Fog. 1. The formation of cloud from water vapour in the air seems to require the presence of nuclei on which the particles may commence to aggregate. These nuclei are afforded by dust in the air, and the resulting cloud varies in appearance with the nature of these dust particles and the magnitude of the aggregations. Its deposition on the dust particles, like its deposition on the ground as dew, will depend on the amount of water-vapour that the air may possess and on the temperature of that layer of air. [Dew.] The cooling of warm air well laden with moisture and containing also a certain amount of dust, will generally cause the formation of cloud, mist, or fog, between which terms no definite distinction can be made. So long as the suspended globules of water remain very small they will be supported by air-resistance and carried about by air-currents; but if they unite and become enlarged they may become too heavy for suspension and fall as rain. Cold water may cool warm air down below the dew-point and so produce fog; this may be seen frequently on marshy land at evening, and is the cause of the coast-fogs of Nova Scotia and other parts. The blackness of London fogs is due to the smoky nature of the particles on which the water-vapour is deposited. The smoke particles are charged with heavy hydrocarbons, and become of a greasy consistency, when they accumulate moisture also. Such fog is highly injurious to animal life, and its prevention is in every way desirable. The simplest method seems to be to prevent the passage of smoke into the air. Smoke is fuel and should not be wasted; its loss in air is uneconomical, and is in the above indirect way dangerous. The aim, therefore, should be to render the carbon-combustion in all furnaces, fires, etc., as complete as possible, so that the products of combustion shall be gaseous and shall contain nothing of the ordinary smoke.
2. In photography, a dark stain which covers a negative, and which may be due to various causes, usually one of the following: over-exposure, use of too much alkali in the developer, a dirty negative plate, exposure to light before or during development. Stains on the plate due to use of a discoloured developer usually disappear during washing, but fog from either of the above causes remains more or less permanent.