat of 210 C. (about 

 400 F.), it is converted into Dextrine, a gummy substance soluble in 

 water, so called from the fact that in solution it rotates the plane of the 

 polarized ray toward the right.* Dextrine has the same chemical com- 

 position with starch, but its physical properties are different, and when 

 treated with iodine it takes a rosy red instead of a blue color. The 

 same transformation of starch is accomplished by boiling with a dilute 

 acid ; the solution becoming in a few minutes clear and liquid, and 

 changing its reaction with iodine. Finally, in the germination of 

 certain starchy seeds, such as the cereal grains, the transformation of 

 starch into soluble dextrine takes place in the presence of moisture at 

 moderate temperatures, under the influence of a nitrogenous ferment. 



2. Starch may be converted into Sugar. When a starch solution or 

 thin starch paste is boiled with a dilute acid, it is first changed, as de- 

 scribed above, into dextrine. But by continued boiling for several 

 hours it begins to be further transformed into sugar, and at last it passes 

 wholly into the saccharine condition. The same conversion takes place 

 during the germination and growth of plants, where sugar makes its 

 appearance at the expense of the starch, as soon as the requisite moist- 

 ure and warmth are supplied. This is the usual source of sugar in 

 vegetable juices, the starch previously stored up being changed into 

 sugar by the molecular actions going on in the vegetable fabric. Finally, 

 various nitrogenous animal substances, like those in the saliva or the 

 .intestinal juices, at the temperature of 38 C., have the same effect. 

 This is the change which normally takes place during digestion. 

 Starchy substances, when taken as food, are changed into sugar in 



* A ray of light which has passed through certain crystalline bodies, such as a 

 li Nicol's prism " of Iceland spar, is found to be polarized ; that is, it has acquired 

 opposite and complementary properties in two different directions. For if received 

 by a second similar prism, which is equally transparent in all positions to ordinary 

 light, the polarized ray will pass through it only when the principal section of the 

 second prism is parallel with that of the first ; but when the second prism is turned 

 round 90, the light is arrested. Now if certain organic substances in solution be 

 placed between the two prisms, it is found that they have the effect of changing the 

 angle at which the second prism must stand in order to arrest or transmit the light 

 from the first. In other words, the plane of polarization of the polarized ray has 

 been deviated or rotated by the organic liquid. Some substances deviate the plane 

 of polarization toward the right, others toward the left. The specific rotary power 

 of each is estimated for a solution of standard strength and quantity, for yellow 

 light, and is indicated in degrees of the circle. The specific rotary power of dex- 

 trine is 118. 



