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 of slate rocks. On 6 June 1856 he lectured on the subject at the Royal Institution (see appendix to Glaciers of the Alps). Huxley, who was present, suggested afterwards that the same cause might possibly explain the laminated structure of glacier ice recently described in Forbes's ‘Travels in the Alps.’ The friends agreed to take a holiday and inspect the glaciers together. The results of the observations made during this and two subsequent visits to Switzerland are given in Tyndall's classical work ‘The Glaciers of the Alps’ (see below). The original memoirs are in the ‘Philosophical Transactions’ for 1857 and 1859. Tyndall, assisted by his friend Thomas Archer Hirst, made many measurements upon the glaciers in continuation of the work of Agassiz and Forbes. He discussed, in particular, the question as to the conditions which enable a rigid body like ice to move like a river. He showed very clearly the defects of former theories, proving by repeated observations on the structure and properties of ice the inefficacy of the generally admitted plastic theory to account for the phenomena. Through the direct application of the doctrine of regelation he arrived at a satisfactory explanation of the nature of glacier motion. The veined structure he ascribed to mechanical pressure, and the formation of crevasses to strains and pressures occurring in the body of the glacier. In assigning to Rendu his position in the history of glacier theories, Tyndall gave offence to James David Forbes [q. v.] A controversy followed, in which the fairness of Tyndall's attitude was fully vindicated.

The expedition to Switzerland, undertaken for a scientific purpose, had a secondary outcome. Tyndall was fascinated by the mountains, and from that time forward yearly sought refreshment in the Alps when his labours in London were over. He became an accomplished mountaineer. In company with Mr. Vaughan Hawkins he made one of the earliest assaults upon the Matterhorn in 1860. He crossed over its summit from Breuil to Zermatt in 1868. The first ascent of the Weisshorn was made by him, in 1861. Tyndall's descriptions of his alpine adventures are not only graphic and characterised by his keen interest in scientific problems, but show a poetical appreciation of mountain beauties in which he is approached by few alpine travellers.

The very important series of researches on ‘Radiant Heat in its relation to gases and vapours,’ which occupied him on and off for twelve years, and with which his name will be always especially associated, were begun in 1859. He was led from the consideration of glacier problems to study the part played by aqueous vapour and other constituents of the atmosphere in producing the remarkable conditions of temperature which prevail in mountainous regions. The inquiry was one of exceptional difficulty. Prior to 1859 no means had been found of determining by experiment, as Melloni had done for solids and liquids, the absorption, radiation, and transmission of heat by gases and vapours. By the invention of new and more delicate methods Tyndall succeeded in controlling the refractory gases. He found unsuspected differences to exist in their respective powers of absorption. While elementary gases offered practically no obstacle to the passage of heat rays, some of the compound gases absorbed more than eighty per cent. of the incident radiation. Allotropic forms came under the same rule; ozone, for example, being a much better absorbent than oxygen. The temperature of the source of heat was found to be of importance: heat of a higher temperature was much more penetrative than heat of a lower temperature.

The power to absorb and the power to radiate Tyndall showed to be perfectly reciprocal. He also established that, as regards their powers of absorption and radiation, liquids and their vapours respectively follow the same order. Thus he was able to determine the position of aqueous vapour, which, on account of condensation, could not be experimented upon directly. Experiments made with dry and humid air corroborated the inference that as water transcends all other liquids, so aqueous vapour is powerful above all other vapours, as a radiator and absorber. These results, questioned by Magnus and by a few later experimenters, but fully established by Tyndall, explained a number of phenomena previously unaccounted for. Since Wells's researches on dew, no fact has been established of greater importance to the science of meteorology than the high absorptive and radiative power of aqueous vapour. Many years later an experiment made in his presence by Mr. Graham Bell suggested to Tyndall a novel and interesting method of indirectly confirming his former results. (See ‘Action of Free Molecules on Radiant Heat, and its Conversion thereby into Sound,’ Phil. Trans. 1882, pt. i.).

Using a dark solution of iodine in bisulphide of carbon as a ray-filter, Tyndall was able approximately to determine the proportion of luminous to non-luminous rays in the electric and other lights. He also found