246 H. von Mohl on the supposed Existence 



exhibited by vegetable structures in polarized light stands in 

 connexion with the chemical composition of these structures. 



Turning from the consideration of the physical qualities of 

 the starch-grains treated with saliva, to their chemical characters, 

 and looking in the first place to their behaviour with water, we 

 find that they are wholly deprived of the property of swelling 

 up strongly and becoming partially dissolved. It must be pre- 

 mised here, that they are totally destitute of the power of swell- 

 ing up strongly in water after having been crushed. This is a 

 property of unaltered starch-grains which I think is very little 

 known*. If dried starch-grains, of the potato, for example, 

 are placed on a glass plate, and strongly compressed with a 

 piece of glass of suitable form, so that each grain assumes the 

 form of a flat disk traversed by cracks, and cold water is then 

 added, the flattened grains in the first moment apparently con- 

 tract, because they first of all swell up again into roundish 

 grains ; but they then begin to expand rapidly, as in hot water — 

 less regularly, however, than when they are boiled, often forming 

 pear-shaped curves, evidently because the crushing pressure acts 

 irregularly, and afiects one part of the grain more strongly than 

 another. This is particularly evident when a grain has been 

 exposed to strong pressure only on one side, as in this case the 

 crushed part is pushed out in the form of a large, apparently 

 vesicular protuberance, while the remaining part is almost un- 

 afi'ectedf. From grains crushed in this way, cold water extracts 

 in solution a portion of the starch-substance, which is coloured 

 blue by iodine, as is seen when solution of iodine is added ; for 

 the iodine precipitates the dissolved substance in the form of 

 blue gelatinous pellicles. 



The starch-grains treated with saliva are equally destitute of 

 the property of swelling up perceptibly when boiled in water. 

 Their lamination remains as distinct as before, and the angles 

 of cracked grains retain all their sharpness. 



If starch-grains, totally deprived of their soluble substance at 

 a tolerably high temperature, are, after thorough cleansing by 

 washing in water, placed in a new portion of saliva, they are 

 found altogether insoluble in it, even when the temperature is 



* This swelling-up of crushed grains in cold water appears to have been 

 noticed for the first time by myself, and is mentioned in the article "Starch" 

 in the * Micrographic Dictionary/ in opposition to the ordinary statement 

 that starch is unaffected by cold water. — A. H. 



t This has been explained by myself as a result of the cracks in the 

 resisting outer lamina3 allowing access of the water to the more soluble 

 inner substance of the grain. A similar local bulging takes place when 

 vyeak acid or potash is allowed to run in upon the shde and attack one 

 side of an uncrushed grain. — A. H. 



