Domestic Economy, Hygiene, Dietetics. AIR. The common air is a fluid composed mainly of two gases, in certain proportions; namely, oxygen as twenty and nitrogen as eighty parts in a hundred, with a very minute addition of carbonic acid gas. Such is air in its pure and right state, and such is the state in which we require it for respiration. When it is loaded with any admixture of a different kind, or its natural proportions are in any way deranged, it cannot be breathed without producing injurious results. We also require what is apt to appear a large quantity of this element of healthy existence. The lungs of a healthy full-grown man will inhale the bulk of twenty cubic inches at every inspiration, and he will use no less than fifty-seven hogsheads in twenty-four hours. Now, there are various circumstances which tend to surround us at times with vitiated air, and which must accordingly be guarded against. That first calling for attention is the miasma or noxious quality imparted to the air in certain districts by stagnant water and decaying vegetable matter. It is now generally acknowledged that this noxious quality is in reality a subtle poison, which acts on the human system through the medium of the lungs, producing fevers and other epidemics. bination just alluded to. Now, carbonic acid gas, in a larger proportion than that in which it is found in the atmosphere, is noxious. The volume of it expired by the lungs, if free to mingle with the air at large, will do no harm; but, if breathed out into a close room, it will render the air unfit for being again breathed. Suppose an individual to be shut up in an airtight box: each breath he emits throws a certain quantity of carbonic acid gas into the air filling the box; the air is thus vitiated, and every successive inspiration is composed of worse and worse materials, till at length the oxygen is so much exhausted that it is insufficient for the support of life. He would then be sensible of a great difficulty in breathing, and in a little time longer he would die. Most rooms in which human beings live are not strictly close. The chimney and the chinks of the doors and windows generally allow of a communication to a certain extent with the outer air, so that it rarely happens that great immediate inconvenience is experienced in ordinary apartments from want of fresh air. But it is at the same time quite certain that, in all ordinary apartments where human beings are assembled, the air unavoidably becomes considerably vitiated, for in such a situation there cannot be a sufficiently ready or copious supply of oxygen to make up for that which has been consumed, and the carbonic acid gas will be constantly accumulating. This is particularly the case in bedrooms, and in theaters, other rooms, and they are usually kept closed during the whole night. The result of sleeping in such a room is very injurious. A common fire, from the draught which it produces, is very serviceable in ventilating rooms, but it is at best a defective means of doing so. The draught which it creates generally sweeps along near the floor between the door and the fire, leaving all above the level of the chimneypiece unpurified. Yet scarcely any other arrangement is anywhere made for the purpose of changing the air in ordinary rooms. Putrid matter of all kinds is another conspicuous source of noxious effluvia. The filth collected in ill-regulated towns, ill-managed drains, collections of decaying animal substances placed too near or within private dwell-churches, and schools. ings, are notable for their effects in vitiating Perhaps it is in bedrooms that most harm the atmosphere, and generating disease in is done. These are generally smaller than those exposed to them. In this case, also, it is a poison diffused abroad through the air which acts so injuriously on the human frame. The human subject tends to vitiate the atmosphere for itself, by the effect which it produces on the air which it breathes. Our breath, when we draw it in, consists of the ingredients formerly mentioned; but it is in a very different state when we part with it. On passing into our lungs the oxygen, forming the lesser ingredient, enters into combination with the carbon of the venous blood (or blood which has already performed its round through the body); in this process about two fifths of the oxygen is abstracted and sent into the blood, only the remaining three fifths being expired, along with the nitrogen nearly as it was before. In place of the oxygen consumed, there is expired an equal volume of carbonic acid gas, such gas being a result of the process of com FOOD. A food is a substance which, when introduced into the body, supplies material which renews some structure or maintains some vital process; and it is distinguished from a medicine in that the latter modifies some vital action, but does not supply the material which tain it can efficiently produce flesh or repair wasted tissue. So important is this distinction, in fact, that one of the divisions of food most generally recognized by physiologists