5^8 ■'' Prof. Grove oti Grotthus's Theoiy of 



meii^6f the molecules of the three gases concerned ri'i'iisit 'be^ 

 perfectly symmetrical. I see in this fact a confirmatioii of ari^ 

 idea which I believe I was the first to announce, of the exist-' 

 ence of an attractive force between mixed gases of a double 

 character; in one sense a chemical force, because it exists' 

 only between the particles of dissimilar gases ; in another 

 sense a mechanical force, because it obeys a mechanical law,* 

 — the law of diffusion as ascertained by Prof. Graham. In' 

 atmospheric air, which is a mixture^ not a chemical combina- 

 tion^ of oxygen and nitrogen, the chemical force is so feeble' 

 as not to interfere with the communication of sound, as it 

 would take place in a homogeneous medium. But in a mix- 

 ture of oxygen, nitrogen and hydrogen, the conditions are 

 changed ; the superior chemical affinity of oxygen for hydro- 

 gen disturbs the mechanical equilibrium of the system ; the 

 molecules cease to vibrate, as in a homogeneous medium; 

 and sound ceases to be communicated, as I infer from the fact 

 announced by Sir David Brewster. 



I cannot better conclude these remarks than by adding to 

 the interesting facts previously quoted, the explanation of a 

 similar phaenomenon which I have never seen noticed before. 

 In Lancashire, where I passed the early years of my life, I 

 soon found that it was, among the farmers and country people, 

 a traditional observation, that when the distant hills appeared 

 unusually distinct, it was a sign of approaching rain. This I 

 attribute to the homogeneity of the atmosphere under such 

 circumstances, with respect to aqueous vapour. 



The whole intervening space between the eye and the di- 

 stant object is saturated with moisture, and consequently 

 vision is more perfect, as there are no series of media of dif- 

 ferent densities to interfere with the direct transmission of 

 light. 



I am. Sir, 



Your obedient Servant, 

 Richmond, October ], 1845, Thomas Starkie Thomson. 



' " — 



LII. On Grotthus's Theory of Molecular Decomposition and 

 Recomposition. By W. R. Giiove, Esq., F.R.S., Sfc, and 

 Professor of Natural Philosophy in the London Itistitution. 



To Richard Phillips, Esq., F.R.S. 

 Dear Sir, 

 '^I^HE great mystery of voltaism, or the phaenomenon of two 

 A constituents of a chemical compound being eliminated 

 at distant points without any apparent disturbance in the in- 

 tervening liquid, is accounted for, as you are well aware, by 



