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CHEMICAL operations are performed every hour in the day in
every household. From the moment when the housemaid strikes the first match in
the morning to the moment when the last candle is extinguished at night the
forces of chemistry are at work ; and even when all is still, and the gentle
breathing of the sleeping inmates is the only perceptible movement in the house,
that very breathing involves a beautiful and complex chemical process. And yet
how very few people know anything of chemistry! The mistress, when she washes
her hands, produces a double decomposition—and does not know it. The housemaid,
in striking the lucifer, is—little as she suspects it—promoting oxidation
through the influence of friction. And the cook may be shortly defined to be a
skilful practical chemist who knows nothing of chemistry.
We hope in this series of articles to illustrate the importance of chemistry in our everyday lives, and the numberless uses to which even a slight knowledge of it may be applied. Systematic instruction in the science would of course be out of place here. The title which we have selected indicates our plan with sufficient distinctness. We only propose to draw attention to those facts in chemistry which have a direct practical bearing upon the welfare of the individual household.
Let us begin with that most interesting, most important,
and most extensive subject, food, and endeavour to glean together some few of
the many facts which science has made known in regard to its nature, its uses,
and the various methods of preparing it which we have at our disposal. It is,
indeed, a very wide subject, for not only have we to consider a multitude of
different substances, prepared and cooked by a multitude of different methods,
but we must also, if we would know anything of the reason of the facts which
come before us, endeavour to learn something of the complex changes which go on
in the body, and the way in which the food we eat conduces towards them.
Why is Food required?—The question seems almost absurd, so familiar is the fact ; and yet the answer to it involves one of the grandest chapters in the history of science. In its simplest form it may be given in three words— food is fuel. We require food frequently for just the same reason that a fire requires coals frequently, and a lamp, oil—because we are burning away. Strange as this may appear, it is a most certain fact. The air that we breathe into our lungs contains oxygen, and this oxygen combines with or burns the muscles and other organs of our bodies just as it does the coals in a fire. The heat produced in a man's body in the course of a day is considerable in quantity, though not very intense in quality. Taking the average, it is enough to raise five and a half gallons of water from freezing point to boiling point, and this is about the heat that would be given off during the burning of a pound of coals. All this heat comes from the slow wasting or burning of the substance of the body, so that it is evident that if we did not make up for this constant loss by eating food, our organs would soon be wasted away and consumed. A moment's thought will show how closely this agrees with well-known facts. Why does an animal become so thin during the slow and painful process of starvation? Clearly because the slow fire in his body is not fed with the fuel of food.
This first simple view of the object of food must, how-[-339-] ever be examined a little more narrowly, and we then find that it requires a slight modification before we can accept it. For after all it is not the food itself, but the substance of the body, which is burnt. We must remember that, even if no food is eaten, the slow burning of the body goes on as long as the life of the animal lasts. It is, therefore, evident that, although it is ultimately burnt, the immediate object of the food is to repair the body—to make up for its incessant losses. Let us look for a moment at the changes which the food undergoes when eaten. It first goes to the stomach, and is there subjected to the beautiful cooking process which is called digestion. We shall have more to say about this farther on, and need only remark here, that by it the food is converted into a creamy liquid. This passes on into the intestines, and hence, by a most elaborate and wonderful process, it is absorbed into and becomes a part of the blood. All the food which acts any useful part in the body is first converted into blood. The blood—the mighty river of life, as it has been called—rushes with amazing force and swiftness through every part of the body. And it is from the blood that the constantly-wasting organs of the body, muscles, bones, nerves, and all derive their nutriment ; it is by it that their losses are compensated. Finally, to make an end of this part of the wonderful story, it is by the blood that the worn-out, burnt, and now useless materials are removed from the organs and thrown out of the body.
Different functions of Food.—Hitherto we have assumed, for the sake of simplicity, that the whole heat of the body is derived from the combustion of the living organs of the body, and, as a consequence, that all the food, after it is converted into blood, is absorbed by and becomes a part of those organs. But this is not really the case, and we are therefore led to take another step onward in our inquiry. The new step will involve a little additional labour of thought, but it is well worth the effort, for it is absolutely necessary if we wish to attain an accurate and scientific knowledge of the nature of food. It has been found that the heat produced by the burning of the organs of the body, is only a small part of the whole heat of the body. The rest of the heat is produced by the direct combustion of the blood itself. So that although all the useful food is converted into blood, only a portion of that blood is employed in repairing the muscles and other organs of the body.