is into nitrogenous and non-nitrogenous, or, as Liebig termed them, the flesh-forming and the heat-producing. Both kinds are essential to the maintenance of life, and it is because vegetables as a whole are deficient in nitrogen that the highest degree of bodily vigor cannot be kept up by them alone. sustains such action. It is essential to the idea of a food that it support or increase vital actions; whilst medicines usually may lessen, increase, or otherwise modify some of them. "Foods are derived," says Dr. Edward Smith, "from all the great divisions of nature and natural products, as earth, water, and air, solids, liquids, and gases; and from substances which are living and organic, or inanimate and inorganic. The popular notion of food as a solid substance derived from animals and vegetables, whilst comprehensive is too exclusive, It is understood that the structures of the since the water which we drink, the air which body are in a state of continual change, we breathe, and certain minerals found in the so that atoms which are present at one hour substance of the earth, are, adopting the defi- may be gone the next, and when gone the nition given, of no less importance as foods. structures will be so far wasted, unless the procIt is, however, of great interest to note how fre- ess of waste be accompanied by renewal. But quently all these are combined in one food, and the renewing substance must be of the same how closely united are substances which seem nature as that wasted, so that bone shall be reto be widely separated. Thus water and min-newed by the constituent elements of bone, erals are found in both flesh and vegetables, and flesh by those of flesh. This is the duty whilst one or both of the components parts of the air, viz., oxygen and nitrogen, are distributed through every kind of food which is alone capable of sustaining life. Hence, not only may we add food to food to supply the waste of the body, but we may within certain limits substitute one for another as our appetites or wants demand. . . . Further, there seems to be an indissoluble bond existing between all the sources of food. There are the same classes of elements in flesh as in flour, and the same in animals as in vegetables. assigned to food,- to supply to each part of the body the very same kind of material that it lost by waste. As foods must have the same composition as the body, or supply some such other materials as can be transformed into the substances of the body, it is desirable to gain a general idea of what these substances are. The following is a summary of the principal materials of which the body is composed: Flesh, in its fresh state, contains water, fat, fibrin, albumen, besides compounds of lime, phosphorus, soda, potash, magnesia, silica, and iron, and certain extractives, whose nature is unknown. Blood has a composition similar in elements to that of flesh. Bone is composed of cartilage, fat, and salts of lime, magnesia, soda, and potash, combined with phosphoric and other acids. Cartilage consists of chondrin, from which gelatine is formed, with salts of soda, potash, lime, phosphorus, magnesia, sulphur, and iron. The brain is composed of water, albumen, fat (so-called), phosphoric acid, osmazome, and salts. "The vegetable draws water and minerals from the soil, whilst it absorbs and incorporates the air in its own growth, and is then eaten to sustain the life of animals, so that animals gain the substances which vegetables first acquired. But in completing the circle the vegetable receives from the animal the air (carbonic acid) which was thrown out in respiration, and lives and grows upon it; and at length the animal itself in whole or in part, and the refuse which it daily throws off, become the food of the vegetable. Even the very bones of an animal are by the aid of nature or man made to increase the growth of vegetables and really to enter into their structure; and being again eaten, animals may be said to eat their own bones, and live on their own flesh." The lungs are formed of a substance called It will be seen from this that animal and veg- connective tissue, from which gelatine is formed etable foods contain precisely the same ele- by prolonged boiling, albumen, a substance ments though in different combinations. At analogous to casein, various fatty and organic the same time they differ sufficiently to make acids, with salts of soda and iron, and water. a due proportion of each necessary to perfect nutrition. One sterling point of difference is, that nitrogen constitutes a much larger percentage of animal bodies than of vegetables. Nitrogen is one of the most important elements of food; only such substances as con The liver consists of water, fat, and albumen, with phosphoric and other acids, in conjunction with soda, lime, potash, and iron. Bile consists of water, fat, resin, sugar, fatty and organic acids, cholesterin, and salts of potash, soda, and iron. Hence, it is requisite that the body should be provided with salts of potash, soda, lime, magnesia, sulphur, iron, and manganese, as |