7. The air gets slowly changed in inhabited rooms where no fires are burning, provided free inlets and free outlets are arranged for its passage. The opening for the outward passage is best made somewhere near the ceiling, because then the light volatile exhalations from the lungs that rise towards the upper part of the room, are the most readily and surely removed. It is a very common mistake, in badly-contrived rooms, to make the upper sashes of the windows fixtures, so that they cannot be drawn down, and to provide no compensating outlets anywhere above. In such rooms the air is scarcely ever in a fit state to be breathed. In making arrangements to secure the perfect ventilation of apartments, it should always be borne in mind that air must go out as well as come in, or the purpose cannot be effected. An opening for the admission of air is of no use whatever, unless some other has been provided, in a convenient position, for the escape of that which is to give place to the new arrival: air will not flow into any room until an equal quantity has been driven out to make way for it. Fresh air will find its way in with equal facility either by high or low openings (if it be colder than the air contained within the apartment), provided there be but a sufficient space afforded for the escape of an equal quantity of the warmer air from the room.

8. Dr. Arnott has shown that a most efficient ventilation may be provided for close apartments by making an opening in the chimney of the room near the ceiling, and adapting to this opening a plate, so balanced as to allow air to flow from the room into the chimney, but to prevent it from moving in the opposite direction. In order, however, to insure the perfect operation of this ventilating valve it is necessary that the throat of the chimney over the fire, should be narrowed, and that there should be a free admission of cold external air, by a low inlet, otherwise the fire is apt to derive a portion of its air-supply from the ventilator, bringing down with it smoke from the chimney. When the inlets and outlets are properly adjusted to each other a portion of the warm air from the room escapes through

the fire, carrying with it carbonic acid, and another portion laden with the lighter exhalations, flows through the ventilating opening. When they are not properly adjusted, and there is a deficient space for supply compared with the outlets, the escape will always be through the fire in preference to the ventilator, and the latter may even be caused to furnish a downward draught.

9. A certain amount of exercise is essential to the perfection of the respiratory act, as well as the presence of uncontaminated air. The capacity of the lungs, the frequency of the respiratory movements, and the properties of the air are so adjusted to each other, that animals should live active lives. The breathing is made more frequent, and the blood is caused to course through its vessels with augmented speed by muscular effort. Consequently, more combustible matter is removed from the blood during such exertion, and more heat is produced. If a deep inspiration is drawn in the place of a shallow one, the air is found to contain at the end of the expiration a considerably higher proportion of carbonic acid than it does after ordinary expirations. The proportion occasionally becomes as high as nine per cent. The gentle and shallow movements of ordinary respiration do not unload the blood of all the combustible matter it ought to lose, but when quickened respiration and circulation are induced, the shallow breathings become deep ones, more blood is forced through the pulmonary vessels, more air is pumped into the air-cells, more oxygen gets into the blood, and more combustible matter is removed from it by exhalation. Brisk exercise in the open air is almost as important an influence as the perfect ventilation of dwellings in the preservation of health.

R. J. MANN.

Food.

BY LYON PLAYFAIR.

LESSON I.

USES OF FOOD.

1. BOTH plants and animals, while they live, require to be supplied with food, for if that is kept from them they soon die. In this chapter we are to read about the food of animals only, and to learn of what it is made up, and to what uses it is applied.

2. When the dead body of an animal is slowly heated, much water flies off as steam, and a dry mass is left which, on being put in the fire, partly burns away, while a small portion remains and cannot be burned. The portion that burns is called organic matter, and contains carbon or pure charcoal, hydrogen and oxygen, the two bodies forming water, and another element called nitrogen, which forms ths of common air, the other th being oxygen. The part of the dead body that did not burn is called inorganic or mineral matter, and is known to exist chiefly in the bones of animals, and in less quantity in their fleshy parts.

3. The organic part of animals consists of many substances, and although each of them is made up of only four elements, these are arranged in various ways, so as to form bodies quite unlike each other; flesh, blood, skin, horns, hair, tendons, feathers, all consist of these four elements, grouped differently.

4. Animals during life are constantly changing and throwing off their parts, which must be replaced by food. But as the latter does not contain flesh, skin, or horns, ready made, there are processes in the body by which simple kinds of food are converted into these complex forms. Science is yet a long way from knowing completely how such changes take place, but enough is known to show what portions either of vegetable or animal food are used for building up the structure of an animal body.

As the organic part of an animal contains the four

elements-carbon, hydrogen, oxygen, and nitrogenany part of the food used to build it up must also contain these four elements. If any one of them were absent, the structure would be imperfect, and could not be completed.

Both in vegetable and in animal food we find two classes of materials. One of these classes contains the whole four elements required for the formation of the animal body; the other contains carbon, hydrogen, and oxygen only, the nitrogen being absent. The latter class can therefore obviously not be used in the structure of the animal organs, and are in fact employed for a different purpose, which will be explained hereafter. In the meantime let us examine a little more closely the class of bodies containing the four necessary elements.

5. The various substances in this class bear a close relation to each other, whether they are found in vegetable or animal food. The fibrin obtained by stirring fresh blood with a stick resembles the fibrin from the juice of a cauliflower; the cheese got by adding vinegar to milk is quite like the cheese formed on adding the same acid to water which has stood over peas or beans; the albumen which forms the white of the egg is nearly the same as that found in a cabbage. The gluten got by kneading flour contained in a muslin bag, exposed to a stream of water, gives a good instance of this class of bodies in vegetables. Although trifling differences may be detected between these substances according as they are had from vegetables or from animals, they may for all practical purposes be considered as really the same among themselves, and as being almost identical in composition with the flesh of animals. By this is meant that if the cheese of milk or of peas, the fibrin of blood or of cauliflowers, the albumen of an egg or of cabbages, were all dried and given to a skilful chemist, along with some dried flesh, he would find in all not only the four elements carbon, hydrogen, oxygen, and nitrogen, but actually in the same proportion to each other, so that if they had not been marked he could not tell one from another, or state

from mere analysis of their organic portion which was flesh and which was vegetable. The general name given to this class of bodies in food is flesh-formers, because their use is to form the main parts of the animal body.

6. The similarity in nature between flesh-formers being known, the process of nutrition appears simple. It was always easy to see how a flesh-eating or carnivorous animal subsisted, for, eating flesh of the same nature as its own body, it was like feeding upon itself. But now when we know that vegetable food contains fleshformers also, the nutritive process in a vegetable feeder, or herbivorous animal, is as simple as in the former case. The animal in fact finds, so far as regards composition, though not as to arrangement, its flesh ready formed in the plant, and eats enough of the latter to supply as much flesh as was wasted from its body during the day. Looking at this a little more closely, it will appear that plants really always form the flesh for animals, for, although there are carnivorous beasts, they subsist on vegetable feeders which had their food from plants, and therefore the latter give really in the first place the flesh both to the herbivorous and the carnivorous animal.

It will be shown afterwards that different kinds of food contain unequal quantities of flesh-formers, but the latter, so far as regards nutrition, are the same, whether they reach the animal through the vegetable or in the apparent form of flesh. In the case of a hunt there is much muscular force exerted both by the hunter and the hunted; but the original structure of both, and the supply of their wasted parts, may come either from the animal or from the vegetable flesh-formers. Thus, an omnivorous animal-a man-mounted on an herbivorous animal-a horse-attended by carnivorous animalsdogs-pursues a vegetable-feeding hare or a flesh-eating fox. Hence we see that flesh-formers, to sustain strength, may either come immediately from vegetables or from animals, although in the first place they must always come from the former.

7. But it was stated that a second class of bodies in food