John Tyndall, a British natural philosopher, born at Leighlin Bridge, county Carlow, Ireland, Aug. 21, 1820. Under the guidance of his father, he received a strict religious training, and early became thoroughly conversant with the Bible. Having mastered Euclid, conic sections, and plane trigonometry, he was employed in the Irish and English ordnance surveys from 1839 to 1844. During the three succeeding years he was a railway engineer, and in 1847 he accepted a post in Queenwood college, Hampshire, which he resigned in the following year to attend the lectures of Bunsen at the university of Marburg. Here, in conjunction with Prof. Knoblauch, he undertook a series of experiments in magnetism and diamagnetism, proving the existence of a relation between the molecular constitution of matter and magnetic force, and demonstrating that the direction of greatest magnetic energy will fall in the line of greatest molecular condensation. The results of their combined investigations were embodied in a paper "On the Magneto-optic Properties of Crystals, and the Relation of Magnetism and Diamagnetism to Molecular Arrangement," published in the "Philosophical Magazine " for 1850. On graduating in 1851, he prepared a mathematical dissertation on screw surfaces (Die Schraubenfläche mit geneigter Erzeugungslinie, und die Bedingungen des Gleichgewichts avf solchen Schrauben). In the same year he removed to Berlin, where for some time he was engaged in the laboratory of Prof. Magnus. Shortly after his return to England he was elected a fellow of the royal society, and in 1852 one of the secretaries of the physical section of the British association.

In June, 1853, he was appointed professor of natural philosophy at the royal institution, which office he still retains (1876). Tyndall first visited Switzerland in 1849, and in company with Prof. Huxley made a second journey in 1856, since which time he has visited the Alps every year. In the winter of 1859 he succeeded in establishing himself on the Montanvert, and determined the rate of winter motion of the Mer de Glace. With the cooperation of Dr. Frankland, he planted several thermometric stations on the slopes and summit of Mont Blanc, and made numerous observations relating to combustion at great altitudes. In 1861 he scaled the hitherto inaccessible peak of the Weisshorn, and in 1868 reached the summit of the Matterhorn, crossing it from Breuil to Zermatt. The results of his glacial investigations were published in the "Philosophical Transactions" (jointly with Prof. Huxley's) for 1858, and subsequently in "Glaciers of the Alps" (London, 1860), and "Hours of Exercise in the Alps" (1871). He opposed the views of Agassiz respecting the occurrence of laminae in glaciers, definitely ascribing the true cause of their formation to mechanicaj pressure.

Through the direct application of the doctrine of regelation, he arrived at a satisfactory understanding of the nature of glacial motion, proving, by carefully repeated observations on the structure and properties of ice, the inefficacy of the generally admitted plastic theory to account for that phenomenon. This discovery led to a protracted controversy with Professor (afterward Principal) Forbes of Edinburgh. (See Glacier, Ice, and Forbes, James David.) In 1863 he published " Heat considered as a Mode of Motion," which placed him in the front rank of scientific expounders. In 1866 he relieved Faraday in his duties at the Trinity house, and on the death of that philosopher in 1867 became superintendent of the. royal institution. To observe the solar eclipse of December, 1870, he accompanied the British expedition to Algeria, and on his return voyage instituted a number of simple inquiries in relation to the color of the ocean. He demonstrated that the change of color frequently observed at different portions of the sea is due to the reflection of certain rays of light from the surfaces of innumerable particles of matter held in mechanical suspension at varying depths of the water's mass.

