BIOGRAPHICAL SKETCH 



whereby the germs, in the order of their 

 development, were successively destroyed 

 before starting into active life, he suc- 

 ceeded 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 

 Tubingen 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 

 p. 12). 



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 (May ipth, 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 floccu- 

 lence" 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 con- 

 tinually 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 February 5th, 1874, is sum- 

 marised in the book on Sound (see 

 p. 12). 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 impor- 

 tant steps in modern lighthouse illumina- 

 tion. Tyndall's inability, during a long 

 series of years, to secure what he con- 

 sidered justice towards Mr. Wigham led 

 him eventually to sever himself from 

 colleagues to whom he was sincerely 

 attached. He resigned his post on 

 March 28th, 1883.* 



As a lecturer Tyndall was famed for 

 the charm and animation of his language, 

 for lucidity of exposition, and singular 

 skill in devising and conducting beautiful 

 experimental illustrations. As a writer 

 he did perhaps more than any other 

 person of his time for the diffusion of 

 scientific knowledge. By the publication 

 of his lectures and essays he aimed espe- 

 cially at rendering intelligible to all, in 

 non-technical language, the dominant 

 scientific ideas of the century. His 

 work has borne abundant fruit in 

 inciting others to take up the great 

 interests which possessed so powerful an 



1 See Nineteenth Century, July, 1888 ; Fort- 

 nightly Review, December, 1888, and February, 

 1889 ; New Review, 1892. 



