160 Intelligence and Miscellaneous Articles. 



glass were not appreciably altered by electrolysis. — Wiedemann's 

 Annalen, No. 4, 1884. 



ON PHENOMENA OF DIFFRACTION IN MOIST AIR. 

 BY M. KISSL1NG. 



In a thorough experimental investigation into the influence which 

 foreign admixtures exert on the formation of fog in moist air — an 

 influence first established by Coulier and Mascart, and then by 

 Aitkin — the author has observed a series of phenomena of diffrac- 

 tion, the development of which, and the law of whose formation, 

 can only with difficulty be made to agree with Fraunhofer's law of 

 the phenomena of diffraction ; they are, on the other hand, in 

 obvious connexion with the phenomena of twilight, which have 

 recently excited such universal interest. The author only gives 

 briefly the most important results of his experimental investigation. 



The phenomena are produced in a tube closed by two parallel 

 glass plates, through which the diaphragm of an Argand burner 

 placed at a suitable distance was observed. 



The author found that Aitkin's law was in general confirmed, 

 namely, that when aqueous vapour is condensed in the air, this 

 always takes place on some solid nucleus. Thus, in the ordinary un- 

 filtered air of a dwelling-room, where the space in which diffraction 

 occurs is but slightly cooled, the fog is so strong that it greatly 

 enfeebles even a powerful source of light. If the unfiltered air be 

 gradually admixed with filtered air, the formation of fog gradually 

 diminishes, while at the same time phenomena of diffraction set in, 

 the intensity of whose colour increases until the quantity of vapour 

 has sunk to a certain, extremely small amount. As the filtration 

 is continued, the coloured phenomena become rapidly less intense, 

 and a formation of fog can no longer be perceived. It must be 

 observed that by "vapour" the author understands the totality of 

 all foreign constituents in atmospheric air with the exception of 

 water. If the degree of moisture in the diffraction-space is dimi- 

 nished, so that it is only slightly removed from its point of satura- 

 tion, an exceedingly rapid decrease of the production of colour sets 

 in. It is found in general that the absolute maximum of the 

 phenomena of diffraction is conditioned by the coincidence of that 

 amount of vapour which is most favourable to light, with the 

 maximum moisture in the layer in question. 



Small admixtures of S0 2 and NH 3 with the unfiltered air of a 

 room produce so strong a fog, that any action of diffraction 

 ceases. 



Finely divided dust of pumice from Ivrakatoa exerted a per- 

 ceptible, though small, influence on the formation of fog. — 

 Beibl'dtter der Physik, vol. viii. p. 514. 



