The old German poet who wound up each verse of his famous drinking song by the assertion that "four elements intimately mixed, form all nature and build up the world," was not so far wrong after all. The jovial song-writer referred to his favourite formula for brewing punch; and according to him the world of conviviality was built up by lemon and sugar, rum and hot water.

Now, it is perfectly true that four elements go a great way towards building up the world; but, setting aside the question of brewing punch, they are called carbon, hydrogen, oxygen, and nitrogen. So universal is their presence in the living and growing parts of animals and plants, that they are always spoken of as "organic elements," and science has ascertained exactly the proportion in which each should exist in a healthy condition of the human body. That body is incessantly, but imperceptibly, undergoing a process which cannot be better described than by the expression of perennial moulting, only that, whereas certain animals cast off certain parts of their body - their skin, their hair, or their feathers - every year, we lose a portion of our weight every day ; that is to say, we should lose it if we did not absorb through our lungs, the pores of our skin, and our stomachs, sufficient oxygen, carbon, hydrogen, and nitrogen, to supply the loss caused by the wear and tear of our daily life. There has even been an attempt made to prove that our vital organs are entirely renewed every forty days or so, but for this calculation there can be no really satisfactory data, although there certainly is constant loss and gain going on within us. The material for repairing this incessant waste which is the inevitable result of the activity of our nervous and muscular system, is not supplied alone by the starch, sugar, water, and fat, nor yet by the milk, meat and vegetables we consume, but by a due combination of food material which shall ensure the proper proportions of albumen, fibrine, and caseine absolutely-required by our changing frames. These are rather hard words, but their meaning will be quite plain if we take as familiar examples of the three indispensable ingredients, the white of an egg, a piece of lean meat, and a bit of cheese. Everyone can understand that, although these things contain the largest proportion of one particular substance, still there may be many other substances in which they are present, all together, and it is just to teach us this, and to explain to us why we should rather give our attention to procuring one form of food than another, that a knowledge of the elements of Practical Chemistry is useful. In reading the accounts of the hardships and sufferings of explorers and travellers, we are often surprised to learn that first one member and then another of the expedition dropped down and died long before the supplies were actually exhausted. This is particularly noticeable in the account of Burke and Wills' attempt to explore the great plains of South Australia, where one by one the travellers died, not so much from sheer lack of some sort of food to eat, as from the unhappy circumstance of the only attainable food being utterly deficient in the ingredients without which the human body cannot be nourished. For instance, there was abundance of an alkaline plant on which the natives almost live at certain times of the year, and occasionally even a few fish were caught.

But these materials taken by themselves were so weak in life-supporting properties, that they failed to repair sufficiently the waste caused by severe exercise and exposure to the weather. A man may be starved to death, and yet scarcely feel hungry; that is to say, he may be able to put food into his mouth which will allay the cravings of his appetite, but which may not have the least power to nourish his body, so that he will die as surely as though he had nothing to eat.

Men's instincts are generally the surest guides, and however much we may have been disgusted to hear of such facts as of Esquimaux and Samoiedes living upon blubber and fat, and even eating 8 lbs. or 10 lbs. of flesh at a meal, Science teaches us that they were unconsciously adopting the very best means of keeping up the supply of carbon and oxygen, or internal warmth, which their cold climate rendered absolutely necessary. So in the same way we often see a sick person take a fancy to some curious kind of food, and perhaps begin to recover from the moment he was allowed to have it. The chances are that if we could bring all the practical chemists in the world into his sick-room, and they were to analyse the component parts of that particular food, and at the same time ascertain exactly which of the organic elements of human life was insufficiently represented in the patient's system, the result of their researches would go to prove that the sick man knew exactly what he wanted to build him up in health, better than anyone else.

