230 Prof. A. De la Rive on tJie Organic Dust of the Air. 



water, or fall to the earth on becoming heavier by the fact of 

 that absorption. Finally, I found in the observations of De Saus- 

 sure, and those of Humboldt, facts relative to the transparency 

 of air which seemed to indicate that the clearness always exists 

 in those places where organic matters are absent, or where the 

 humidity annuls their influence. 



These considerations led me to believe, as stated in the memoir 

 communicated to the Academyof Sciences in Paris (Comptes Rendus, 

 vol. lxiv. p. 1221, and Ann. de Chim. et de Phys. vol. xii. p. 243), 

 that it would be interesting to embrace the transparency of the 

 atmosphere in the number of meteorological elements which are 

 subjected to regular determination, and thus to connect it in a pre- 

 cise manner with the other elements, such as the pressure, the 

 temperature, the degree of humidity, the direction of the wind, 

 and, above all, the hour of the day, the period of the year, or 

 the seasons. This class of observations, I added, possessed in- 

 terest, not only for science properly so called, but perhaps also 

 for medicine, as regards ins hygiene of epidemic diseases. It is 

 very probable indeed, I said, that miasmas, the hydrogenous 

 nature of which was demonstrated in 1834 by M. Boussingault, 

 are due to these organic germs, whose presence would be thus 

 established in a manner tolerably exact by the degree of trans* 

 parency, more or less, of the air. 



I expressed myself thus in 1867 ; but for the purpose of veri- 

 fying my conclusions, I had previously sought to appreciate 

 the variations of transparency by a photometer of a new construc- 

 tion, the description of which was given at the Meeting of the 

 Societe Helvetique in Geneva in 1865. (See my discourse on 

 opening the Society, August 23, 1865, Ann. de Chim. et de 

 Phys. vol. xxiv. p. 55.) This instrument, the detailed descrip- 

 tion of which is to be found in the memoir above referred to, 

 has since served me in my regular observations. It confirms 

 the idea which I entertained regarding the influence on the 

 transparency of the air of organic matter, which floats in greater 

 or less quantity according to the hour of the day, the period of 

 the year, &c, and the effect of which is more or less abolished 

 by the humidity of the air. 



I intend to publish immediately the result of these obser- 

 vations, which continue to be regularly made at the Observa- 

 vatory of Geneva. Perhaps the Editors of the Philosophical Ma- 

 gazine would be good enough to make them known. They might 

 be preceded by a description of the instrument which I name 

 an atmospheric photometer, and by means of which the observa- 

 tions were made. 



Auguste De la Rive. 



