bristles, hairs, flesh, &c.) possess the property of communicating a yellow tinge to flame.

This metal is taken into the system, principally in the form of chloride, which contains 40 per cent. of the metal. This salt is used at our table as a condiment, and is a constituent of most animal foods. Thus it is contained in both the white and the yolk of egg, in milk, and in flesh. It is not an ordinary constituent of plants, unless they grow in the neighbourhood of the sea or other salt water. Minute quantities of it are found in most of our common waters.

Sodium is expelled from the system both in the form of chloride and of oxysalt. In the urine of flesh-eating animals it exists in the form of sulphate and phosphate of soda.

11. CALCIUM. This metal is a component part of all animals. In the higher classes it exists principally in the form of subphosphate of lime. Thus, the bones of the vertebrata contain this salt mixed with a small portion of carbonate of lime. But the shells and crusts of invertebrated animals, as lobsters, oysters, &c., consist of carbonate principally, but mixed with a little subphosphate of lime. Muscles, nervous matter, the liver, the thyroid gland, and, indeed, all the animal solids, as well as the blood, contain calcium in the form of subphosphate of lime.

Calcium is a constituent of the white, the yolk, and the shell of eggs; and it is probable that the calcium found in the skeleton of the chick, when it quits the shell, was derived from one or more of these sources.*

* This, however, is denied by Dr. Prout (Phil. Trans. 1822, p. 399). "I think I can venture to assert," says he, " after the most patient and

It is likewise a constituent of milk, and from this source the young mammal derives the requisite subphosphate of lime for deposition in his bones.

We derive the calcium of our system from the animal, vegetable, and mineral substances which we consume as food. Thus bones, flesh, viscera, blood, and milk of animals, yield us this metal. To these sources must be added eggs, as above mentioned. Most vegetables also contain it. Thus subphosphate of lime is found in cereal grains, onions and garlic; the oxalate exists in the stalks of garden rhubarb used for making tarts and puddings; the tartrate is found in grapes; gum and unrefined sugar yield ashes containing calcium. Another source of calcium is common water (well and river water), which usually contains both bicarbonate and sulphate of lime.

"The Chinese," says Mr. Medhurst *, use great quantities of gypsum [sulphate of lime], which they mix with pulse, in order to form a jelly, of which they are very fond."

In some conditions of system a morbid appetite for calcareous substances exists. "Physicians," says Liebig," are well acquainted with the fact, that children who are not well supplied with a sufficient quantity of lime in their food, eat that which they

attentive investigation, that it [the lime of the skeleton of the chick] does not pre-exist in the recent egg; certainly not, at least in any known state. The only possible sources, therefore, whence it can be derived, are from the shell, or transmutation from other principles." I have before (p. 4) noticed Dr. Prout's opinions as to the origin of the lime of the chick when it leaves the shell.

* China, its State and Prospects, p. 38. Lond. 1838.

collect from the walls of houses, with the same appetite that they have for their meals." Such cases are, according to my experience, very rare; and there is no evidence to prove Liebig's assertion, that in these cases the food was deficient in its ordinary proportion of lime.

12. MAGNESIUM. Small quantities of this metal are found in the blood, teeth, bones, nervous matter, thyroid gland, and other parts of the body. It exists in combination with oxygen and phosphoric acid, and often with ammonia also. (See Phosphorus).

It is a constituent of both vegetable and animal foods. Thus it is found in cereal grains, potatoes, flesh of animals, milk, eggs, &c.

13. POTASSIUM. Minute traces of potassium exist in blood, the solids, and several of the secretions of animals.

Liebig* states, that "without an abundant supply of potash, the production of milk becomes impossible;" but I know not on what authority he makes this statement, for Schwartz † found only seven parts of chloride of potassium (equivalent to 3.68 parts of potassium) in 10,000 parts of human milk-a quantity apparently too minute to be of much importance.

Potassium is a constituent of both animal and vegetable food. Most plants which grow inland contain it; thus, it is found in grapes and potatoes. Its presence may be readily detected: burn a grape stalk in the candle-the minute ash obtained at the point

• Animal Chemistry, p. 164.

+ Gmelin, Handbuch der theoretischen Chemie, vol. ii. p. 1403.

of the burnt stalk will, if introduced into the outer or almost colourless cone of the flame, communicate a violet tint; thus demonstrating the presence of potassium or potash.

Nitrate of potash is sometimes used in the preparation of salted meats. This, therefore, is another source of potassium in the system. Moreover, common salt contains minute traces of this metal.

14. FLUORINE.-Berzelius detected minute quantities of fluoride of calcium in the bones and teeth of animals; but, more recently, Dr. G. O. Rees failed to detect it. If fluorine be a normal constituent of the body, it is doubtless introduced into the system in the small portions of the bones of animals occasionally swallowed with their flesh, for it cannot be derived from plants, since it has never been detected in these bodies. It is remarkable, however, that fluoride of calcium is abundant in fossilized bones, and in the human bones found at Pompeii and Herculaneum.

CHAP. II.-Of Alimentary Principles.

Two or more of the undecompounded bodies, described in the last chapter, form, by their union with each other, certain compound substances, termed Alimentary Principles, or Simple Aliments; and, by the combination or mixture of the latter, our ordinary foods, called Compound Aliments, formed.

are

Some alimentary principles contain two elements only, as Water. Others contain three, as Sugar and Fat. Proteine is formed of four elements, while Fibrine and Albumen contain six.

Some alimentary principles, as Water and Common Salt, are derived from the Mineral Kingdom: others are obtained from the Organised Kingdom.

Dr. Prout arranges alimentary principles in four great classes or groups, viz., the aqueous, the saccharine, the oleaginous, and the albuminous. The types of these groups are found in milk, the only article of food actually furnished and intended by nature as food for animals. Thus this secretion contains water, sugar, butter, and caseum (an albuminous substance).

This arrangement is a very excellent one; but several reasons induce me to adopt another. Milk

* On the Nature and Treatment of Stomach and Urinary Diseases, p. vi., Lond. 1840.