Radiant heat has been considered by most writers the best means of warming; it heats the body without heating the air,' and of course there is no possibility of impurity being added to the air.

The disadvantages of radiant heat are its cost, and its feebleness at any distance. The cost can be lessened by proper arrangement, but the loss of heat by distance is irremediable. The effect lessens as the square of the distance-i.e., if, at 1 foot distance from the fire, the warming effect is said to be equal to 1, at 4 feet distance it will be sixteen times less. A long room, therefore, can never be warmed properly by radiation from one centre of heat only.

It has been attempted to calculate the amount of air warmed by a certain space of incandescent fire, and 1 square inch has been supposed sufficient to warm 8.4 cubic feet of air. But much depends on the walls, and whether the rays fall on them and warm them, and the air passing over them.

Radiating grates should be so disposed as that every ray is thrown out into the room. The rules indicated by Desaguliers were applied by Rumford. Count Rumford made the width of the back of the grate one-third the width of the hearth recess; the sides then sloped out to the front of the recess; the depth of the grate from before backward was made equal to the width of the back. The sides and back were to be made of nonconducting material; the chimney throat was contracted so as to lessen the draught, and insure more complete combustion. The grate was brought as far forward as possible, but still under the throat.

The open chimney, which is a necessity of the use of radiant grates, is so great an advantage that this is per se a strong argument for the use of this kind of warming, but, in addition, there can be little doubt that radiant heat is really the healthiest.

Still the immense loss of heat in our common English fire-places must lead to a modification, and radiant heat must be supplemented by

Convection and Conduction.

The air is heated in this case by passing over hot stones, earthenware, iron or copper plates, hot water, steam, or gas pipes. The air in the room is thus heated, or the air taken from outside is warmed, and is then allowed to pass into the room, if possible at or near the floor, so that it may properly mingle with the air already there. The heat of the warming surface should not be great, probably not more than 120° to 140° Fahr.; there should be a large surface feebly heated. The air should not be heated above 75° or 80° Fahr., and a large body of air gently heated should be preferred to a smaller body heated to a greater extent, as more likely to mix thoroughly with the air of the room.

It does not matter what the kind of surface may be, provided it is not too hot. If it is, the air acquires a peculiar smell, and is said to be burnt; this has been conjectured to be from the charring of the organic matter. Some have supposed the smell to be caused by the effect of the hot air on the mucous membrane of the nose, but it is not perceived in air heated by the sun. Such air is also relatively very dry, and absorbs water eagerly from all substances which can yield it.

I Dr. Sankey has made experiments which show that the temperature of the air of a room heated by radiant heat is really lower than the temperature indicated by the thermometer, because the bulb is warmed by radiation. When this is prevented by enclosing the bulb in a bright tin case the thermometer falls.

If the air is less heated (not more than 75°) it has no smell, and the relative humidity is not lessened to an appreciable extent. Haller's experiments, carried on over six years with the Meissner stove common in Germany, show that there the relative moisture is not lessened with moderate warming,' and the same result has been found with the Galton stoves. On the other hand, when the plates are too hot, the air may be really too much dried, and Dr. Sankey states that while he never found the difference between the dry and wet bulbs in a room warmed by radiant heat to be more than 8° Fahr., he has noticed in rooms warmed by hot air a difference of 15° to 17° Fahr., which implies a relative humidity, if the temperature be 60°, of only 34 per cent. of saturation, which is much too dry for health. In this case the air is always unpleasant, and must be moistened by passing over water before it enters the room, if possible; some heat is thus lost, but not much. Of the various means of heating, water is the best, as it is more under control, and the heat can be carried everywhere. Steam is equally good, if waste steam can be utilized, but if not, it is more expensive. Hot water pipes are of two kinds: pipes in which the water is not heated above 200° Fahr., and which, therefore, are not subjected to great pressure; and pipes in which the water is heated to 300° or 350° Fahr., and which are therefore subjected to great pressure. These pipes (Perkin's patent) are of small internal calibre (about inch), with thick walls made of two pieces of welded iron; the ends of the pipes are joined by an ingeniously contrived screw. In the low-pressure pipes there is a boiler from which the water circulates through the pipes and returns again, outlets being provided at the highest points for the exit of the air. In Perkin's system there is no boiler; one portion of the tube passes through the fire.

