cold, if added to one of the element Iodine, in water, will immediately produce a deep blue color, which is esteemed by the chemist as an exceedingly marked characteristic of starch.

If a portion of this blue compound, or as it may be called, "iodide of starch," be thrown into boiling water, the blue color instantly disappears, and very seldom returns when the water cools; hence the experienced chemist, when testing for starch by iodine, or vice versâ, always takes the precaution of employing the solutions cold, or he will fail in obtaining the blue color, even though abundance of both substances may be present.

Pure starch is perfectly white, but is apt to assume more or less of a yellow tinge in the course of time; hence the manufacturer of this proximate principle for domestic use, confers upon it a slightly blue tint, by the addition of a small quantity of "smalt," or "oxide of cobalt;" this renders it perfectly unfit for all refined chemical purposes, and for such the pure white starch of wheaten flour, or of arrow-root, must be invariably used.

An aqueous solution of iodine is easily made, by placing one grain by weight of such solid element in a pint bottle filled with rain-water, and leaving it for a few hours; if, then, a portion of this solution, which has the color of brown sherry, be poured into another and smaller bottle, and a few grains of wheat or barley thrown into it, even upon agitation for several minutes no blue color will be produced, and yet we are certain that the respective grains must contain starch; then why does the iodine fail in immediately detecting its presence?

For a reason that often presents itself to the chemist during his manifold experiments upon organic and in

organic matter, and that has more than once been presented during our rapid and general inquiry; viz., that mechanical structure or attraction of aggregation opposes chemical attraction or affinity, and in the case now pre-sented it is that of the thin hard coating of bran or husk around the grains which prevents the contact of their starch with the iodine.

Place the mixture of iodine and grains of wheat or barley in a mortar, and crush them with the pestle thus mechanically destroying their attraction of aggregation, that of the solid iodine being already destroyed by chemical solution,-and then, the starch that they contain will meet with the iodine, the two substances will combine, and produce the blue color, or iodine of starch.

Again, if perfectly dry iodine, and perfectly dry starch, be rubbed in a mortar, no blue color will appear, because neither of the substances is sufficiently divided by this mechanical proceeding to exert chemical attraction; but the addition of a few drops of water will effect a chemical division, or a solution of part of the iodine, and then it will immediately and chemically combine with the mechanically divided starch; it is also probable that a slight solution of the starch simultaneously takes place.

Like as with "soda-water powders," consisting of dry tartaric acid and carbonate of soda, though both are mechanically divided, they exert no chemical action upon each other; but when thrown into water, they are simultaneously and chemically dissolved, and in such state of solution far exceeding in attenuation the utmost exertion of mechanical communication; they are immediately brought into perfect and mutual contact, and then display their chemical incompatibility; "as

vinegar upon nitre," (natron,) by evolving a form of carbonic acid.

Bread contains gluten and starch; the latter may be detected by a very simple experiment:—place a thin slice of bread on a plate, and moisten it with an aqueous solution of iodine; and from being white or nearly so, it will become dark blue, because in bread, the starch, though still solid, is neither dry, nor protected as it was in the grain.

If the blue-stained bread be now placed on another plate, and covered with a solution of "ammonia” or strong"hartshorn," the blue color will vanish, in consequence of a very extraordinary affinity that some of the elements of the ammonia have for the iodine, which is greater than its affinity for the starch; or, in other words, in the one case a compound is produced, and in the next it is decomposed.

Starch may be decomposed by the mere application of a moderate heat, as in baking or roasting; it then becomes brown, and soluble in cold water, and yields no blue color with iodine; in fact, the heat has disturbed. the original arrangement of its proximate elements, Carbon, Hydrogen, and Oxygen, and caused them to group together as Gum; reference is made to its composition at page 34.

A piece of bread is insoluble in water, and therefore confers no taste; if a piece of bread be toasted brown, and then thrown into water, it forms the beverage well known as "toast and water," which has a mucilaginous taste, and for this reason, the heat of the fire has converted a portion of the original starch of the bread into Gum; this dissolves in the water, and forms mucilage.

At the same time, during the toasting of the bread, a

portion of its original gluten, starch, and recently-formed gum, are decomposed into volatile matters, which cause the smoking, and peculiar smell of the toast-after the first dampness or steam of the bread has passed away,but a portion of carbon does not volatilize; it remains, forming the "charred" or "burnt-toast;" and this, when thrown into water, forms a dark brown extract, which colors "the water."

If the bread be more perfectly toasted, so as to become very much blackened and burnt, the resulting charcoal is not useless, as might be supposed; for, upon throwing it into a portion of water unfitted for beverage by long keeping, the charcoal will deprive it of all offensive odor and taste, and render it palatable, yet the theory of this fact is not understood.

By the long-continued action of diluted sulphuric acid upon starch, the chemist can further derange the natural proportions of its elements, and produce Sugar, a proximate principle likewise consisting of Carbon, Hydrogen, and Oxygen, as shown at page 34.

A small proportion of Sugar exists in wheaten flour, and likewise another principle called Albumen; but these require a very considerable amount of chemical knowledge for their eduction, and the details of the process would encroach too much upon the prescribed limits of this examination; reference, however, must be made to the fact, that wheat flour, in addition to its organic principles, gluten, starch, sugar, and albumen, contains notable traces of the inorganic compounds, called phosphate of lime and phosphate of magnesia; and it is a most remarkable fact, that these inorganic substances are transferred from wheaten and other food

in which they exist, into the constitution of the bones of man, and of the higher order of animals.

The extreme utility of manuring wheat lands with recent "bone-dust," consists in the fact, that the organic, or tough matter of the bone called Albumen, is extremely prone to putrefy, and thus render soluble the phosphates of lime and magnesia, with which it is associated; these are accordingly rendered into a state suitable for absorption by the rootlets of the growing grain, and are chiefly transferred to the ear, whilst the silica and potash are arrested, to form the glistening straw, as mentioned at page 143.

Bones, when calcined or burnt, have all their really organic matters, such as albumen, fat, and oil, destroyed and dissipated, and nothing but their inorganic phosphates remains; such "bone-ash," although invaluable to the chemist as a source of phosphorus, is not so to the agriculturist, because it is insoluble, and not easily or perfectly received by the rootlets of plants, and therefore it requires solution, or rather decomposition, by the agency of diluted sulphuric acid, before it is added to the soil.

Bone-ash is the source from whence the chemist elicits all his supplies of phosphorus,-about twenty thousand pounds weight per annum,-for the manufacture of“ Lucifer matches," he generally effects the decomposition of the bone-ash by submitting it to the action of charcoal and sand at a red heat; these agents deprive the phosphoric acid of oxygen, and elicit phosphorus, as the "red lead" deprived hydrochloric acid of hydrogen, and elicited chlorine, at page 218.

Another most important office of the organic matter of bone, and of animal and vegetable manures in which