142 STARCHES. 



than the number of water molecules, the chemical constitution 

 being usually expressed by the symbol, Co Hio O5, or a probable 

 isomer C18H30O15. 



Starch, when heated to 158*^ F., swells up and forms a paste, 

 from which alcohol precipitates a white powder consisting of 

 soluble starch. Heat alone dries the granules, and at a tempera- 

 ture of 320° F. converts them into a soluble modification called 

 dextrine. This dextrine differs from starch in being soluble in 

 cold water, and in producing a reddish brown colour with iodine 

 instead of the bluish purple so characteristic of true starch. This 

 test, above all others the most delicate for starch, can most readily 

 be applied under the microscope, all that is required being to 

 place a small portion of the material to be examined on a slide 

 with water under a covering glass, and by means of a delicate 

 pipette introduce a solution of iodine in potassium iodide, to the 

 edge of the cover. A slip of bibulous paper now placed on the 

 opposite edge will, by capillary attraction, draw out the water 

 from under the cover, and cause the iodine to run in gradually, 

 tinting any starch granules blue, the colour gradually deepening as 

 the action of the re-agent is the more prolonged. Dextrine, under 

 similar circumstances, takes a copper colour. Inulin, however (a 

 variety of starch to be obtained from the tubers of certain Compo- 

 sitce^ as dahlia and elecampane) is not thus affected by iodine. 



The composition of starch cannot be considered alone, for 

 during the growth of a plant we find in it certain other consti- 

 tuents having the same composition as starch. Of these, dextrine, 

 gum, and sugar need only be mentioned here as connected with 

 our subject, as we shall find it probable that in the life of the 

 plant these three substances, or others analogous to them, are the 

 immediate principles from which starch granules are produced. 

 In this connection I may mention that starch boiled with dilute 

 acid loses its power of becoming blue with iodine, and is, in point 

 of fact, first of all converted into dextrine, and ultimately into 

 glucose, a form of sugar. This same change is brought about 

 naturally in the plant by the action of diastaste. 



Having spoken thus far on the chemistry of starch, we will 

 next proceed to investigate its physical characters, and herein not 

 a step could be taken without the aid of the microscope, which. 



