1874.] On Experiments with a Fireman's Respirator. 359 



IV. " Further Experiments on the Transmission of Sound." 

 By JOHN TYNDALL, D.C.L., LL.D./ Professor of Natural 

 Philosophy in the Royal Institution. Received May 21, 1874. 



The author describes a number of experiments made with heterogene- 

 ous atmospheres obtained by saturating alternate layers of air with the 

 vapours of various volatile liquids. Starting from his observation on the 

 transmission of sound through a snow-storm on the Mer de Grlace, in 

 the winter of 1859, he shows the extraordinary power of sound to pass 

 through the interstices of solid bodies as long as the continuity of the 

 air is preserved. Sound, for example, penetrates through twelve layers 

 of a silk handkerchief, while a single layer of the same handkerchief 

 dipped into water, so as to fill the interstices, cuts off the sound. 



He also describes numerous experiments with artificial fogs of a 

 density so great that a depth of three feet sufficed to intercept the con- 

 centrated beam of the electric light ; the effect of such fogs on sound 

 was sensibly nil. Experiments were also executed on the illumination 

 of such fogs by sudden flashes, obtained by the combustion of gun- 

 powder or gun-cotton, or by the alternate extinction and revival of the 

 electric and other lights. Such flashes promise to be extremely useful 

 as fog-signals. 



The author corrects the mistake of supposing that, in the experiments 

 at the South Foreland, the lower trumpets were not compared with the 

 higher ones. This, in fact, was the first step of the inquiry. 



He also communicated an extraordinary instance of the interception of 

 sound during one of the battles of the late American war. 



In these experiments the author has been ably aided by his assistant, 

 Mr. John Cottrell. An account of the experiments \\ill be found in a 

 paper now printing for the Philosophical Transactions. 



V. " On some recent Experiments with a Fireman's Respirator." 

 By JOHN TYNDALL, D.C.L., LL.D., Professor of Natural 

 Philosophy in the Royal Institution. Received May 21, 1874. 



In vol. clx. of the ' Philosophical Transactions,' 1870, p. 337, 1 refer to 

 certain experiments on the " floating matter of the air," which were 

 afterwards considerably expanded and in part described in my * Frag- 

 ments of Science/ These experiments, in which my object was to obtain 

 optically pure air by filtration through cotton-wool, suggested to me the 

 notion of a fireman's respirator. Cotton-wool had been previously 

 employed by Schroeder and Pasteur in their experiments on spontaneous 

 generation. 



I had heard that smoke was a formidable obstacle to the fireman, and 

 that cases of suffocation were not rare ; hence the desire to construct a 



VOL. xxn. 2 F 



