I have heard of a good old lady who, when nearing the end of her earthly existence, said she did not mind the dying if she could only breathe. Now this goodly person had doubtless spent all the years of her life without observing the fact that every plant or animal however small or simple in structure must have, if nothing else, the organs for breathing, and when that function is suspended or destroyed, life ceases. The respiratory organs may be reduced to a single cell, wall, or membrane. The forms of these organs, however, are exceedingly variable, elaborate, and sometimes complicated.
In the sea, plants and animals have a compensatory relation to each other. The plant exhales oxygen and the animal exhales carbon. That is to say, the carbonic acid which is mixed mechanically with the water coming in contact with the cell, wall, or membrane, covering the plant, the atom of carbon is appropriated, freeing the two atoms of oxygen, which in turn are appropriated by the animal.
Not only is this process of breathing compensatory and reciprocative—an interchange of commodities—the plant giving two atoms of oxygen for one of carbon, and the animal bringing its single but equally valuable atom of carbon for two atoms of oxygen, but without this interchange, neither could plant or animal live, and our world of life would become as dead as the moon is supposed to be.
The process of breathing is so common that we seldom think about it, unless there is an interference in some way. Each one of us sitting quietly in this room would breathe about 1000 times in an hour, requiring over 100 gallons of air to sustain the proper supply of oxygen for the blood. During this time we have taken from the air a certain amount of oxygen and have returned to it an equal amount of something else, which we call carbon oxide, or carbonic acid gas. The oxygen has burned the effete material which is cast out of the blood in the process of breathing, and it is returned to the atmosphere as a kind of coal. The fundamental principle is the same in animals that breathe water as those that breathe air, only the apparatus is different. Animals that breathe water have a fine capillary network of blood-vessels spread out on gills, branchia or projections arranged so that the water shall pass rapidly over them, and thus the carbon is carried away and the oxygen taken into the circulation.
Animals that breathe air through lungs have little air cells, so very small that a human lung is said to contain 600 millions of them; and these lie in contact with the capillary circulation of the lung which receives the oxygen and gives out the carbon. Some air-breathers have no lungs, but merely spiracles or minute holes in the body through which the air enters, coming in contact with the circulation.
In all cases, whatever the form, size, or character of the animal the object is to bring the air in contact with the circulation that oxygen may be received in exchange for the burnt material—the carbon oxide—which, when once formed, is poisonous, and must be expelled from the animal.
Now if we look over the earth we shall find immense deposits of coal. Here in the United States we have nearly 200,000 square miles of coal deposits. In other countries there is a like proportion of these carbon deposits, such as petroleum, bitumen, and paraffine. Then there are great forests and other vegetable growth. These have stored up the carbon set free by the animal, and have kept the air comparatively free from carbonic acid gas, which but for the vegetables would in a little while have rendered our atmosphere unfit for animal use. What is true of the air in this respect is also true of the sea.
Thus it comes about that by the process of breathing, principally, we have the immense coal fields, the wide spread forests, and the herbage that covers almost the entire globe. For in the air and the water there exist the germs of animal and vegetable life so profusely, so universally, that the proper conditions of heat and light will develop contemporaneously, both the organic kingdoms. If we should take ten drops of water from the middle of the Pacific Ocean, near the surface, and add them to a small tube, say two ounces, of water that had been deprived of life by boiling, and kept sealed for a number of years, and place the tube in favorable conditions, we should in a few days see a little universe spring, as it were, into existence. There might not be a great variety of forms, but who can say that there might not be enough to populate or re-populate some world just entering into the conditions of such life as our earth contains, or some other world that had suffered a reverse, or cataclysm, by which all life was destroyed.
Mr. Lloyd, Superintendent of the Birmingham Aquarium, says he kept for eight years a bottle of sea water, well corked and covered with paper, and that when he opened it the water was perfectly clear, free from smell, and of the same appearance as when taken from the sea. But when exposed for eight days to light in a window an abundance of microscopic plants and animals began to grow, and soon covered the sides of the bottle, and darted about in the fluid.
Having occasion some ten months ago to use some sea-water, I brought to my house a demijohn full and placed it on the north side where the sun seldom shines, and where it is nearly always cool; although the temperature sometimes goes as high as 75° and 80° Fahrenheit in the afternoons. There was no particular effort to exclude light and air; the cork fitted loosely, and the wicker work was not unusually close. And yet, whenever I have examined this water it is clear and free from smell, and there are no plants or animals growing in it. But by exposure of a small quantity to the light and warmth of a window, these have rapidly developed. It is a fact, then, easily demonstrated in our own rooms and houses, that by excluding light from water and keeping it in a cool place we can arrest the growth of organisms. This is the case with springs. The microscope fails to discover germs in spring water until it has been exposed to the light for some time.