This doctrine of Aristotle is interesting because modern science, calling to its aid all the multitudinous inventions that ingenuity can devise, has reached the conclusion that heat is a condition of motion of the particles of material bodies. Yet the resemblance between this result and the speculation of the old philosopher, though noticeable, is merely superficial, and no certain progress was made in the study of heat till philosophers learned to submit their guesses to the test of experiment.

The progress of science is not a steady advance, there are continual haltings by the way, and even temporary retreats; a long period of stagnation may precede some brilliant discovery or powerful and far-reaching generalization that will at once rouse investigation and usher in a period of great progress; this was true in a marked degree of the study of light.

Early speculation taught that light was an emanation thrown out in straight lines from the luminous body. But during the seventeenth century the theory that light consisted of waves or undulations coming from the heated body was powerfully advocated by Huyghens. Newton, however, who made many interesting and important investigations in light, strongly advocated the emission theory, and the weight of his great authority turned the scale against the wave theory, in consequence of which it was in disrepute for nearly a hundred years, during which time very little progress was made in the knowledge of light.

During the life of Newton it had been established by Roemer, a Danish astronomer, that it took a certain time for light to pass from a heated body to the eye; for by calculations based on the times when the moons of the planet Jupiter were observed to be eclipsed, he had found that light traveled at the rate of 185,000 miles in a second.

Just at the beginning of the present century Thomas Young, an English scientist, brought forward new and convincing evidence of the truth of the wave theory, and showed how waves of light could be made to interfere with each other and produce darkness. This was the opening of a period of great progress. Immediately succeeding Young came Fresnel, the great French physicist, who contributed more than any one else to the development of the wave theory, and whose labors, together with those of such men as Arago and Foucault, at once brought the science of light almost to the position it occupies to-day.

But it is not true that all waves coming from a hot body are visible; even if it were not hot enough to give out waves of light it would send off waves which, though invisible, are capable of giving the sensation of warmth. These invisible waves, or heat radiations as they are sometimes called, have been made the subject of many careful investigations, and prominent among those who have devoted themselves to their study we find Professor John Tyndall, whose studies in radiant heat and diamagnetism have given him an honored place in the scientific world.

Tyndall was born in the village of Leighlin Bridge, Ireland, in 1820. His parents were poor, and this poverty brought with it the usual gifts in developing the mind and ingenuity of the little lad who was to owe all his success in life to his own individual efforts.

Like his little companions in the same condition of life, he played about the village streets, made excursions into the surrounding country, and found life a pleasant thing; for poverty to the country child brings with it none of that sordid wretchedness which so early leaves its blighting impress on the soul of the city child, to whom it comes without any grace or brightening charm.

Thus circumstanced, in spite of his parents' humble means, the boy's life passed pleasantly enough; and the lessons which nature taught him in his wanderings around Leighlin Bridge were the most useful he could have learned. He grew up a part of the beautiful world around him, and the songs of the birds, the blossoming of the flowers, and the thousand experiences of life with which he was always familiar, seemed to belong to him as much as the coloring and perfume were a part of the wild flowers he gathered.

And, besides this love and appreciation of nature, the boy was fortunate in the books which he read as a child, and which left an indelible mark on his character. His father was a man of strong religious principle, and the volumes in the family library included, with the Bible, the principal works of the most celebrated writers on theology; and, although this subject would have ordinarily no charms for a child, yet the fervid imagination, the poetic and above all the high ideality which made the duties of common life seem a religious ceremony, could not fail to make a lasting impression on the mind of a sensitive and imaginative child; while the Bible, with its wonderful imagery and powerful descriptions of nature, together with its human interest, all tinged with the deepest religious inspiration, was no less a source of fruitful teaching to the child, who read and re-read the glowing pages until he knew the volume almost by heart, and the sublime style of the Hebrew prophets had grown as familiar to him as the voice of Nature in the outdoor world.

Thus, when at seven or eight his school-days began, young Tyndall started up the hill of learning with two priceless aids—a loving intimacy with nature, and a familiarity with the grandest literature that the world has ever known.

