Effects produced by Fog and Vapour on the Intensity of Sound. 277 



terest. Hitherto it has been held that the haloid salts of silver 

 were modified chemically only by the rays which they absorb to 

 a marked extent (Schultz-Sellack, 'Reports of the German Che- 

 mical Society/ 1871, p. 211); and, moreover, the influence of 

 "sensitizers" has been partially called in question (Schultz- 

 Sellack, ' Photographic Communications/ seventh annual publi- 

 cation, p. 301). 



My investigations show that, with respect to the sensibility of 

 photographic plates to the action of light, not only does the op- 

 tical aptitude for absorption on the part of the sensitive silver 

 salts themselves, but also the optical aptitude for absorption on 

 the part of the substances mixed therewith play a prominent 

 part. 



Further experiments on this point are in progress. 



Berlin, October 1873. 



XXXIII. A Theory of the Effects produced by Fog and Vapour 

 in the Atmosphere on the Intensity of Sound, By Professor 

 Challis, M.A., F.R.S., F.R.A.S.* 



THE experimental results obtained by Br. Tyndall relative to 

 the intensity of sound under various conditions of the 

 atmosphere (as published in the Proceedings of the Royal Society, 

 No. 149, pp. 58-68) are of so much scientific importance that it 

 seemed to me desirable to endeavour to account for them theore- 

 tically. The questions they raise are clearly hydrodynamical ; 

 and I think I shall be able to show that the appropriate answers 

 may substantially be given by reference to propositions in hydro- 

 dynamics which I have previously discussed in the pages of this 

 Journal. 



It is evident from the experiments that the phenomena to be 

 explained are of two kinds — those resulting from Fog, or the pre- 

 sence in the air of vapour in a visible form, and those due to the 

 admixture with the air of invisible vapour. As to Fog, or Haze, 

 I shall at once assume that it is to be attributed to the suspen- 

 sion in the air of extremely minute globules of water. This 

 supposition is justified by phenomena of the Fog-bow, which in- 

 dicates by its form an action of spherical drops of water on light 

 the same as that which takes place in the case of the primary 

 rainbow ; and its freedom from colour gives evidence of the ex- 

 tremely small size, as compared with that of rain-drops, of the 

 suspended globules. 



Let us conceive aerial vibrations capable of producing sound 

 to be generated and transmitted in air loaded with an immense 



* Communicated by the Author. 



