ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 237 



which would be fog - on the ground, depends on the height at which the 

 dew-poiut of the air is reached, or else on the height of the boundaries 

 of a lower and upper current differing in temperature. The lower air 

 is too dry to permit the condensation of vapor within its borders. A 

 warmer and moister upper current condenses vapor by contact with 

 the cold upper boundary of the lower air. The cloud canopy prevents 

 excessive loss of heat from the surface of the earth. 



A mist, in the usual meaning of the term, is the name given to very 

 small rain, or to a cloud of which the globules are large enough to fall 

 perceptibly. Near the surface of the earth it seldom, if ever, grows 

 from radiation fog or from the haze of an ticy clonic conditions, but 

 very frequently is a result and direct growth from wet or mixture fogs. 

 It may be considered as fine rain, which falls from a cloud undergoing 

 cooling and consequent aggregation of particles. In hilly country 

 near the sea, where the wind arrives after having blown over a large 

 breadth of warm ocean, misty rain is very common. 



At Kingairloch the number of dust particles was always very Ioav in 

 such weather, showing that the majority were being used up by the 

 mist. The transparency of the air, or " visibility," so often preceding 

 rain is due first to the paucity of dust particles brought by an ocean 

 wind which is made purer than it otherwise would be by the clouds 

 and rain of the area from which it blows; secondly, to the homogeneity 

 of the air and the tendency to form large cloud globules or drops of 

 rain when near saturation, the proportion of vapor to dust particles 

 being high. 



In quiet winter weather, a long-continued damp mist or else a very 

 fine steady rain has, in the present writer's experience in England, 

 preceded intense cold, and may be supposed with great probability 

 to be caused by the gradual descent of very cold air upon the lower 

 strata. 



PARTICLES SUSPENDED IN THE AIR. 



The atmosphere contains an immense number of substances sus- 

 pended in it in the form of visible and invisible dust, but only a small 

 proportion of these require attention as affecting human life. Deserts, 

 dry and sandy tracts, and wind-swept plains yield a continual supply 

 of fine motes of silica, aluminium silicate, calcium carbonate, calcium 

 phosphate, etc. Volcanoes pour forth sand, fine mud, sulphur, sul- 

 phuric acid, silicon glass, etc., into the upper air, by which they are 

 carried over all quarters of the globe. Meteors and small aerolites 

 burn up as they daily pass through the high and rare atmosphere at 

 heights from 30 to 200 or even 300 miles, and the products of their 

 combustion, iron oxide, magnesia, silica, or other fine dust, fall impal- 

 pably toward the ground • Clouds of unburnt carbon perpetually rise 

 from towns, factories, steamships, and scattered houses; in manufac- 

 turing districts and towns particles of iron, steel, stone, and clay are 

 abundant; so are fragments of vegetable tissue, cotton, hair, wool, 