Nature is our surest guide after all, only unfortunately our civilization has blunted our instincts, and rendered us more or less artificial, so that we can hardly tell what is Nature, and are obliged to call in the aid of Science to teach us. Those who live in hot countries do not require to provide their systems with internal warmth by means of food, and we shall generally find that they prefer a diet which will contain very little carbon. But it often happens that an Englishman travelling or living in such places will become terrified at his loss of relish for meat and heating food, and will fly either to his doctor for tonics, to his cook for pickles to incite his flagging appetite, or, still worse, to wine or brandy for stimulants to repair his imaginary weakness. Nature, thus thwarted in her arrangements, turns sulky, and the man falls ill, accusing the climate of the fault springing from his own ignorance and folly. In his own country he knows much better what is good for him; and in mixing bacon with his beans, or in taking, like the Irishman, cabbage with his potatoes, or, like the Italian, a strong kind of cheese with his maccaroni, he exhibits so many purely chemical ways of preparing mixtures nearly similar to each other in composition and nutritive value.

In the rudest diet, and in the luxuries of the most refined table, the main cravings of animal nature are never lost sight of. Besides the first taste in the mouth, there is an after-taste of the digestive organs, which requires to be satisfied if we want to arrange a perfect diet. It is not necessary that a food should yield every kind of material which the body requires to nourish it, for then one sort of food might be sufficient for the wants of man. Each sort must fulfil one or more of the body's requirements, so that by a wise combination the whole of its wants may be supplied. It is also to be borne in mind that our nourishment is not only the solid food which we actually take into our stomachs, according to the popular idea on the subject, but comprises the water we drink and the air we breathe. But as these pages should treat simply of the nourishment for our bodies, which nourishment must needs be submitted to the action of fire, it is only with the cooking of food we have to deal.

In considering the question of the best and cheapest food, and the most wholesome mode of cooking it, we must keep steadily before us the principle, that it is not the quantity of food received into the human body which nourishes it, but the proportion which can be digested of such food. All else is sheer waste - an encumbrance worse than useless - whose presence clogs and throws out of gear the delicate mechanism appointed to deal with it.

It is generally agreed by scientific chemists, that in casting around for something like a form of food which could be taken as a type of all others, there is none so perfect as milk. During the period when the young of animals as well as of human beings are fed entirely on milk, they grow very rapidly in the size of every part of their bodies. From this we infer that milk must contain all the essentials which go to build up muscle, nerve, bone, and every other tissue. The first lesson we learn from taking milk as an example of perfect natural food, is that there should be a certain proportion of liquid mixed with the substances we consume as food, though, as the animal attains its full size and there is only waste to be made up, not growth to be provided for, the necessity for the liquid form of food diminishes.

Of the flesh-forming substances contained in milk, caseine is the most important, and in the largest proportions ; therefore it is with milk in the form of cheese that it can best be dealt with as human food in this place. Now, there is a popular theory that cheese is unwholesome, and it certainly is an indigestible substance, but still it need only be avoided by those who suffer from weak digestions. The hardworking man who labours with his muscles in the open air, and whose stomach is in the best possible condition to digest his food, does wisely to spend, as he generally does, what little money he may possess in cheese, for cheese contains nearly twice the quantity of nutritive matter he would get in the same weight of cooked meat. Even with delicate feeders, a small quantity of cheese taken with other food facilitates digestion, for caseine is easily decomposed or put in a condition which causes other things to change. When, therefore, we eat a piece of cheese after a meal, it acts like yeast in bread, and starts a change in the food; for the chances are that the stomach in trying to digest the cheese will digest the rest of its contents at the same time. The mouldy cheese which some people's instinct leads them to prefer, acts more quickly in this way than fresh cheese. When cheese is spoken of as a nourishing article of food, especially to those who labour in the open air, it is only cheese in which the cream has not been previously separated from the milk, for the actual nutritive value will depend on the amount of butter material left in it. The cheap skim-milk cheeses of South Wales yield so little nourishment in this respect, that they are of but slight value as flesh-formers, whereas the rich cheeses from Cheddar, Stilton, and Ayrshire are not only infinitely cheaper than meat, but are also very nourishing.

It will perhaps only be necessary to take bread and beef as samples of food which contain in themselves every element required to build up the human frame, to repair the daily waste, and to preserve all the conditions of perfect health. The generality of mankind have found out the value of these substances for themselves without the aid of science ; but it may be as well to learn something about bread and beef, for the simple reason that as we cannot always, under all circumstances, make sure of having them as food, we may be able to select those substances which come nearest to them in nutritive value, if we understand the component parts which make them so important.