Page:South African Geology - Schwarz - 1912.djvu/33

 collected from the roof's of the houses was put under the microscope, and a quantity of quite recognizable organisms were discovered, the commonest being Sarcina coli, a bacterium which lives in the intestines of animals.

Generally, the atmosphere extends upwards for about 45 ml., at which height the barometer would stand at less than 1 in.; meteors burn at heights up to 200 ml., but the air is so rarefied beyond 45 ml. that it is scarcely worth while considering it. If concentrated to a uniform density, such as it has at sea level, the atmosphere would extend to a height of 5 ml. The pressure of the atmosphere is taken as unity: it is equal to a pressure of 14.73 lb. on every square inch, and it will support a column of water 34 ft. high (30 in. of mercury, as mercury is 13.59 times heavier than water).

The origin of the atmosphere, on the Planetismal Hypothesis, is due to the gases brought to the earth in the meteorites out of which it is formed. Actual meteorites contain a large amount of gas, principally carbon monoxide, carbon dioxide, marsh gas, hydrogen, and nitrogen, and this may be taken to represent the constitution of the original atmosphere. On the fixation of carbon from the carbon oxides and marsh gas, free oxygen and hydrogen were produced, which, uniting, gave water, and the surplus of oxygen and the nitrogen remains in the atmosphere to-day.

The watery envelope, or hydrosphere, is the liquid covering of the earth. When the water vapour condenses from the atmosphere it falls as rain; about a quarter of this runs off into the drainage channels, eventually forming rivers, which flow to the oceans, or into depressions on the land, where the water evaporates. About