Mr. Hood states that 5 feet of a 4-inch pipe will warm 1,000 cubic feet in a public room to 55°. In dwelling-houses for every 1,000 cubic feet 12 feet of 4-inch pipe should be given, and will warm to 65°. In shops, 10 feet, and in workrooms 6 feet per 1,000 cubic feet are sufficient. If Perkin's pipes are used, as the heating power is greater, a less amount does, probably about two-thirds, or a little more.

Steam piping is now also much used, and in some cases is more convenient even than water. The Houses of Parliament are warmed by steam pipes in a chamber under the floor; the radiating surface of the pipes is increased by soldering on to them at intervals a number of zinc or (preferably) small copper plates. If it is wished to lessen the amount of heat, the pipes, where provided with thin plates, are simply covered with a woollen cloth.

The easy storing up and conveyance of heat to any part of the room or house by means of water pipes, the moderate temperature, and the facility of admission of external air at any point by passing the fresh air over coils, or water leaves, make it certain that the plan of warming by hot water will be greatly used in time to come, although the open fire-place may be retained for comfort.

Mr. George has devised a gas stove (called the Calorigen), which appears to be a decided improvement on the common gas stove. Gas is burnt in a small iron box, and the products of combustion are carried to the open air by a tube. Another coiled tube runs up through the box; this communicates below with the outer air, and above opens into the rooms. As the fresh air passes through this tube it is warmed by the heat of the

Die Luftüng und Erwärmung der Kinderstube und des Krankenzimmers, von D. C. Haller, 1860, pp. 29-38.

gas stove. Mr. Eassie speaks very well of this stove, which he has put up in several places. He says he has known one to be persistently capable of registering fifteen degrees above the external temperature during a very severe winter, and that too in a room of over 1,700 cubic feet, with the roof and three sides constructed of glass. A coal calorigen is also made which seems to answer well. Dr. F. T. Bond's Euthermic stove is also a very good contrivance.

1

A plan which was proposed 130 years ago by Desaguliers is now coming into general use, viz., to have an air-chamber round the back and sides of a radiating grate, and to pass the external air through it into the room. Thus a great economy of heat, and a considerable quantity of gently warmed air, passes into the room. In Captain Galton's grate, and in the plan proposed by Mr. Chadwick for cottages, the lower part of the chimney is also made use of. The advantages of these grates are that they combine a good amount of cheerful open fire, radiant heat, and chimney ventilation, with supplementary warming by hot air, so that more value is obtained from the fuel, and larger spaces can be more effectually warmed. A great number of patents have been taken out for grates of this kind. The air-chamber should not be too small, or the air is unduly heated; the heated surface should be very large; fire-clay sometimes gives a peculiar odor to the air, which iron does not do if the surface of iron be very large and disposed in gills; a combination also of iron and fire-clay is said to be good, and to give no odor. The conduit leading to the air-chamber should be short, and both it and the chamber should be able to be opened and cleaned, as much dust gets in. The room opening of the air-chamber should be so far up that the hot air may not be at once breathed, and there should be no chance of its being at once drawn up the chimney. The action of all stoves of the kind is liable to considerable variation from the action of the wind; and sometimes the current is even reversed and hot air is driven out.

Attention has been lately directed, both in France and America, to the fact of the comparative ease with which gases pass through red hot castiron. Mr. Graham showed that iron heated to redness will absorb 4.15 times its volume of carbon monoxide; and the experiments by MM. Deville and Troost, made at the request of General Morin, proved that in a castiron stove heated with common coal there passed through the metal in 92 hours 589 C.C. of carbon monoxide,' or from .0141 to .132 per cent. of the air which was slowly passed over the hot surface. In America Dr. Derby" has directed particular attention to this point, and has adduced very strong reasons for believing that the decidedly injurious effects produced by some of the plans of warming houses, especially by air passing over a castiron furnace heated with anthracite is due to an admixture of carbon monoxide. Professor Coulier, of the Val de Grâce,' has contended that the amount of carbon monoxide passing through in the experiments of Deville and Troost is really so small, that if mixed with the air of a room which is fairly ventilated, it would be quite innocuous; and he believes (from direct

1 Sanitary Arrangements for Dwellings, 1874, p. 140.

2 Comptes Rendus de l'Acad., Jan., 1868. These experiments were first undertaken in consequence of a statement by Dr. Carret, that in the department of Haute-Savoie an epidemic occurred which affected persons only in the houses where iron stoves were, and not porcelain.

