Among the several plausible hypotheses are the following: (a) That the cloud particles are jostled together by currents of air, or that the larger ones fall fast enough to overtake the smaller ones, so that in either ease larger par ticles are formed which, as they descend, grow by the accretion of small particles that lie in their path. (1) That some of the smaller particles are positively and others negatively electrified, and consequently by attraction are made to coalesce. (c) That some particles, being larger than others, have different surface ten sions, and that the larger ones are thereby enabled to grow at the expense of the smaller ones. (d) That the original cloud particles consist of vapor that has condensed upon par ticles of dust or foreign substances in the air, and that this condensation takes place more readily upon some nuclei than others, as is known to be the case from the observations of Wilson, Aitken, Barns, and others. (e) That the atmosphere within a Cloud, being saturated, has no remaining nuclei upon which condensation can take place, and as the air continues to rise and cool it conies to a state of supersaturation and intense molecular strain, which is finally relieved by a violent condensation upon groups of cloudy particles already existing: this violent condensation takes place in such a way as to sweep many of the smaller cloud particles to gt•ther into one large drop: C'. T. II. Wilson has
shown that these larger drops can only be formed when the air in the dustless cloud has been expanded and cooled at least one-third more than is required for ordinary dusty air. (f) son has lately shown the plausibility of a slight modification of the preceding method: he finds that dustless air virtually acquires new nuclei on which condensation takes place when a beam of ultra-violet light or of the Roentgen rays, the radiation from uranium. or even ordinary sun light, passes through the moist air, that in fact such nuclei are being formed in it all the Professor J. J. Thomson's observations on the formations of 'ions'—namely, the breaking away from a molecule of some one of its integral com ponents which he calls 'corpuscles'—suggest that the atmospheric 'ions' thus formed are active in producing cloudy condensation, and that the negative 'ions' attract moisture to themselves more readily than the positive, therefore they grow to he larger drops, and descending to the earth with their negative give it nega tive electricity, while the atmosphere is left essentially positive.