PROPERTIES OF STARCH. 321 



upon the globule are then seen slowly to unfold, and may at 

 times be traced into the ruga> of the frill. Finally, the granules 

 swell up to twenty or thirty times their original bulk, present- 

 ing the appearance of a flaccid sac.* 



Grains of potato-starch, when illuminated by polarised light, 

 a Nicol's prism being interposed between the object and the 

 eye, show a well-marked black cross, the centre of which cor- 

 responds with the hilum. There being no such appearance in 

 the grains of wheat-starch, a means is thus afforded of detect- 

 ing the fraudulent addition of potato starch or flour to wheat- 

 flour. 



Starch, as found in commerce, is a white glistening powder, 

 which, when pressed between the fingers, produces a peculiar 

 sound, while a feeling of elasticity is discerned. It is insoluble 

 in cold water, alcohol, and ether. In hot water it undergoes 

 a peculiar change : the exterior layer of the granules absorbs 

 water, they swell up, and the mixture takes that viscid character 

 which fits it for stiffening linens. The starch cannot be re- 

 covered in its first state from this solution. When dry starch 

 is considerably heated it passes into British gum, which is 

 identical with dextrine. 



Starch appears to retain a small proportion of saline matter, 

 which in part consists of potash ; and it even contains, in the 

 integument of the grains, a minute quantity of azotised matter. 



The deep-blue colour which starch gives with free iodine is 

 the means of detecting its presence even in the most minute 

 quantity. When the solution of starch and free iodine is boiled, 

 the colour disappears, but returns again when cooling takes 

 place. Starch dpes not ferment with yeast, but when mixed 

 with chalk and cheese, and kept at the temperature of 100 F., 

 it is said that alcohol is developed after several weeks. 



The most abundant sources of starch in the vegetable king- 



* Busk, ' Quart. Journ. Micros. Society,' vol. i. 

 X 



