Page:Dictionary of National Biography volume 57.djvu/440

 that the obscure rays collected by means of a rock-salt lens would ignite combustible materials at the invisible focus; while some non-combustible bodies, exposed at the same dark focus, became luminous or calorescent. The astounding change in the deportment of matter towards heat radiated from an obscure source which accompanies the act of chemical combination, and many other points of equal importance, were first established by these researches, for which Tyndall received the Rumford medal in 1869. Nine memoirs on these subjects were published in the ‘Philosophical Transactions,’ and many additional papers in other journals. They have been gathered together in ‘Contributions to Molecular Physics in the Domain of Radiant Heat’ (see below). This volume also includes a series of striking experiments on the decomposition of vapours by light, in the course of which the blue of the firmament and the polarisation of sky-light—illustrated on skies artificially produced—were shown to be due to excessively fine particles floating in our atmosphere.

While engaged upon the last-mentioned inquiry, Tyndall observed that a luminous beam, passing through the moteless air of his experimental tube, was invisible. It occurred to him that such a beam might be utilised to detect the presence of germs in the atmosphere: air incompetent to scatter light, through the absence of all floating particles, must be free from bacteria and their germs. Numerous experiments showed ‘optically pure’ air to be incapable of developing bacterial life. In properly protected vessels infusions of fish, flesh, and vegetable, freely exposed after boiling to air rendered moteless by subsidence, and declared to be so by the invisible passage of a powerful electric beam, remained permanently pure and unaltered; whereas the identical liquids, exposed afterwards to ordinary dust-laden air, soon swarmed with bacteria. Three extensive investigations into the behaviour of putrefactive organisms were made by Tyndall, mainly with the view of removing such vagueness as still lingered in the public mind in 1875–6, regarding the once widely received doctrine of spontaneous generation. Among the new results arrived at, the following are noteworthy: bacteria are killed below 100° C., but their desiccated germs—those of the hay bacillus in particular—may retain their vitality after several hours' boiling. By a process which he called ‘discontinuous heating,’ whereby the germs, in the order of their development, were successively destroyed before starting into active life, he succeeded in sterilising nutritive liquids containing the most resistant germs. This method, since universally adopted by bacteriologists, has proved of great practical value. The medical faculty of Tübingen gave Tyndall the degree of M.D. in recognition of these researches. The original essays, written for the ‘Philosophical Transactions,’ are collected in ‘Floating Matter of the Air’ (see below).

In 1866 Tyndall had succeeded Faraday as scientific adviser to the Trinity House and board of trade. He held the post for seventeen years, and it was in connection with the elder brethren that his chief investigations on sound were undertaken, with a view to the establishment of fog signals upon our coasts. Many conflicting opinions were held as to the respective values of the various sound signals in use when Tyndall began his experiments at the South Foreland (19 May 1873). Very discordant results appeared at first, but all were eventually traced to variations of density in the atmosphere. Tyndall discovered that non-homogeneity of the atmosphere affects sound as cloudiness affects light. By streams of air differently heated, or saturated in different degrees with aqueous vapour, ‘acoustic flocculence’ is produced. Acoustic clouds, opaque enough to intercept sound altogether and to produce echoes of great intensity, may exist in air of perfect visual transparency. Rain, hail, snow, and fog were found not sensibly to obstruct sound. The atmosphere was also shown to exercise a selective and continually varying influence upon sounds, being favourable to the transmission sometimes of the longer, sometimes of the shorter, sonorous waves. Tyndall recommended the steam siren used in the South Foreland experiments as, upon the whole, the most powerful fog signal yet tried in England. His memoir on the subject, presented to the Royal Society on 5 Feb. 1874, is summarised in the book on ‘Sound’ (see below). Passing mention should be made of the beautiful experiments on sensitive flames described in the same volume.

It was likewise in his capacity of scientific adviser that Tyndall was called upon, in 1869 and on many subsequent occasions, to report upon the gas system introduced by Mr. John Wigham of Dublin, the originator of several important steps in modern lighthouse illumination. Tyndall's inability, during a long series of years, to secure what he considered justice towards Mr. Wigham led him eventually to sever himself from colleagues to whom he was sincerely attached. He resigned his post on 28 March 1883 (see Nineteenth Century, July 1888; Fortnightly