We will now pass to the consideration of the foods we do actually eat and drink.

In the first place, almost all these foods are mixtures of several of the substances I have been describing as essential food substances, but there is only one of these things that contains all the substances that are necessary for nourishing an animal, in proper proportions, and that one is milk, which is the natural nutriment of all young animals of the order mammalia for a certain time. It contains about 87 1/2 per cent of water, and about 12 1/2 per cent of solid matters. The milk from town-fed cows is richer than country milk, because the cows are mere machines for making milk; they do not go out into the fields and spend their energies, and therefore the town milk contains about 14 1/2 per cent of solid matters instead of 12 1/2 per cent.

What are the solid matters in the milk? In the first place it contains salts, which are in the proportion in which they are wanted, salts of lime, especially phosphate of lime, salts of soda, potash, magnesia, and others I need not mention, and it contains also iron. It contains fats which are in the form of an emulsion, and when the milk is allowed to stand these fats partly rise to the top in the form of cream. One of these fats contains phosphorus, and such a fat is found in the nervous tissues of all animals. It contains sugar - viz., lactose or sugar of milk, and it contains nitrogenous substances, the chief of which goes by the name of casein; it forms the curd of milk, and is the most important ingredient in cheese.

The milks of various animals have slight variations in their composition.

I will now describe to you a simple way by means of which you can tell for yourselves pretty accurately whether a specimen of milk is genuine or not; of course an accurate analysis of milk can only be made by chemical processes, performed by an experienced person in a chemical laboratory, but it can be examined sufficiently to arrive at a pretty accurate conclusion in this way: - Milk contains solid substances in solution; it is heavier than water, and has a density of about 1030, water being 1000, and an instrument has been made, called the lactometer, which shows the density of the milk in which it is placed. It consists of a hollow glass bulb, having some mercury at the bottom of it, and a stem which is graduated in degrees from 1035 downwards to 1000. Immediately this instrument is plunged into milk, it is clear that it will sink, and that the depth to which it will be submerged will be regulated by the greater or less density of the milk in which it is placed.

This is not sufficient of itself, because milk that contains a very large proportion of cream is lighter than milk that contains less, and so sometimes in very rich milk this instrument will stand very low down, and, on the other hand, substances could be added to increase its density. However, for all practical purposes it may be fairly assumed that milk is only tampered with by adding water, or by removing cream. A second test is easily made by means of an instrument called the creamometer: it consists of a tube divided into 100 parts, and it should be filled up to the top mark with milk, which should be allowed to stand for at least 12 hours, for the cream to rise; the amount of cream varies a great deal, but it should not be less than ten per cent by volume. If a sample of milk gives less, and at the same time the lactometer sinks low in it, then you may be almost certain that water has been added; if, on the other hand, with too little cream the lactometer stands at 1030 or higher, the milk has not been watered, but has been skimmed.

The food that most nearly approaches milk in its capabilities of nourishing an animal is an egg of a bird. You will at once understand this, as a young bird is formed of it; the egg must contain all the substances that are necessary to make an animal. It does not, nevertheless, follow from this, that it contains all the substances in their proper proportions to keep an animal alive; as the reverse is the case. An egg is deficient in water, and in calorific substances, especially of those in the division we call carbo-hydrates - starches, for instance - and these are replaced by the heat that is derived from the act of hatching; and without the shell the egg is also deficient in mineral salts, as these are supplied by the gradual dissolving of the shell which during the formation of the chick continues dissolving until only a thin crust remains.

The white of an egg consists of almost pure albumen, an exceedingly digestible substance when lightly boiled; but if boiled hard it becomes more indigestible, and is not so readily attacked by the gastric juice.

The yolk of an egg contains about 31 per cent of fat. but it contains also a considerable portion of nitrogenous substances in a highly digestible form.

Eggs get lighter by being kept, because some of the water evaporates through the shell, and air passes through the shell and takes its place. This air that passes into the egg facilitates the process of decomposition that goes on in stale eggs.

A fresh egg is heavier than a stale one. If you dissolve an ounce of common salt in ten ounces of water and place in the fluid an egg, if it be fresh it will sink, but if stale it will float. Sometimes stale eggs will get so light as to float even in water.

Eggs may be preserved in various ways: one of the simplest means is to put them in brine, another to smear them with butter, but it must be borne in mind that they very readily acquire the flavour of the substances in which they are kept.

A fresh egg is also somewhat translucent, and if you look through an egg at a candle the light will get more and more faint as the egg gets staler.