ulum to vegetable life." This was but the next year after Priestley's discovery of oxygen itself; yet to this day there lingers in our common thought an undefined impression that the carbonic acid of the air is just an impurity, tolerated because there is only a little of it, but an impurity that it were as well to be rid of altogether. Now, if the redundant resources of life were at our human disposal, we might be in danger, some day, in the sheer forgetfulness of self-regard, of throwing away as an impurity the very foundations of sustenance. Some one, perhaps, would set forth that this gas when not diluted is immediately fatal to human life; another would declare, "Once a poison, always a poison;" and another would ask why we should imperil our own health for the sake of the plants.
Oxygen was named next, among the primary resources, redundant in supply. It is a prominent constituent of all living tissues, forming seventy-two parts in a hundred of the human body with its fluids. It is taken in two conditions: first, in combination, chiefly by the plants; second, in the elemental state, by animals. In combination, it is taken by the plants from carbonic-acid gas, just noticed as a source of carbon; from water, to be considered as a source of hydrogen; and, in smaller quantities, from a considerable number of other substances. The greater part of the oxygen in animal tissues is obtained in the products elaborated by the plants.
But for all animal life the most imperative demand is for oxygen in the elemental state.
The other elemental resources are available only in their compounds; oxygen does its best service when alone. The others serve life as materials for its bodily tissues; oxygen has an additional duty, the maintenance of operations giving warmth and strength. The activities of life consume various materials, but most constantly of all they demand a raw material of inorganic nature, a simple material in its primitive condition. This supply of elemental oxygen, a necessity for all animal life, is a necessity that is imminent in direct proportion to vital activity, and for man is absolutely imperative. When supplied with oxygen, we can subsist days without other food; when deprived of oxygen, life fails in a few minutes. It is scarcely a figure of speech to say that the breath is the life. The energy of oxygenation is told in every stroke of the heart. The food that is eaten does not raise an iota of bodily strength without the help of the pound and a quarter of pure oxygen that is daily inhaled. To breathe poorly is to faint; to eat richly and breathe poorly is to suffocate and perish.
The supply of elemental oxygen is certainly impartial and bountiful without reservation. It is more than given—it is pressed upon us; to escape from it is a work of toil and difficulty. No one is poor from want of it, or rich from gain of it. Were it furnished for pay, all that a man hath would he give for an hour's supply of it. The poor, taken together, fare best in its use; while the wealthy, in their elabo-