We are thus led to perceive that there are two different uses to which the food which is eaten and converted into blood has to minister. The first is the repair of the organs, and the second the direct production of heat in the blood. And it is a very singular and interesting fact, that side by side with this distinction between the offices of food, there is an equally well marked distinction between the qualities of different kinds of food. Some of the most important constituents of food are utterly different in composition from the solid organs of the body. They cannot, under any circumstances, be employed in the body for the repairing of those organs. They are only valuable for the heat they produce when they are burnt in the blood ; in other words, they are mere fuel. Food of this kind is conveniently described as heat-producing food.
On the other hand, there is another great class of food-ingredients, which consists of articles almost identical in composition with the organs which have to be repaired. Let us, for the sake of simplicity, confine our attention to those very important organs, the muscles, which constitute what is generally called the flesh of the animal. A large portion of all food has almost exactly the same chemical composition as flesh. It cannot be doubted that the main purpose of food of this kind is to form flesh, and it is therefore known as flesh-forming food. Nevertheless, it is important to remember that this term does not convey the whole truth in regard to this kind of food. For some part of the so-called flesh-forming food does not become converted into flesh, but is burnt in the blood, like the heat-food. And even that part of it which does become flesh is afterwards, as we before explained, wasted away and burnt, so that although heat-food can never act as flesh-food, flesh-food can, and does act as heat-food. Dr. Savory fed some rats for a considerable time entirely on flesh-forming food, and he found that they remained in good health and retained their ordinary heat ; but this experiment, though very interesting as illustrating the double office of flesh-food, must not be understood as proving that heat-food may be dispensed with, for most animals would suffer very seriously upon such a diet, and require a properly-balanced proportion of the two kinds of food.
Work done by the Body.—There is yet another aspect or this subject which must not be left unnoticed. The body is not only a producer of heat—it is a very powerful engine. The muscles of the body are in reality machines for doing work. And the work they do is much greater than most people have any idea of. A strong man can easily do in a day as much work as though he lifted 350 tons a foot high. The heart itself, the most powerful and the most untiring of the muscles, pumps out the blood which passes into it with a force which appears almost incredible. At every beat it throws out five or six ounces of blood, and in twenty-four hours from fourteen to nineteen tons! The force required to do this would lift fourteen sacks of coals to the top of the Monument at London Bridge.
The whole of this enormous daily work is done at the expense of the food consumed, as certainly as the work done by a steam-engine is done at the expense of the coals burnt in the boiler-fire. And it appears probable that the parallel is still closer, for as in the steam engine the work is done, not by the coals, but by the heat produced from the coals, so the work of the animal body is done by means of the heat developed in it, and the whole of this heat, as we have already seen, comes ultimately from the food.
It was, until lately, believed that all the work of the body was done by the burning of the muscles themselves. If this were true, it is evident that flesh-forming food would be the only kind which would be of any use for the doing of work. But this has been clearly shown to be a mistake, and it is now held to be highly probable that both kinds of food, inasmuch as they both produce heat in the body, are alike serviceable for the doing of work. The practical importance of the question will be perceived at once. Men who do hard work eat more than others, and unless they know the right kind of food to eat, it is obviously possible that they may be cramming themselves with large quantities of food which is of little or no real use to them. We shall have more to say on this subject hereafter.
Classification of Food.—We may now attempt to form a classification of the constituents of food, which, without pretending to purely scientific completeness, shall yet be sufficient for the practical purposes which we have in view. It is very difficult to frame a thoroughly satisfactory definition of food. Perhaps the simplest is that which includes under it everything which is assimilated in the body, and which is necessary or useful to it. Taken in this wide sense, the term must be applied to some substances which are not generally reckoned under it. W ater for instance, common salt, and, even medicine, must in this view be regarded as food, and, in accordance with it, we will divide the materials of food into the four following heads :-
3. Mineral food.
4. Stimulants, spices, flavours, &c.
The two last of these may be dismissed for the present with very few remarks. We shall have much to say hereafter about water, and something about salt. The [-400-] mineral substance called phosphate of lime is an essential ingredient of food, because the bones of animals consist chiefly of it. All the most important articles of food contain it. Our fourth head is of necessity very vague. Under it we include alcohol, tea, coffee, spices, essences, and many other things which are useless, or nearly so, for the actual nourishment of the body, but which in many cases have a high special value of their own. The first two heads require a somewhat closer examination.
