B. TERRESTRIAL PHYSICS AND METEOROLOGY. 85 



diminution of temperature. When, then, the cold air of the 

 upper regions is carried down little by little in its whirling 

 movement into the lower and moister strata, it causes all 

 about it the formation of slight fog. This latter, like an ex- 

 terior envelope or veil, renders visible thereby the contour 

 of the mass of cold air. Doubtless the air in its descent, be- 

 ing subjected to compression, is warmed up somewhat, but 

 it must remain always colder than the surrounding air, and 

 the envelope of fog will be produced as long as its tempera- 

 ture remains below the dew-point of the general mass of 

 air." To this view Peslin objects that, "according to the 

 general mechanical theory of heat, the increasing pressure to 

 which the mass of air is subjected as it is brought lower and 

 lower into the atmosphere, ought to cause it to be, at any 

 moment, warmer than the surrounding air, and that it ought, 

 therefore, to be always impossible for such descending cold 

 air to condense the vapor of the atmosphere with which it 

 comes in contact." Faye states that the views of Peslin in 

 this latter regard are not new to him, having been explicitly 

 elaborated by Espy, and by the Commission of the French 

 Academy that in 1841 made a formal report on Espy's theo- 

 ries; and he had fully considered this objection before he 

 wrote his recent memoir, which has attracted so much at- 

 tention, and that the truth of his own view w^ill appear if, in- 

 stead of confining ourselves to theory only, we combine 

 therewith the observation of what actually takes place in 

 nature. 6 B, LXXX., 658. 



ox THE AMMONIA IN THE ATMOSPHERE. 



Schlosing, in a memoir on the presence of ammonia in the 

 atmosphere, states that the origin of this substance is to be 

 looked for in the action of atmospheric electricity on the ni- 

 tro2:en and in the emanations from the soil, althou2:h it is 

 doubtful whether the latter can be a very efficient cause. The 

 direct assimilation of gaseous nitrogen by plants, and the emis- 

 sion of ammonia, is no longer admissible. The entire process 

 is a sort of circulation. Nitric acid is produced by the elec- 

 trical reactions in the atmosphere, and arrives sooner or later 

 in the ocean. There, after having passed into combination 

 with sea-water and its salts, it is converted into ammonia, 

 and now the nitrogen compound is in a form fit for diffusion. 



