ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 205 



day's rain on tlie low ground, prove how commonly rain is melted ice 

 or snow. 



Other solid particles always present in great numbers in the lower air, 

 and of great importance in relation to human, animal, and plant life, are 

 various kinds of microbes, fungi, molds, and spores. At certain sea- 

 sons the pollen of plants is very abundant. In some countries the air 

 is thick in the dry and windy season with the dust of the soil. Agri- 

 cultural fires cause a thick haze over parts of Germany, the United 

 States, and other countries at certain times of the year. After great 

 volcanic eruptions the air over many thousand square miles has been 

 affected by a dense haze. This was notably the case in the summer 

 of 1783, when, after an eruption in Iceland, terrestrial and celestial 

 objects were dimmed by "dry fog" in western and central Europe dur- 

 ing several weeks. In 1883, on the other hand, after the eruption of 

 Krakatoa, near Java, the upper air, between 40,000 and 120,000 feet in 

 altitude, was overspread with a semitransparent haze of a very remark- 

 able character, consisting mainly of finely divided, glassy pumice. 

 This haze stratum in the upper sky extended over all known countries 

 and remained visible for several months. 



Cloud globules are the most obvious and widely present liquid ingre- 

 dients of the atmosphere. They possess properties of great interest 

 in connection with the recently discovered ubiquitous atmospheric dust, 

 with optical phenomena, and with the formation and distribution of 

 rain. 



The other familiar forms of water in the air are dry and damp fogs, 

 mist, and rain. Haze is in most instances, at least so far as the pres- 

 ent writer's observations go, in the south of England, a phenomenon 

 depending on very small particles of water and on the presence of dust 

 particles as nuclei. 



Ozone, an allotropic and unstable form of oxygen, has been found to 

 be constantly present, in very small quantities, in the open air in nat- 

 ural conditions, but can not be traced in the impure air of great towns, 

 and is no doubt always greatly diminished where dwellings are thick 

 together. Ozone consists of molecules, each supposed to contain three 

 molecules of oxygen. 



Peroxide of hydrogen is also supposed to exist in slight traces in the 

 general atmosphere. 



Minor impurities, arising from animal life, from manufacturing 

 processes, and from the combustion of coal, are mostly not perceptible 

 to the senses, except in the neighborhood of places where they are 

 given off very abundantly. 



The principal functions of all these various elements and substances 

 of which the atmosphere is composed, may now be regarded in detail 

 with special reference to their influence upon human life and welfare. 



OXYGEN. 



Oxygen, that wonderful element which constitutes very nearly half 

 of the solid crust of the globe, combined as most of it is with the 



