We are continually, as you know, getting rid of certain substances from our bodies. We get rid, in the first place, by means of all our excretory organs, of a considerable quantity of water; we get rid of that from the lungs, skin, and kidneys. We get rid of mineral salts especially in the secretion of the kidneys ; of carbonic acid especially by the lungs, but also by the kidneys and skin ; and of matters containing nitrogen -viz., a substance called urea, and another called uric acid-almost entirely by means of the kidneys.

It is necessary that these losses should be replaced. Besides this we are continually exerting force in various ways, and it is necessary that, in some way or another, this force should be generated.

So we see the necessity of taking certain substances into our bodies, and we have already seen how one substance, viz., oxygen, is obtained.

We will now go On to consider the other substances that are taken into our bodies. These we get from the mineral world, the vegetable world, and the animal world, but we may say that we get them all indirectly from the mineral world, because vegetables build up their structures from the substances they get from the mineral world, and we eat vegetables, or we eat the flesh of animals that have themselves fed upon the vegetable world.

Now, since the substances that our bodies are made of are exceedingly various in their composition, and since all parts of our bodies waste during use, and require to be repaired, it is quite clear that the food we eat must be of a mixed kind. It must contain a large number of substances ; no one substance at any rate, and not even two or three, would be sufficient for food for us, or for any animal.

So, then, we eat foods that are made up of a great variety of substances. The substances of which these foods are made up may be classified, and the first group includes substances belonging to the mineral world, viz., water and mineral salts. Since two-thirds of the weight of the body consists of water, and as we get rid of water by all our excretory organs, it is quite clear that we must take a considerable quantity of water in our foods; but this will be more fully considered in a separate lecture. We take, also, mineral salts in our food, directly and indirectly ; we take them directly in the form of condiments, and the most important of the mineral salts for us to take is common salt, or chloride of sodium. This salt is found in all the tissues, in the blood, and in all the fluids of the body. No doubt, one action of it is to promote the flow of saliva in the mouth, but that is not the only action; it is a food that is necessary to our existence, and to the existence of animals generally. Animals, in countries where common salt is scarce, go for hundreds of miles to have a lick at the salt rocks.

We get common salt, in some countries, from the sea-water, which contains a considerable quantity of it. In Norway it is got by freezing sea-water. It is obtained in some countries by evaporating sea-water. It is, however, got in much larger quantities from rocks containing salt. In this country there are enormous salt-works in several counties, notably in Cheshire and Worcestershire, where it is obtained either by digging it out, or more generally by pumping out the salt water contained in the fissures of the rooks. This brine is then evaporated, and the salt remains.

Other salts may be used as condiments instead of common salt, but they cannot replace it as a.food.

Then we require salts of lime, and especially phosphate of lime, because almost all our tissues contain salts of lime and phosphate of lime. All animals contain a certain quantity of phosphoric acid.

We get phosphates indirectly, and we get them chiefly in the grains that we eat, especially in the grains of cereals-wheat, barley, oats, maize, rice, and so on-and they get it in turn from water. The rain-water which falls on the soil dissolves phosphate of lime, and these plants collect it and enclose it in their grains. We get phosphates, too, to a certain extent in the meat we eat. We require a great number of other salts, but only one or two others that I wish to mention-salts of potash, from green vegetables, and salts of iron. Iron is a necessity for the existence of animals, because it is one of the constituents of the red corpuscles of the blood, and we obtain it especially in the red meat that we eat, also to a certain extent in most of the other foods. These, water and mineral salts, go by the name of inorganic food substances.

We have, besides, a class of food substances that we call organic food substances, because we get them from the organic world.

Now, some of these substances are composed of carbon, hydrogen, and oxygen; they contain no nitrogen, and so they are called non-nitrogenous foods. They are of two kinds-one class includes fats and oils, and the other class includes such foods as starches, sugars, and gums. Fats and oils contain a very large quantity of carbon and hydrogen, but very little oxygen; but starches, sugars, and gums contain a large quantity of oxygen, on an average amounting to half their weight; and besides these there is a small division which consists of certain organic acids that we get from the green vegetables-citric acid, tartaric acid, eta ; and lastly, alcohol must be mentioned under this head.

Organic foods that contain nitrogen are called nitrogenous foods; they contain carbon, hydrogen, oxygen, nitrogen, and very often sulphur or phosphorus. Now these nitrogenous food substances are also divided into two. large classes, and one smaller and less important class. The first class, which is the most important of all, includes substances like albumen and fibrine, substances that contain a considerable proportion of nitrogen, and that are called albuminous bodies or protein compounds; and the second class includes one important substance-viz., gelatine, and a few less important ones. There is also a smaller class containing nitrogen, which includes the essential principles of tea, coffee, cocoa, etc.