pened that Priestley, who resided near a brewery in the town of Leeds, in England, accidentally observed that the beer during its fermentation in the vats gave forth a remarkable aërial substance. The flame of a lighted stick immersed in it was at once extinguished, and the smoke floating on the top of the stratum showed that it was very heavy, a result which was perfectly confirmed by the observation that, invisible and intangible as it was, this air could be poured from vessel to vessel like water, and in the vats in which it originally occurred it would overflow their edges and descend to the door, along which it would run like a stream, its course being readily tracked by the expedient of putting a lighted stick into it, and observing the extinction of the flame. Moreover, he found that it would dissolve in water, for, if dishes of that liquid were placed where it had access, an agreeably acidulous and sparkling fluid, soda-water, was formed. And that the agent which brought all these results about possessed a physiological potency, was proved by the fatal fact, too often known in such manufactories, that if, by accident it was breathed, death at once took place.
The substance which Priestley thus first encountered was that known to us as carbonic-acid gas; it had already been studied under other circumstances by Black and older chemists. I mention it here because it led Priestley to that long-continued investigation of factitious airs, which was crowned by the great discovery of oxygen gas.
We have seen with what acuteness Priestley detected differences between the gas just mentioned and common air. It is a striking fact, verified over and over again in the history of science, that the most imposing results may be presented to the acutest mind, and their significance and value remain undetected. Priestley, in 1771, having exposed some saltpetre to the fire, disengaged oxygen, experimented with it, and even showed its energetic power of supporting the flame of a candle, and yet the value of these truths entirely escaped him. Three years subsequently he submitted one of the compounds of quicksilver to the force of the sun's rays, converged by a burning-glass, oxygen again escaped, and this time he secured his discovery.
He was not long in recognizing its importance. One after another, as the properties were developed, the value of their consequences was apparent. First, a lighted candle, far from being extinguished, burnt with increased brilliancy, and substances commonly reputed incombustible, such as iron and other metals, were consumed as though they were wood. The doctrine of vitiated airs disappeared at once. Here was a substance possessed of all the chemical energies of the atmosphere, only in an incomparably more intense degree. If there were vitiation at all, the air itself was a vitiated form of this gas. Then, too, he found that it could sustain completely the breathing of animals, and that, in reality, it was absolutely essential to the discharge of that function, a fact which led him to apply to it the epithet "vital air;"