3 Anthracite and Health, by G. Derby, M.D., Professor of Hygiene in Harvard University.

'Mem. de Med. Mil., Sept., 1868, p. 250.

experiment) that the headache and oppressive feeling produced by these iron stoves are really owing, as was formerly believed, to the relative dryness of the air. But evidence is adverse to this now. The gas passes with much greater difficulty through wrought-iron, or through stoves lined with fireclay.'

A great number of grates and stoves have been proposed, which it is impossible here to notice. In Germany many excellent stoves are now used, which not only economize fuel but warm the outside air, which is admitted round or under them." The medical officer's advice will be sought, first, as to the kind; and second, as to the amount of heat. He will find no difficulty in coming to the conclusion that in most cases both methods (radiation and convection) should be employed; the air warmed. by plates or coils of water-pipes being taken fresh from the external air and thereby conducing to ventilation. He will be also called on to state the relative amount of radiant and convected heat, and to determine the heat of the plates, and of the air coming off them, and the degree of humidity of the air. The thermometer, and the dry and wet bulbs, will give him all the information he wants on these points.'

Dr. Bond has recommended a coating of silicate as a preventive against the passage of deleterious products through an iron stove.

2

See a good account in Roth and Lex's work (op. cit., p. 365).

Mr. Chadwick has lately called attention to the old Roman plan of the Hypocaust, where the floor of the room is warmed by pipes, or by carrying smoke-flues under it, and he has contrived some ingenious plans to carry out the idea. There can be no doubt of the great comfort of this plan, although it appears to be expensive. Attention has been called, of late years, to heating on the whole house system, and there can be no doubt that this is an excellent plan, if properly carried out and carefully supervised. Drs. Drysdale and Hayward in this country (Health and Comfort in House Building, London, 1872), and Dr. Griscom, of New York, have devised ingenious plans for the purpose. In colder countries, such as Russia, the plan is in general use, but apparently with little or no regard to proper supply of fresh air, or carrying away of foul air.

CHAPTER XII.

EXERCISE.

A PERFECT state of health implies that every organ has its due share of exercise. If this is deficient, nutrition suffers, the organ lessens in size, and eventually more or less degenerates. If it be excessive, nutrition, at first apparently vigorous, becomes at last abnormal, and in many cases, a degeneration occurs which is as complete as that which follows the disuse of an organ. Every organ has its special stimulus which excites its action, and if this stimulus is perfectly normal as to quality and quantity, perfect health is necessarily the result.

But the term exercise is usually employed in a narrower sense, and expresses merely the action of the voluntary muscles. This action, though not absolutely essential to the exercise of other organs, is yet highly important, and indeed, in the long run, is really necessary; the heart especially is evidently affected by the action of the voluntary muscles, and this may be said of all organs, with the exception perhaps of the brain. Not only the circulation of the blood, but its formation and its destruction, are profoundly influenced by the movement of the voluntary muscles. Without this muscular movement health must inevitably be lost, and it becomes therefore important to determine the effects of exercise, and the amount which should be taken.

SECTION I.

THE EFFECTS OF EXERCISE.

(a) On the Lungs-Elimination of Carbon.-The most important effect of muscular exercise is produced on the lungs. The pulmonary circulation is greatly hurried, and the quantity of air inspired, and of carbon dioxide expired, is marvellously increased. Dr. Edward Smith investigated the first point carefully, and the following table shows his main results. Taking the lying position as unity, the quantity of air inspired was found to be as follows::

The great increase of air inspired is more clearly seen when it is put in this way: under ordinary circumstances, a man draws in 480 cubic inches