242 Intelligence and Miscellaneous Articles. 



distance of seven leagues. Humboldt observes that the peak of Te- 

 neriffe is visible at extraordinary distances, either immediately or 

 even several hours after an abundant rain. 



The cause of this phenomenon is not to be sought in an optical 

 effect arising from a mixture of air and of aqueous vapour, as was 

 formerly believed, but simply in the fact that aqueous vapour par- 

 tially dissolves the impurities mixed with the air and makes it more 

 translucid. This opinion, originally expressed by Colonel Jackson 

 in 1832, always seemed well based, but it needed to be more pre- 

 cisely expressed and supported by experiment. M. Pasteur's re- 

 markable researches, in showing that the atmosphere, more espe- 

 cially the layers nearest the soil, is filled with a mass of organic 

 germs, seemed to furnish a key to these phenomena, at the same 

 time that they have enabled me to explain the circumstances other 

 than moisture which influence the transparence of the air. 



All organic germs form, when the atmosphere is dry, a sort of 

 slight mist which intercepts some of the light from distant objects ; 

 but when a general moisture sets in the mist disappears, either because 

 the germs become transparent from absorbing aqueous vapour, or 

 because the water which they have absorbed rendering them heavier 

 makes them fall to the ground. This, as 1 think, is the most fre- 

 quent cause of those striking changes in the transparence of the at- 

 mosphere which are sometimes manifested in the most unexpected 

 manner, but which always coincide with variations in moisture. 



Further, if the presence of aqueous vapour makes the atmosphere 

 transparent when it contains organic germs, this presence is no 

 longer necessary in the absence of these germs. This explains why 

 mountains in winter are visible even when the air is very dry, why 

 the air is transparent on snow fields, why, again, as has been ob- 

 served by Humboldt, the atmosphere of the Peak of Teneriffe is trans- 

 parent with an east wind which brings the African air ; for this, 

 having brought no organic exhalations from the deserts whence it 

 comes, or from the sea over which it has passed, has no need of 

 moisture to be transparent. It is, on the contrary, in the hot season 

 and in the months in which organic life is most active that the air is 

 most charged with this kind of dry vapour, which in serene weather 

 diminishes so materially the visibility of distant objects. 



These considerations have led me to the conclusion that it would 

 be truly interesting to include the transparence of the atmosphere in 

 the number of meteorological elements which are regularly deter- 

 mined, so as to establish precise relations between this particular 

 element and others, such as pressure, temperature, degree of mois- 

 ture, direction of the wind, and especially of the time of day and of 

 the year (that is, the seasons) . This kind of observations would be 

 interesting, not merely as a question of pure science, but also per- 

 haps for medicine, from the point of view of hygiene and epidemic 

 maladies. It is probable, in fact, that the miasmata which M. Bous- 

 singault, in a beautiful research published in 1834, had found to con- 

 tain hydrogen, are due to these organic germs, whose presence in the 

 atmosphere and falling to the ground would be indicated in a suffi- 

 ciently exact manner by the greater or less transparence of the air. 



