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COMPOSITION AND CHEMICAL CHARACTERS OF STARCH. 33 
have seen, they vary much in form and size under the same 
circumstances ; those, however, which are derived from the 
same plant are more or less uniform in appearance, so that a 
practised observer may distinguish under the microscope the 
different kinds of starch and refer them to the particular plants 
from whence they have been derived. Sometimes there is more 
than one hilum in a starch granule, and then as growth takes 
place round each, compound granules are formed, as mentioned 
on page 30. 
Composition and Chemical Characteristics of Starch, 0,H,,9;. 
—The starch granule consists of the true starch-compound and 
water, The starch-compound is again formed of two substances, 
which are intimately blended together, viz. granwlose and cellu- 
lose. The granulose makes up by far the greater part of the 
starch-compound, being in the proportion of 95 to 5 of the cellu- 
lose. It is capable of being dissolved out of the cellulose by 
saliva and dilute acids, and it is to this granulose that the 
starch granule owes the violet-blue colour which it assumes 
when treated with a solution of iodine. The cellulose on the 
other hand, being not soluble, is left behind as a skeleton, and 
is not coloured blue by the iodine solution. 
Starch is, therefore, composed chemically of carbon and the 
elements of water; it never occurs, however, naturally in a 
perfectly pure condition, but always contains a very small 
quantity of mineral constituents, and also a certain proportion 
of the peculiar secretions of the plant from whence it has been 
derived. These impurities can never, under ordinary circum- 
stances, be entirely removed, and from their varying amount 
in commercial starches arises in a great degree the differences 
in their value for food and other purposes. 
Starch is insoluble in cold water, alcohol, ether, and oils. 
By the action of boiling water it swells up and forms a mucilage 
or paste ; and if to this when cooled iodine be added, a deep 
blue colour is produced ; but this colour is at once destroyed 
again by the application of heat or alkalies. If starch be ex- 
posed to a temperature of about 320° F. for a short time, it is 
converted into a soluble gummy substance, called dextrin or 
British gum. A similar change is produced in starch by the 
action of diluted sulphuric acid, and also by diastase, a peculiar 
nitrogenous substance occurring in germinating seeds. Starch 
was formerly considered as peculiar to plants, and its presence 
therefore was regarded as an absolute distinctive mark between 
them and animals. Of late years, however, as already noticed 
(page 4), a substance presenting the chemical reactions and 
general appearance of starch has been found in some animal 
tissues. Such a distinctive character, therefore, can be no 
longer absolutely depended upon. 
Rapuipes.—This name is now more generally applied to all 
inorganic crystals of whatever form which are found in the cells 
D 
