THE ATMOSPHERE 197 



accustomed Flagellaria, which normally live in water at 15 C., 

 to live in water at 70 C. To destroy the spores of bacilli 

 temperatures of 110 to 120 C. are required. On the other 

 hand, algae which produce red snow flourish at a temperature 

 below C., and bacteria can withstand cold exceeding 200 C. 

 produced by the evaporation of liquid air. Some organisms 

 only flourish in the purest sea-water, while others multiply in 

 conditions of putrescence where there is entire absence of free 

 oxygen. Nevertheless, in Mars, which is but half the diameter of 

 the earth, and still more in the other planets, the conditions of 

 heat, moisture, light, and gravity must incalculably transcend all 

 terrestrial variations, and render the existence of protoplasm 

 there most improbable, and certainly forbid the evolution of 

 terrestrial forms of life. It is none the less conceivable that 

 in the infinity of space other suns exist attended by planets 

 where the physical conditions are similar to those on the earth, 

 and where life exists. As the structure of the atom is now 

 recognised to resemble a miniature universe, so our universe 

 may be but an atom in one infinitely greater. 



The chief source of kinetic energy on the earth with the 

 exception of the tides, which depend on the rotation of the 

 earth on its axis is the light and heat rays of the sun. 

 Currents of air, winds, and storms arise from unequal warming 

 of the different layers of the air, the ocean currents from 

 unequal warming of the sea. By the sun's heat water vapour 

 is formed and lifted into the higher and colder regions of the 

 atmosphere. There the vapour condenses and the kinetic energy 

 of the ethereal waves appears as the kinetic energy of the falling 

 rain, and of the flowing brooks and streams. Sunlight drives 

 alike the ships and windmills, the water-wheels and the electric 

 motors at the Niagara falls, the sunlight which illuminated the 

 earth millions of years before burns in our fires, and drives 

 our factory machines and locomotives. The sunlight generates 

 the energy of the whole living world. 



The composition of the atmosphere and the nature of the 

 process of combustion and of respiration have only been known 

 for some six score years, while geological evidence points to the 

 existence of man for years which probably number hundreds of 

 thousands, and archseological evidence shows that the chemical 

 handling of metals had advanced in Egypt so far as the fashion- 



