HYDROCARBONACEOUS PROXIMATE PRINCIPLES. 



Fig. 5. 



STARCH GRAINS OF INDIAN CORN. 



are often marked with crossed or radiating lines, as if from partial 

 fracture. 



Starch derived from all these different sources has essentially the 

 same chemical composition, and may be recognized b}" the same tests. 



It is insoluble in cold water, but 

 if it be treated with about 

 twenty times its weight of boil- 

 ing water its granules swell, 

 become gelatinous and amor- 

 phous, combine with a certain 

 proportion of water, and fuse 

 into a thick opaline liquid, which 

 is thinner or thicker according 

 to the quantity of water present. 

 After that they cannot be made 

 to resume their original form, 

 but on cooling they solidify into 

 a nearly homogeneous paste, 

 retaining the water in union with 

 the starchy matter. The starch 

 is then said to be amorphous or 

 *' hydrated." By this process 



it is not essentially altered in its chemical properties, but only in 

 its physical condition. If starch be treated with 100 or 150 parts of 

 boiling water, it makes an opaline liquid which does not gelatinize ; but 

 on standing, the imperfectly liquefied portions, containing the insoluble 

 cellulose, subside to the bottom as a turbid deposit, while the soluble 

 starch remains above, forming a clear, colorless, and perfectly fluid solu- 

 tion. 



Starch is especially distinguished by its property of striking a blue 

 color by contact with iodine. This reaction will take place even when 

 the starch is in the raw condition, and starch granules may be readily 

 recognized under the microscope by this means. It is still more prompt 

 when the starch is hydrated and especially when it is in solution. 

 A very minute quantity of tincture of iodine added to a solution of 

 starch will cause the whole to assume at once a very deep and rich blue 

 color, which may be largely diluted without losing its characteristic 

 tinge. The mixture of the two substances, however, must, in the first 

 place, be made at a moderate temperature. If the solution be hot, no 

 visible reaction will occur ; and even after it has taken place if heat be 

 applied the blue color will disappear, to return again after cooling down 

 to the original temperature. Secondly, the iodine must be in a free 

 state. If it be used in the form of a soluble iodide, it will produce 

 no effect, since the starch has not sufficient affinity for it to withdraw 

 it from its union with other matters. No third substance, furthermore, 

 must be present in the mixture, which would be capable of combining 

 with the iodine and thus preventing its action upon the starch. All the 



