Prof. A. De la Rive on the Organic Dust of the Air, 229 



chloride of silver is exposed to the sun's light, the precipitate 

 changes its nature, the line is extremely distinct, but the phe- 

 nomena of polarization are far less definite than before. 



I do not see how these facts can be due to any other cause 

 than that of particles in suspension. 

 Believe me, 



My dear Tyndall, 



Yours very sincerely, 



L. Soret. 



XXIX. On the Organic Dust of the Air. 

 To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



I RECEIVED last night and translated rapidly this morning 

 the following letter from M. De la Rive, which I trust will 

 reach you in time for your forthcoming Number. I may add 

 that the current Number of Eraser's Magazine contains a brief 

 historic summary of this subject, to which I would refer the 

 reader. I look forward with extreme interest to the publication 

 of M. De la Rive's photometric observations. 



John Tyndall. 



Royal Institution, 

 February 24, 1S70. 



My dear Tyndall, Geneva, February 22, 1870. 



I have read with great interest the analysis of your lecture at 

 the Royal Institution on the 21st of January, and admired the 

 experiments by which you succeed in demonstrating that all the 

 dust floating in the air of London is of an organic nature. Your 

 conclusions as regards the probable cause of epidemic diseases 

 remind me that I had announced the same consequences in an 

 inquiry in which, by a process different from yours, I sought to 

 prove the presence of organic dust in atmospheric air. Permit 

 me to state in a few words the results of this inquiry. 



I had followed with care the labours of Pasteur, who, like you, 

 had deprived the air of its organic matter, sometimes by passing 

 it through a red-hot tube, and sometimes by the employment of 

 cotton wool. I had been struck by the fact that the air on high 

 mountains, and more particularly on the summits covered with 

 snow, does not contain any such matter. I was thus led to be- 

 lieve that the remarkable transparency of atmospheric air when 

 it is charged with humidity, both before and after rain, arises 

 from the fact that the organic particles which float constantly in 

 the air, at least over plains, become transparent by absorbing 