Prof. Tyndall visited the United States in 1872, and delivered a course of lectures in some of the principal cities of the east, the proceeds of which, $13,000, were given to the establishment of a fund designed for promoting the study of the natural sciences in America. In the "Contemporary Review" for July, 1872, Prof. Tyndall published with commendation a letter addressed to himself, wherein the writer proposed that the efficacy of prayer should be tested by making one ward of a hospital the special object of the prayers of the faithful for a term of years, and then comparing its rate of mortality with that of other Wards during the same time. This gave rise to a widespread controversy, and was popularly denominated " Tyndall's prayer test." In August, 1874, while presiding over the annual meeting of the British association, he delivered the famous inaugural known as the "Belfast Address," which was denounced as a declaration of materialism. - The labors of Prof. Tyndall, though more particularly directed toward the examination of the molecular constitution of matter, have not been confined to any special branch of physics.

Between 1849 and 1856 he was mainly occupied with the prosecution of his experiments in magnetism and electricity, in the course of which he conclusively settled the question of diamagnetic or reversed polarity, the existence of which, originally asserted by Faraday, and reaffirmed by Weber in 1848, had been subsequently denied by the former. In 1859 he initiated a remarkable series of researches in radiant heat, which were extended over a period of more than ten years. The diathermancy of simple and compound gases, as well as of various vapors and liquids, was experimentally tested, and the degrees of their opacity to radiant heat determined with great precision. Dry atmospheric air, which had hitherto afforded but negative results to Melloni, was ascertained to have an absorptive power about equal to that of its main elementary components, and but a mere fraction of that of aqueous vapor; a discovery which, in its bearings on terrestrial and solar radiation, has exerted a marked influence on the progress of meteorology.

The principle of the physical connection of the emission and absorption of undulations (first enunciated by Euler), which formed the basis of Angstrom's experiments on the radiation and absorption of incandescent solids, and which laid the foundation for the science of spectrum analysis, was applied by Tyndall to gases and vapors some time previous to the publication of Kirchhoff's more specialized generalizations respecting refrangibility. Tyndall's investigations on obscure and luminous radiations, and on the nature of calorescence, or the transmutation of heat rays, form some of the most noteworthy of his contributions to molecular physics. By means of a filter composed of a solution of iodine and the bisulphide of carbon, so constituted as to intercept all but the ultra-red rays of any luminous source of heat, he has ascertained that the visible thermal rays emanating from any particular body bear but a small ratio to the total number of thermal rays emitted by that body. He has also shown, by experiments made on his own eyes, that the calorific energy of a concentrated electric beam, capable of raising platinized platinum foil to vivid redness, and of instantaneously exploding gunpowder at an absolute dark focus, is incompetent to excite the sense of vision in the human retina.

The subject of gaseous conductivity (which led to views antagonistic to those entertained by Magnus), the action of odors and colors on radiant heat, and the various laws governing acoustic and optical phenomena, have also engaged his attention. To him is due the beautiful interpretation of the azure color of the firmament, as well as of the changing tints accompanying the morning and evening twilight. (See Light.) Since 1873 his labors have been more generally related to those of the Trinity house, in connection with inquiries made into the causes which affect the acoustic transparency of the atmosphere. Prof. Tyndall is a strenuous advocate of the doctrine of evolution. His vigorous language and felicitous method of exposition have given him the highest position among scientific lecturers. Besides the works already mentioned, he has published "Mountaineering in 1861" (1862); "On Radiation" (1865); "Sound, a Course of eight Lectures" (1867; 3d ed., embracing his important observations on acoustic opacity, 1875); " Faraday as a Discoverer" (1868); "Natural Philosophy in Easy Lessons" (1869); " Notes of a Course of nine Lectures on Light" (1870); "Researches on Diamagnetism and Magne-crystallic Action" (1870); "Notes of a Course of seven Lectures on Electrical Phenomena and Theories" (1870); "Essays on the Use and Limit of the Imagination in Science" (1870); "Fragments of Science for Unscientific People" (1871); "The Forms of Water in Clouds and Rivers, Ice and Glaciers" (1872); and "Contributions to Molecular Physics in the Domain of Radiant Heat" (1872). Some of these have been translated into various European languages.

His work on "Sound" has been published in Chinese at the expense of the Chinese government.