338 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



vessel filled with smoke, widened the "dust-free coat" enormously, and 

 the whole box was cleared of smoke. A discharge from a Yoss or 

 Wimshurst machine through smoke causes a very rapid aggregation in 

 masses or flakes along the lines of force, and the soot is left on the 

 sides and floor of the vessel. The most effect is produced when the air 

 itself is electrified, but a knob acts less quickly than a point. 



A piece of rubbed sealing wax held about a yard distant from a fall- 

 ing water jet broken into small drops causes the drops at once to 

 cease to scatter, and unites them into large drops as of a thunder 

 shower. A cloud of steam turns into " Scotch mist;" a spherule of 

 water amalgamates with a large mass at the first opportunity; if there 

 be the slightest difference in size or in electrification, the repulsion is 

 exchanged for attraction before actual contact. The opposed surfaces 

 come into collision with considerable violence, even when the relative 

 motion of the centers of the masses is small. Surface tension is over- 

 come, and thus violence of contact promotes the coalescence of drops. 



The whole subject is of deep interest, not only in connection with the 

 causes of rain and conditions of cloud formation, but with the physics 

 of the atmosphere generally. 



OVERCOOLING, ETC. 



Other matters deserving fuller investigation than they have yet 

 received, although they have been the subject of valuable memoirs by 

 Dufour, Von Bezold, and others, are the capability of vapor existing in 

 the atmosphere beyond the normal degree of saturation, "overcooling," 

 as it has been termed; and, secondly, the degree of temperature and 

 other conditions in which small drops of water and cloud globules can 

 exist unfrozen. These questions are of great interest both meteorolog- 

 ically and in relation to physics in general. 



With regard to the supersaturation of air, this has been proved to be 

 possible in the laboratory to a remarkable degree when dust is absent, 

 but has not yet been proved in the atmosphere, It seems highly prob- 

 able that occasionally, especially in very moist air, when much rain 

 and cloud has been long continued, or in the intervals between thunder 

 clouds at a great height, there may be spaces of the atmosphere in 

 which dust is so rare and moisture so large that the ordinary point of 

 saturation may be passed. The accumulation upon drops or snowflakes 

 passing through such a space would be heavy. 



The latent heat of condensation from vapor upon cold drops of ice 

 has been supposed, owing to its very considerable amount, to make the 

 growth of such drops or hailstones to a large size by deposition from 

 vapor impossible. But rapid passage through cold air may be found to 

 dispose very quickly of the heat thus set free. Experiment is needed 

 on this point. 



With regard to the liquidity of droplets below the freezing point, the 



