388 THE POPULAR SCIENCE MONTHLY. 



pened that Priestley, who resided near a brewery in the town of 

 Leeds, in England, accidentally observed that the beer during its fer- 

 mentation in the vats gave forth a remarkable aerial 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 expe- 

 dient 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 diflerences 

 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 sig- 

 nificance and value remain imdetected. Priestley, in 1771, having ex- 

 posed 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 quick- 

 silver 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 incombus- 

 tible, 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;" 



