You will remember that when the fresh air is drawn into your lungs as you breathe, it enters a large quantity of little cavities or chambers, which have, all of them, a fine net-work of the supply-pipes stretched out upon their walls; and that as the blood rushes on in its course through these supply-pipes, it sucks air into itself from the air-cavities, and carries it, in its own streams, to all parts of the living structure. Air goes with the blood to that strong force-pump, the heart, and is then pumped out with the blood to every crevice and fibre of the body. Every part of the body therefore receives, by means of the supply-pipes and in the blood, heat-fanning air, as well as supporting food.
When air reaches the living flesh and nerves, by thus flowing to them in the blood-streams of the supply pipes, it sets up those changes of substance in their structures which lead to the production of movement, and feeling, and other kinds of living power. When it reaches the dissolved fuel, contained in the blood and in the various little furnace-chambers of the laboratory, it sets up those changes in the fuel which lead to the production of warmth. The fuel is slowly burned in the blood and in the chambers of the frame, and there gives out warmth, as a fire does whilst it is burning in a grate. This warmth consequently heats the blood, and the warm blood carries its heat wherever it goes. The entire body thus becomes as warm as the blood, or nearly so.
Now, where do you think all the heat originally comes from, that is procured from burning fuel? The heat is stored away in the fuel, as one of the ingredients of its composition, until it is burned. But where was the heat obtained from, which is stored up in the fuel? Of course, when the fuel was made, that heat-store had to be supplied to it, as well as its other ingredients. First let us see when and how the fuel was made, and perhaps we shall then be able more perfectly to understand this matter of its warming qualities and power.
In the case of coal, it is not a very difficult task to trace the stored-up heat to its source. But what a surprising truth it is, which becomes apparent when the task has been performed. The heat is, so to speak, bottled-up sunshine! Coal is dug up from deep mines hollowed out in the earth. But at one time it was wood, growing on the outer surface of the globe, and covered with foliage which was spread out into the genial air. Traces of the leaves and stems from which it has been made, are still discovered in its substance. Long centuries ago, the vast forests containing these trees, were overthrown by some tremendous earthquake, and swept away by strong floods of water, and so the tree-stems were at last deposited in hollow basins, and were there buried up by millions and millions of tons of heavy rock and soil. There, where they were buried, they have remained, turning more and more black and dense through the process of slow decay, until they have been dug up piece-meal to feed the furnaces and fires of the existing generation of men.
Now, you know very well that trees only grow in warm weather, and in sunshine. In winter time their branches stick out stiff and bare, and do not increase in the slightest degree. But in summer time they clothe themselves with beautiful masses of foliage, and suck in from both the air and the soil large quantities of vapor, of liquid food, and of sunshine. All these they combine together into fresh layers of timber. All these therefore were buried in the ground as timber, when those old forests were overthrown which form the coal-beds. Timber cannot be made in cold weather, because heat is one of its necessary ingredients. But as all the warmth of the weather comes from the sun, it is the sun’s warmth which is stored away in the coal, and which is set free and made useful when the coal is burned.
The grand source of all warmth on the earth is that brilliant light which God has placed in the sky to rule over the day. In a summer’s day you sit down in the bright sunshine, and bask in its warmth. In winter time, when the sky is covered with clouds, and ice and snow lie thick over the ground, you place yourself indoors near the glowing fire; but strange to say, it is still the sun’s genial warmth that you experience. If the fire be of coal, it is warmth which was borrowed from the sun centuries ago. Reflect for an instant upon this marvellous arrangement entered upon, for your comfort, ages before you were yourself called into being! When those coal-making forests spread their broad masses of foliage out in the sunshine, there were no human creatures existing upon the earth; and, indeed, not even the flocks and herds, which are so essential to man’s welfare, had been framed. Neither cattle nor sheep could have found pasture on the plains which yielded them support. The great duty of those forests must have been to store up genial warmth for then uncreated generations of beings, who in due season were to appear, and to avail themselves of the provision thus made.
But suppose that you had neither fresh nor stored-up sunshine to fall back upon, and had to depend entirely for your warmth upon that furnace which is carried about in your living laboratory, and kept alight by the puffing of your breath. Still that internal heat comes originally out of the sunshine. Just before the time when man was placed upon the earth, the beautiful family of plants was created, which fills the gardens with roses, and which yields the apple, the pear, the cherry, the plum, the apricot, the peach, the almond, the strawberry, and the raspberry. Just at the very time was planted on the globe, the vegetable tribe which furnishes the different kinds of nourishing grain, and which provides pasture for grazing animals. The fruits, the grasses, and the grain were all commissioned to extract power and warmth from the sunshine, and to store it up in such a form that the influences could conveniently be introduced into the interior of the living body. Living animals which are warmed by the fuel contained in their food, procure their heat from sunshine that was stored up, as it were, but yesterday. When animals live upon flesh, and get their strength out of the lean fibre, and their warmth out of the fat of this food, still it must be remembered that the flesh has been fed on the grass of the field just before. The main office of the plant in creation is thus to store up in a fixed and convenient form supplies of active energies which can be turned to account by animated frames. The plant effects this end by preparing the food upon which animals live;—that food which, besides keeping the body in repair, serves also to furnish it with warmth, and to give it strength and power. How admirable and beneficent is this plan, whereby the genial influence of life-quickening sunshine is economized and preserved for the service of one-half of creation, by the instrumentality of the other half!
In the far distant regions of the north, there are places on the earth to which no daylight or sunshine comes for four long months at a time. During this gloomy period the ground goes on, from hour to hour, scattering more and more of its heat, until it is almost as cold as the chill space in which the great world is poised, and has indeed more than 100 degrees of frost. The land and the water alike get covered up by one broad and thick sheet of never-melting ice and snow. There is not a leaf, or a grass blade, or a vegetable stalk any where in the wide white desolation. But there are animals and human beings, who are born and die, who maintain a prolonged existence in it. Let us just look in upon one of the households in this drear frost land, and see what the odd community is like.
In the midst of a broad snow waste, through which the sharp wind is howling with a fearful sound, there is a small mound nearly covered by the snow-drift. We perceive this mound by faint starlight, the only gleam that comes down from the sky. A few feet away from the mound we discover a small hole blocked up by a lump of snow. We move the lump aside, and stretching ourselves out at full length on the ground, we squeeze into the hole head foremost, and crawl along a narrow passage, burrowed out in the firm snow for about a dozen feet. We then find ourselves in a vault ten feet wide and fifteen feet long, and so low that we can scarcely sit upright within it. This is the inside of the mound. It is the interior of a hut, or dwelling-place, of these people of the drear frost land. The walls of the hut are built of large stones piled together, with a padding of frozen moss covered over them, and with thick ice and snow covered over the moss.
There are twelve living individuals, men, women, and children, huddled together in this close vault. They have no fire to keep them warm. Indeed, there is neither coal nor wood which they could use to light a fire, within many hundred miles. There is in one corner of the hut a broad shoulder-blade of a large quadruped laid flat, and in the hollow of this blade there is some crushed seal’s blubber, and some soft moss, with long cotton-like rootlets. The end of the moss is burning with a small, dull, smoky flame. This is the only artificial source of light and warmth within the hut.