Let us consider, first, a moist warm atmosphere. It may be very moist, because warm air can hold a considerable quantity of moisture in solution. On account of the warmth of the air, its lightness, and the amount of moisture contained in it, almost all internal actions go on slowly. Whenever we breathe in light air, we do not at each inspiration breathe in the same weight of air that we do when we breathe in heavy air. When, in addition to being warm, the atmosphere is charged with moisture, the evaporation from the skin goes on to a much less extent, because the air has as much moisture in it as it can have already; less exercise is taken, and so the respiration and circulation go on slower, and blood of an inferior quality is circulated throughout the body, and supplies, among other parts, the nervous system, so that a general laxity and want of tone are the result, which is the state we know as due to the atmosphere of what are called relaxing places, and it is in such atmospheres that certain diseases, especially contagious fevers, spread most easily and rapidly.

If the atmosphere is cold and very moist, most of the moisture is suspended, for the air may not only have moisture dissolved in it, but it may have it suspended in it in the form of mist and fog. Now, the fact of the air being moist makes it seem colder than it is. It is a universal experience that moist cold air seems colder than dry cold air, which may really be much colder. Heat is more readily extracted from our bodies in a moist cold atmosphere than in a dry one, and there is a difficulty in getting rid of moisture from the respiratory organs and from the skin, because the air is already charged with moisture. On account of this chilly tendency of a moist cold atmosphere, the diseases prevalent in such weather are lung diseases, kidney diseases, and rheumatism.

In large towns the mists and fogs have, besides, special deleterious effects, because they hold suspended in them the. impurities of the air-soot, acid vapours from manufactories, etc., and so it is desirable in such weather, especially in large towns, to wear something in the way of a respirator or comforter round the mouth and nose to prevent suspended particles in the air, and even the chilly suspended particles of moisture from getting into the lungs.

With hot dry air we have increased action of the skin, because there is less resistance to the flow of blood in the skin when the air is dry and warm; all the small arteries bringing blood to the skin are relaxed, and the blood also flows quicker in the capillary vessels in the skin. There is a large amount of blood in the skin, and the action of the perspiration glands becomes increased by the excessive amount of blood supplied to them. Evaporation from the surface of the skin takes place very fast, but generally in a hot dry atmosphere it does not take place sufficiently fast to get rid of the excessive perspiration. On the other hand, the action of all the internal organs except the liver is diminished; for instance, the mucous membrane of the mouth becomes dry, and the same is true of the other mucous membranes, the action of the salivary glands is lessened, and the action of the gastric glands is also lessened ; these are some of the most important organs connected with digestion, so that the power of digestion is lessened and the appetite is lessened, because the want of food is less, and so the lessened power of digestion does no harm if we do not try and eat too much in hot dry weather.

The action of the liver is increased in hot atmospheres, and it is well known that liver diseases are prevalent in hot climates.

You will remember I told you there are two reasons why we eat food-the first is to repair the waste of the body which is continually going on, and the second to supply animal heat, which is necessary for our existence, and which is transformed into various kinds of force which we exert. The second of these is required to a much less extent in hot dry weather, because the air around is already so hot that we are not losing heat and may gain it, so we need take little or no food for the purpose of keeping up our heat.

In cold dry air we have precisely the reverse effects. For cold dry air chills the skin, making the small arteries which bring blood to the skin contract, because cold is one of the agents which makes muscular tissue contract; so the small arteries by which the blood comes to the skin contract and prevent the blood coming to the skin; besides that, the flow of blood in the capillary vessels is slackened in obedience to the known physical law that the flow of liquids in capillary tubes is lessened by cold. Less blood then comes to the skin, and this is very fortunate, because remember that we lose a great deal of heat from the skin, and so the fact that less blood comes to the skin becomes important, for if as much blood came to the skin in cold weather as in warm we should lose heat too fast, and the blood would get chilled too much.

The blood, then, is chiefly in the internal organs, so there is an increased action of the internal organs (except the liver) in cold weather, an increased power of digesting food, and an increased need of food; and in cold weather we eat more than in hot weather, both to repair the greater amount of tissue waste that is going on and to keep up our animal heat.

The diseases prevalent in cold weather are what you would expect from the fact that the blood is thrown from the skin into the internal organs, and the action of the skin is lessened; if the action of the skin is lessened, and the lungs and kidneys have to do some of its work as well as their own, they may get too much to do, and when the blood is thrown back upon them, congestion of one of those organs, and subsequent inflammation, may take place, and lung and kidney diseases are therefore prevalent in cold weather, and so in cold weather we have always the highest death-rates.