CELL-CONTENTS. STARCH. 



425 



granules as it comes in contact with them. The course of the acid is 

 to be followed by moving the object gently upwards ; and when, from 

 its diffusion, the re- agent begins to act slowly, the peculiar changes 

 in the starch-granules can be more readily witnessed. In pressing or 

 moving the glasses, the starch disk becomes torn, and is then dis- 

 tinctly seen, especially in those coloured blue, to consist of two layers, 

 an upper and a lower one ; and the collapsed vesicular bodies of an 

 extremely fine but strong and elastic membrane." Mr. Busk believes 

 the hilum to be the central opening Into the interior of the ovate vesicle. 



Nitric acid communicates to wheat-starch a fine orange-yellow 

 colour; and recently-prepared tincture of guaiacum gives a blue colour 

 to the starch of good wheat-flour. 



Pure wheat-flour is almost entirely dissolved in a strong solution of 

 potash, containing twelve per cent of the alkali ; but mineral substances 

 used for the purpose of adulteration remain undissolved. 



Wheat-flour is frequently adulterated with various substances ; and 

 in the detection of these adulterations, the microscope, together with a 

 slight knowledge of the action of chemical re-agents, lends important 



fig-. 204. 

 Wluat-Flmr Starch-granules, mth a small portion of its celMose. 



diameters. 



Magnified 420 



assistance. It enables us to judge of the size, shape, and markings on 

 the starch-grains, and thereby to distinguish the granules of one meal 



