few centuries later, however, events occurred which were originated and put upon the scene by means of the art of printing, and which greatly diminished the blessings of the invention on account of the almost total destruction of national prosperity during the Thirty Years' War. The art itself meanwhile had not improved. The prints of those times are poor and wretched compared with the excellent works of Gutenberg. The importance of the art by far exceeds its intellectual merit. Many inventions have since been made, which involve far higher intellectual endowments than the invention of printing. The Jacquard loom, the stocking-frame, the carding-machine, the watch, the chronometer, and other inventions, unquestionably involve rarer gifts of combination and executive force; yet, as regards influence, none of them can even remotely be compared with the printing-press; none would at that time have come to light without the press.
This vast capital handed down to us by former generations, modern humanity has immeasurably increased, even doubled and trebled. The inventions and discoveries mentioned thus far are fully known, as to their immense bearing upon the direction of human life.
In addition to these, let us record two events of the second half of the last century, which, more promptly and thoroughly than even any of the preceding, changed the entire social conditions of humanity: one an invention, that of the steam-engine; the other a discovery, that of oxygen.
The importance of the steam-engine requires no comment. Man derives power from the rays of the sun which were stored up as carbon in the vegetable kingdom from time immemorial. The steam which today gushes from the locomotive is an equivalent of the rays that decomposed the carbonic acid of the huge marine plants of those early periods, and accumulated the carbon as a source of power—a sleeping affinity, a lifted weight. In combining this carbon again with oxygen, we produce precisely as much heat as disappeared during the growth of those plants. The steam generated by this heat we allow to push against a movable obstacle, and to this obstacle we attach the resistances to be overcome—a train of cars, a number of looms or hammers, grindstones or rolls. The power is neither given us nor is it generated. It disappears with the wood or the coal.
The discovery of oxygen has an altogether different importance. We are confronted by an apparently insignificant fact which Destiny seemed for a time to have permanently assigned to the chemist's laboratory. It was on the 1st of August, in 1774, that Priestley, an English clergyman and a naturalist, for the first time performed the celebrated experiment which up to the present day is repeated in nearly every course of lectures on experimental chemistry. He heated red oxide of mercury in a small glass retort, and obtained an invisible, colorless gas together with drops of liquid mercury. To collect the gas he employed the same means which we still use to-day. He took