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TYNDALL 91 flache mit geneigter Erseugungslinie, und die Bedingungen des Glelchgewichts auf 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 sec- retaries of the physical section of the British association. In June, 1853, he was appointed Erofessor of natural philosophy at the royal istitution, 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 vis- ited 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 sev- eral 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 in- accessible peak of the Weisshorn, and in 1868 reached the summit of the Matterhorn, crossing it from Breuil to Zermatt. The results of his gla- cial investigations were published in the " Phil- osophical Transactions " (jointly with Prof. Huxley's) for 1858, and subsequently in "Gla- ciers 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 mechanical pressure. Through the direct application of the doctrine of regelation, he arrived at a satis- factory understanding of the nature of glacial motion, proving, by carefully repeated obser- vations on the structure and properties of ice, the inefficacy of the generally admitted plas- tic theory to account for that phenomenon. This discovery led to a protracted controversy with Professor (afterward Principal) Forbes of Edinburgh. (See GLACIEK, 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 re- turn voyage instituted a number of simple in- quiries in relation to the color of the ocean. He demonstrated that the change of color fre- quently observed at different portions of the sea is due to the reflection of certain rays of light from the surfaces of innumerable parti- cles 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 establish- ment 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 du- ring the same time. This gave rise to a wide- spread controversy, and was popularly denom- inated " Tyndall's prayer test." In August, 1874, while presiding over the annual meet- ing of the British association, he delivered the famous inaugural known as the "Belfast Address," which was denounced as a declara- tion of materialism. The labors of Prof. Tyn- dall, though more particularly directed toward the examination of the molecular constitution of matter, have not been confined to any spe- cial branch of physics. Between 1849 and 1856 he was mainly occupied with the prose- cution of his experiments in magnetism and electricity, in the course of which he conclu- sively settled the question of diamagnetic or reversed polarity, the existence of which, ori- ginally asserted by Faraday, and reaffirmed by "Weber in 1848, had been subsequently denied by the former. In 1859 he initiated a remark- able series of researches in radiant heat, which were extended over a period of more than ten years. The diathermancy of simple and com- pound gases, as well as of various vapors and liquids, was experimentally tested, and the de- grees of their opacity to radiant heat deter- mined 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 analy- sis, was applied by Tyndall to gases and va- pors some time previous to the publication of Kirchhoff 's more specialized generalizations re- specting refrangibility. Tyndall's investiga- tions on obscure and luminous radiations, and on the nature of calorescence, or the transmu- tation 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 car- bon, 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