Flesh-formers —The solid part of the flesh of all animals consists chiefly of a substance called fibrin. Fibrin stands naturally at the head of the list of flesh-formers, for nothing could be more suitable for the repair of the flesh than flesh itself. Albumin, which is found in the juice of flesh, in the white of egg, and in the blood, is another flesh-former, very similar to fibrin in composition and properties. It is equal to fibrin as a flesh-former, and as it is soluble in water in its natural condition, it is more easy of digestion. It has, however, the curious power of becoming insoluble when boiled. White of egg consists almost entirely of albumin, and every one knows how entirely it is altered by a few minutes' exposure to the heat of boiling water. Milk contains a third important flesh-former called Casein. It forms the curd of milk and constitutes the greater portion of cheese.
These three substances, almost identical in composition with one another, and with the flesh, are the most important animal flesh-formers. But it has been found that compounds very similar both in composition and properties occur in those vegetables which are used as food. If a little flour be tied up in a small linen bag and squeezed under water with the fingers for some minutes, a fine white powder, called starch, is squeezed out and a sticky mass is left behind, well known as bird-lime. This is called glutin. It is a very important and valuable flesh-former. Lastly, peas, beans, and some other vegetable substances contain a compound called legumin, which is similar to, some say identical with, casein in composition and properties, and we have therefore in the most important articles of food five distinct, though very similar, flesh-formers.
Heat-producers.—The substances which are exclusively destined for the maintenance of the animal heat, and thereby to the production of work, are more numerous than the flesh-formers, with the exception of gelatin, of which we shall speak presently, and their chemical composition is much better understood. Leaving gelatin out of the question for the present, they may be roughly divided into the three following classes :-
1. Fats and oils.
2. Starches and gums.
Fats and oils form a very well-marked and important class. They are found both in animal and vegetable foods, and differ but slightly in composition in all cases. The most important examples among animal food are the fat of butchers' meat, the suet, lard, and dripping which are obtained from it, and butter, which in the form of cream is one of the most valuable ingredients of milk. Most of the staple articles of vegetable food contain a greater or less proportion of fat or oil, and they are, in particular, found in all seeds.
Starches and sugars are mainly derived from the vegetable kingdom, though examples of both occur in the animal body. They all have about the same composition, and although they contribute largely to the heat of the body, they are not as valuable, considered merely as fuel, as the fats and oils, which, we shall hereafter find, give out more heat in their burning than an equal weight of any other article of food.
Another common constituent of food is known as gelatin. It occupies a somewhat ambiguous position in our classification. It can only be obtained from certain animal substances, and does not exist ready-formed ever in them. When the tendons, skin, and similar parts of animals are boiled for a length of time in water, they gradually become soluble and then constitute gelatin. Bones behave in a similar manner, but the mineral matter remains behind unchanged. Glue and size are prepared in this way from the hides of animals. They consist of somewhat impure gelatin. Isinglass is nearly pure gelatin, and it is found in a less pure state in calves' feet jelly, and in the substance which is sold under the name of gelatin in the shops.
Gelatin is so like the flesh-formers in composition that we should naturally be inclined to class it among them and it is almost always considered by persons ignorant of science as a nutritious and valuable food. But it appears to be nearly certain that it has no value whatever as a flesh-former, and it must, therefore, be classed amongst the heat-givers. This fact is of greater practical importance than we should at first sight imagine. How often do people judge of the quality of soup or broth by the stiffness of the jelly which it forms on cooling. The test is utterly fallacious, because the stiffness is entirely due to gelatin. It is very easy to make the poorest soup set to a firm jelly by merely adding to it a little isinglass. One pound of gelatin will convert ten gallons of water into jelly, but this jelly has very little value as food, and is utterly useless for the production of flesh.
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