Read CHAPTER XI of History of the Intellectual Development of Europe‚ Volume II, free online book, by John William Draper, on



The Age of Reason in Europe presents all the peculiarities of the Age of Reason in Greece. There are modern representatives of King Ptolemy Philadelphus among his furnaces and crucibles; of Hipparchus cataloguing the stars; of Aristyllus and Timochares, with their stone quadrants and armils, ascertaining the planetary motions; of Eratosthenes measuring the size of the earth; of Herophilus dissecting the human body; of Archimedes settling the laws of mechanics and hydrostatics; of Manetho collating the annals of the old dynasties of Egypt; of Euclid and Apollonius improving mathematics. There are botanical gardens and zoological menageries like those of Alexandria, and expeditions to the sources of the Nile. The direction of thought is the same; but the progress is on a greater scale, and illustrated by more imposing results. The exploring voyages to Madagascar are replaced by circumnavigations of the world; the revolving steam-engine of Hero by the double-acting engine of Watt; the great galley of Ptolemy, with its many banks of rowers, by the ocean steam-ship; the solitary watch-fire on the Pharos by a thousand light-houses, with their fixed and revolving lights; the courier on his Arab horse by the locomotive and electric telegraph; the scriptorium in the Serapion, with its shelves of papyrus, by countless printing-presses; the “Almagest” of Ptolemy by the “Principia” of Newton; and the Museum itself by English, French, Italian, German, Dutch, and Russian philosophical societies, universities, colleges, and other institutions of learning.

First, then, as respects the atmosphere, and the phenomena connected with it.

A foundation for the mechanical theory of the atmosphere was laid as soon as just ideas respecting liquid pressures, as formerly taught by Archimedes, were restored, the conditions of vertical and oblique pressures investigated, the demonstration of equality of pressures in all directions given, and the proof furnished that the force of a liquid on the bottom of a vessel may be very much greater than its weight.

The equatorial calms and trade-winds accounted for on physical principles, it was admitted that the winds of high latitudes, proverbially uncertain as they are, depend in like manner on physical causes.

With these palpable movements there are others of a less obvious kind. Through the air, and by reason of motions in it, sounds are transmitted to us.

In like manner much light was cast on the meteorological functions of water. It was seen that the diurnal vaporization from the earth depends on the amount of heat received, the vapour rising invisibly in the air till it reaches a region where the temperature is sufficiently low. There condensation into vesicles of perhaps 1/50000 of an inch in diameter ensues, and of myriads of such globules a cloud is composed. Of clouds, notwithstanding their many forms and aspects, a classification was given cirrus, cumulus, stratus, etc. It was obvious why some dissolve away and disappear when they encounter warmer or drier spaces, and why others descend as rain. It was shown that the drops can not be pure, since they come in contact with dust, soluble gases, and organic matter in the air. Sinking into the ground, the water issues forth as springs, contaminated with whatever is in the soil, and finds its way, through streamlets and rivers, back to the sea, and thus the drainage of countries is accomplished. Through such a returning path it comes to the receptacle from which it set out; the heat of the sun raised it from the ocean, the attraction of the earth returns it thereto; and, since the heat-supply is invariable from year to year, the quantity set in motion must be the same. Collateral results of no little importance attend these movements. Every drop of rain falling on the earth disintegrates and disturbs portions of the soil; every stream carries solid matter into the sea. It is the province of geology to estimate the enormous aggregate of detritus, continents washed away and new continents formed, and the face of the earth remodelled and renewed.

But a false theory always contains within itself the germ of its own destruction. The weak point of this was, that when a metal is burnt the product ought to be lighter than the metal, whereas it proves heavier. At length it was detected that what the metal had gained the surrounding air had lost. This discovery implied that the balance had been resorted to for the determination of weights and for the decision of physical questions. The reintroduction of that instrument for, as we have seen, it had ages before been employed by the Saracen philosophers, who used several different forms of it marked the epoch when chemistry ceased to be exclusively a science of quality and became one of quantity.

Yet this is only a part of what the Italian experiment, carried out by modern methods, has actually done. Could there be a more brilliant exhibition of their power, a brighter earnest of the future of material philosophy?

The beautiful colours of soap-bubbles, at first mixed up with those of striated and dotted surfaces, were traced to their true condition thickness. The determination of thickness of a film necessary to give a certain colour was the first instance of exceedingly minute measures beautifully executed. These soon became connected with fringes in shadows, and led to ascertaining the length of waves of light.

That light consists of undulations in an elastic medium was first inferred in 1664. Soon after, reflexion, refraction, and double refraction were accounted for on that principle. The slow progress of this theory was doubtless owing to Newton’s supremacy. He gave a demonstration in the second book of the “Principia” (Pro that wave motions must diverge into the unmoved spaces, and carried popular comprehension with him by such illustrations as that we hear sounds though a mountain interpose. It was thought that the undulatory theory was disposed of by the impossibility of seeing through a crooked pipe, though we can hear through it; or that we cannot look round a corner, though we can listen round one.

The present century finally established it through the discovery of interference, the destruction of the emission theory being inevitable when it was shown that light, interfering under certain circumstances with light, may produce darkness, as sound added to sound may produce silence results arising from the action of undulating motion. The difficulties presented by polarization were not only removed, but that class of phenomena was actually made a strong support of the theory. The discovery that two pencils of oppositely polarized light would not interfere, led at once to the theory of transverse vibrations. Great mathematical ability was now required for the treatment of the subject, and the special consideration of many optical problems from this new point of view, as, for example, determining the result of transverse vibrations coming into a medium of different density in different directions. As the theory of universal gravitation had formerly done, so now the undulatory theory began to display its power as a physical truth, enabling geometers to foresee results, and to precede the experimenter in conclusions. Among earlier results of the kind was the prediction that both the rays in the biaxial crystal topaz are extraordinary, and that circular polarization may be produced by reflexion in a rhomb of glass. The phenomena of depolarization offered no special difficulty; and many new facts, as those of elliptic polarization and conical refraction, have since illustrated the power of the theory.

From almost any of the branches of industry facts might be presented illustrating the benefits arising from the application of physical discoveries. As an example, I may refer to the cotton manufacture.

It deserves to be remarked that the cotton manufacture was first introduced into Europe by the Arabs. Abderrahman III., A.D. 930, caused it to be commenced in Spain; he also had extensive manufactures of silk and leather, and interested himself much in the culture of the sugar cane, rice, the mulberry. One of the most valuable Spanish applications of cotton was in the invention of cotton paper. The Arabs were also the authors of the printing of calicoes by wooden blocks, a great improvement on the old Indian operation of painting by hand.

The foregoing paragraphs were written in 1859. Since that time Italy has become a nation, Rome is its capital, Venice belongs to it. In 1870-71 I was an eye-witness of the presence of Italian troops in the Eternal City.