water again to be held as vapour. Fogs, like mists, are composed of small spheres of water condensed from the atmosphere by cooling; but in these unwelcome visitors the region of cooling extends to a higher level, and the lowering of temperature is due to other causes than contact with the cold surface of the earth. In our populous cities, the density of the fogs is accentuated by the presence of large quantities of solid particles in the atmosphere, which arise from the smoke from coal fires, and the abrasion of the roads by traffic. We can make a city fog in our flask. I blow in some tobacco smoke, and then pump in air as before. You will notice that the smoke, which is now disseminated through the air in the flask, is scarcely visible; but now, on opening the tap, the interior becomes much darker than was the case in our previous experiment. We have produced a genuine yellow fog, that is, a dense mist coloured by smoke. When we have learned how to abolish smoke, and how to prevent dust arising from the streets, our worst fogs will be reduced to dense mists, such as are now met with on the sea or on land remote from large centres of habitation. There is one feature common to the spheres which compose a mist or fog, or indeed to any kind of drop resulting from the condensation of moisture in the atmosphere. As shown by the deeply interesting researches of Aitken and others, each separate sphere forms round a core or nucleus, which is usually a small speck of dust, and hence an atmosphere charged with solid particles lends itself to the formation of dense fogs immediately the temperature falls below the dew-point. But dust particles are not indispensable to the production of condensed spheres, for it has been shown that the extremely small bodies we call “ions,” which are electrically charged atoms, can act as centres round which the water will collect; and much atmospheric condensation at high elevations is probably due to the aid of ions.² Near the surface, however, where dust is ever present, condensation round the innumerable specks or motes is the rule. Here, for example, is a jet of steam escaping into air, forming a white cloud composed of a multitude of small spheres of condensed water. If now I allow the steam to enter a large flask containing air from which the dust has been removed by filtration through cotton wool, no cloud is formed in the interior, but instead condensation takes place at the end of the jet, from which large drops fall, and on the cold sides of the flask. The cloud we see in dusty air is entirely absent, and the effect of solid particles in the process of condensation is thus shown in a striking manner. Clouds are masses of thick mist floating at varying heights in the atmosphere. On sinking into a warmer layer of dry air the particles of which clouds are composed will evaporate and vanish from sight. If the condensation continue, however, the spheres will grow in size until the friction of