His school days reached to his nineteenth year, during which time he pursued the usual course of study, and showed no particular talent for anything, excepting perhaps a taste for which developed itself during the last two years of his school life. He began the study of civil engineering after leaving school, intending to make it his profession, and for three years diligently studied the preparatory course, meeting with the most gratifying results.

But in 1842 he attended a course of lectures at a Mechanics' Institute, which, combined with a desire for larger study which had come to him the year before, opened wider fields of thought and gave him a deep interest in subjects unconnected with his special work.

But for five years longer he kept on in the way he had marked out for himself, completing his course of study and practicing engineering with marked success. Then, in 1847, he was appointed teacher in Queenswood College, Hampshire, and during the year that he spent in this place he became so interested in chemistry and other branches of physical science, that he determined to leave England and take a course of scientific study at some German university.

Marburg, in Hesse-Cassel, was chosen as the place of study, and here, in company with the friend whose lectures in chemistry had first interested him in natural science, Tyndall spent two years engaged in absorbing study. His student life was of the simplest kind, as money was scarce, and the end he had in view, the acquiring of knowledge for its own sake, did not point to any large remuneration from a material stand-point in the future. He studied sometimes sixteen hours a day, and although his hopes of success were sometimes overclouded by the gloomy doubts which often visit the imaginative mind, his resolve never faltered; and if his life at Marburg had borne no other fruit, it yet would have been rich in the development of that loftiness of purpose and stern devotion to duty, which at this period became such marked characteristics of the young student.

But Marburg did bring other and great prizes to him. He was under the teaching of Bunsen, the celebrated chemist, whose lectures on electro-chemistry, or the chemical changes which occur through electricity, attracted Tyndall at once, and at the same time he attended an illustrated course of lectures on radiant heat, or heat which comes in rays from the heated body, in the same manner that the heat of the sun reaches the earth. These studies were in the direct line of experimental research, and Tyndall was thus easily led to a point where he began independent investigation.

Faraday's important discovery of diamagnetism, was then attracting great attention in the scientific world. Faraday had shown that all matter could be influenced by magnetism, and had divided bodies into magnetic and diamagnetic. A bar of a magnetic substance when suspended between the poles of a magnet would point in the direction of the line joining the two poles. But if the bar were diamagnetic, it would set itself cross-wise, so that its two ends were as far away as they could get from the poles of the magnet.

But further investigation had brought to light the fact that certain substances which were diamagnetic, ceased to be so when discovered in the form of crystals. Thus, a piece of bismuth suspended between the poles of a magnet would point across the line joining the two poles, showing that bismuth was a diamagnetic substance, but a crystal of bismuth when suspended did not follow this direction, and the same was found to be true of many other substances.

In 1849 Tyndall began the study of this interesting phenomenon, and for several years carried on experiments in magnetism and electricity with the hope of arriving at some satisfactory conclusion; and, by 1855, he may be said to have reached results which were so important as to place his name foremost in the ranks of those who have studied this subject.

Crystallization, or the mysterious force by which charcoal becomes a diamond, common clay a sapphire or ruby, and by which other transformations are effected, had been an interesting subject of study from the time that science had first revealed that the same substance might exist either in the crystalline or non-crystalline state, and it was in this field of thought that Tyndall labored in his experiments on diamagnetism.

He claimed that the apparently contradictory actions of some diamagnetic substances and their crystals, were due to the structure of the substance or crystal, or the peculiar ways in which the particles forming the body were joined together. This property or peculiarity he stated was not simply characteristic of certain substances, but that, as nature acted by general laws, it would be possible, by following out the suggestions contained in this fact, to arrive at the most important discoveries in relation to the structure of the earth, and its magnetic actions; and that just as the fall of an apple suggested to Newton the theory of gravitation, so the refusal of a crystal to act in accordance with the laws that governed the uncrystallized substance might point to a law of nature which, if discovered, would unravel many of the mysteries which puzzle the scientific mind.