plants, especially of the palm tribe; in unripe apples and pears, according to M. Liebig; and lastly, in several lichens. It rarely occurs in the stems and branches of dicotyledonous plants. In whatever part of the plant starch exists, it is contained in the cavities of the cellular substance, not attached to the cell, but perfectly isolated and surrounded with an aqueous liquid.

Several varieties of starch are found in commerce presenting slight modifications in their physical characters, according to the plants from which they are derived, and the processes by which they are extracted. The various sorts have hence received particular designations. But chemically speaking, there is only one kind of starch, which is found to be possessed of the same chemical characters and the same composition, from whatever plant or part of the plant it is derived: so that the several varieties must not be viewed as distinct species, but as mere structural modifications of the same chemical substance.

The principal varieties of starch which have received applications either in the arts or in domestic economy, are the following:

Common starch, derived from the grains of cereals, especially of wheat.

Potatoe starch, derived from the tubers of the potatoe.

Arrow-root, derived mostly from the roots of the maranta arundinacea.

East Indian arrow-root, chiefly from the tubers of the curcuma angustifolia.

Sago, derived from the pith of palms of the genus sagus.

Cassava and tapioca, derived from the tuberous root of the Jatropha manihot.

Indian corn starch, derived from the Zea mais. Salep, supposed to be derived from the roots of different species of orchis.

Tous les mois, supposed to be derived from canna coccinea, and

Rice starch.

§ I. PROPERTIES AND COMPOSITION OF STARCH.

In a state of purity, starch is a fine, white, tasteless, and inodorous powder. It gives a peculiar sound when squeezed between the fingers, and feels slightly crystalline, especially tous les mois and potatoe starch. It is insoluble in cold water, in alcohol, and in ether. In boiling water, starch resolves itself into a mucilaginous liquid which forms a jelly on cooling. Pure starch is soluble in dilute acids, and forms a transparent solution which undergoes a remarkable series of changes by ebullition. The recent solution is coloured deep blue by an aqueous solution of iodine, but after a short boiling iodine produces a purple colour, and on continuing the boiling iodine at last ceases to produce any colour. In this experiment the starch is converted first into a substance having the properties of gum, called dextrin, and afterwards into a variety of sugar, distinguished as starch sugar,

grape sugar.

Starch is soluble in strong solutions of the caustic alkalies. If brayed in a mortar with a concentrated solution of potash, starch unites with the alkali, forming a transparent gelatinous compound, which is pro

bably of definite constitution, and not a mere mixture of starch and alkali. This compound is soluble in water and in alcohol, and starch is precipitated from such solutions by acids. When a solution of starch is mixed with lime-water, with a solution of caustic barytes, or with a solution of subacetate of lead, a precipitate is produced consisting of a definite combination of starch with the base applied. A solution of starch is also precipitated by an infusion of galls and by a solution of borax. When heated with moderately strong nitric acid, starch is oxidated and converted into oxalic acid, and at least one other acid, the nature of which has not been exactly determined. No mucic acid is produced, as happens when gum is heated in nitric acid.

Starch dried at the temperature of 212° has the following composition:

The composition of starch dried at common temperatures is C12 H12 O12; by exposure to a temperature of 212°, it loses, therefore, two equivalents of water.*

12

9

12

* The compound of starch and oxide of lead was found by M. Payen to have the composition C12 H, O, + 2 PbO, which renders it probable that the composition of anhydrous starch is C12 H, O,: the hydrate obtained by drying at the ordinary temperature will then contain three equivalents of water; and that dried at the boiling point, one equivalent of water. We have then the following series:

Though possessing the chemical characters of a pure and distinct principle, starch, singularly enough, is possessed of an organized structure. When examined by the microscope, it presents the form of rounded grains, the size and shape of which differ not only in starch procured from different plants, but even in that from the same plant at different times, and from different parts of the same plant at the same time. Thus, the diameters of the granules of potatoe starch vary from the seven-thousandth of an inch to the two-hundredth of an inch. Fig. 20 is a

Fig. 20.

0000

representation of the ordinary appearance of potatoe starch when viewed by the microscope with a power of two hundred diameters. The largest granules obtained from the seed of the millet (fig. 21, with the same magnifying power) do not exceed in diameter, according to Raspail, the four-hundredth part of a millimetre, that is, very nearly, the ten-thousandth of an inch. The granules of starch obtained from wheat (fig. 22), are more regular in form than those from the potatoe.

The size of the granules increases with the age of the vegetable, and in certain organs the shape also changes. In the month of June, starch extracted

Fig. 23.

from the tuberous root of the iris (iris florentina) presents the appearance of fig. 23, with a power of one hundred diameters; but on allowing the tubers to repose in a shaded place in the open air for fifteen days, the starch then appears of the irregular forms in fig. 24. In the tuber vegetating in the earth, the starch does not acquire this appearance before autumn (Raspail, Chimie Organique, t. i. p. 452, 2me ed.). The granules of starch in the seed of the pea are three or four times as large as those in the expressed juice of the stalks (Liebig).

Fig. 24.

Leeuwenhoeck had observed in 1716, by means of the microscope, that each granule of starch is composed of an envelope differing in its characters from the interior globule. This observation, after having been entirely forgotten, was revived in 1825 by M. Raspail, who was led, by a series of experimental researches on the nature of amylaceous principles, to the following conclusions:

1o. The granule of starch is composed of an insoluble tegument and a soluble gummy interior, which is the substance now known as dextrin (page 99). 2o. The effect of a gentle heat on starch is first the expansion of the granule, and afterwards the bursting of the envelope; the rupture may occur in dry or wet starch. The application of many chemical agents, such as sulphuric acid and hydrate of potash, is attended with the rupture of the envelope merely on account of the heat produced by the combination of these bodies with the humidity of the atmosphere or of the starch. 3o. When starch