STARCH. 59 



animal fluids more especially, such as the serum of blood, saliva, mucus, 

 urine, contain ingredients which prevent the reaction of starch with 

 iodine, and may even dissipate the blue color after it has been once pro- 

 duced. These substances, therefore, must be removed from the fluid 

 before the application of the test, or else the iodine must be added in 

 sufficient excess to allow a surplus for action upon the starch. With 

 these precautions it forms a striking and 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 about 210 (about 

 4000 F.), it is converted into Dextrine, a gummy substance freely solu- 

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

 of the polarized ray toward the right. 1 Dextrine has the same chemical 

 composition with starch, namely C 6 H 10 5 , but its properties are changed, 

 and it will no longer produce a blue color with iodine. The same 

 transformation is very quickly accomplished by boiling starch with a 

 dilute acid; the opaline arid gelatinous solution becoming in a few 

 minutes clear and liquid, and losing at the same time its power of 

 reaction with iodine. Finalty, in the germination of starchy seeds, 

 like the cereal grains, a nitrogenous substance is produced termed 

 " diastase ;" and this has the power, when supplied with moisture at a 

 moderate temperature, of effecting the transformation of the starch into 

 soluble dextrine. 



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

 a thin starch paste is boiled with a dilute acid, it is rapidly changed, 

 as already mentioned, into dextrine. If the boiling be continued for 

 several hours it is still further transformed into sugar ; and at last the 

 whole of it passes over into the saccharine condition. This also hap- 

 pens in the process of germination and growth in plants, where sugar 

 makes its appearance under influence of the diastase, and at the expense 

 of the starch, as soon as moisture and warmth are supplied in the 

 requisite degree. This is the usual source' of sugar in vegetable 



1 A ray of light which has passer! through certain crystalline bodies, such as a 

 " 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 it be 

 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 prin- 

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

 second prism is turned round 90, the light is entirely 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 passing 

 through the organic liquid. Some substances deviate in this way the plane of 

 polarization toward the right, others toward the left. The specific rotatory 

 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 rotatory power 

 of dextrine is 118. 



