B. TERRESTRIAL PHYSICS AND METEOROLOGY. 8i) 

 THE FOEMATION OF CLOUDS. 



Mtihry has lately presented, in a very impressive manner, 

 the conclusions deducible from some observations published 

 by Meissner, in 1863, on the formation of vapor vesicles and 

 of clouds. The researches of Meissner were mainly directed 

 to the relations of ozone and antozone, and it was only as 

 one of the incidental results of his work that he announced 

 that, without the presence of oxygen in tlie air, there could 

 be no clouds. In regard to this important point Meissner's 

 researches have apparently not attracted the attention that 

 is due them, and Milhry urges that meteorologists and physi- 

 cists are not yet to consider that the question of the exist- 

 ence of vesicles of vapor has been settled in the negative. 

 Basing his conclusions on Meissner's researches, Miihry says 

 that the condensation and precipitation of aqueous vapor 

 would take place immediately, in the form of small drops, if it 

 were not for the presence of oxygen in the air; that this gas 

 itself brings about the transition stage the vapor vesicle. 

 The experiments of Meissner consisted in confining within the 

 receiver of an air-pump a mixture of aqueous vapor and the 

 gas to be exjjerimented on. By a rapid stroke of the piston 

 the mixture is then quite suddenly expanded, and the cool- 

 ing due to expansion produces a precipitation of a portion of 

 the inclosed vapor. The faint cloud that is seen by close ob- 

 servation within tlie receiver continues but a few minutes, 

 and was first observed with special care by Saussure, in 1783. 

 Meissner, however, has shown that when other gases replace 

 the air within the receiver, the condensation in general takes 

 place not in the form of a cloud, but of fine, light drops that 

 fall directly to the bottom, the cloud being produced only 

 wiien oxygen is present, either pure or mixed with other gas- 

 es. These experiments have been repeatedly and very care- 

 fully made by Meissner witli air, nitrogen, hydrogen, carbonic 

 acid gas, and in pure aqueous vapor alone, and in various 

 mixtures of these gases. Meissner further measured the ex- 

 act degree of expansion needed to produce these vapor clouds, 

 and found that saturated air at 30 inches deposited its vapor 

 when the pressure is suddenly reduced to 21.4 inches; by a 

 second step he passed from saturated air at 21.4 inches to 

 16.1 inches, when a somewhat fainter cloud was formed; the 



