56o NORMAN LOCKYER LECTURE 



It appears probable that the primitive Earth must have remained hot 

 sufficiently long for most of its initial atmosphere to have been lost. It 

 was pointed out by Russell and Menzel that in the stars and the nebulae 

 neon is as abundant as argon, M^hereas in the Earth's atmosphere argon is 

 five hundred times more abundant than neon. Nitrogen is far less 

 abundant on the Earth than in the stars ; it is ten thousand times more 

 abundant in the Sun than on the Earth. These large « differences in 

 relative terrestrial and solar abundance demand explanation, because in 

 general the relative abundance of elements on the Earth is in close agree- 

 ment with their relative abundance in the Sun and other stars. These 

 facts can be accounted for on the supposition that the rate of loss of 

 atmosphere was very rapid when the Earth was hot. When the cooling 

 had proceeded sufficiently far for the escape of the atmosphere to cease, 

 neon had been depleted to a mdch greater extent than the heavier argon. 

 If this supposition is correct, much of the original oxygen, nitrogen, and 

 water-vapour and all the original helium and free hydrogen must have 

 been lost. As the molten Earth cooled, great quantities of water-vapour, 

 carbon dioxide and other gases must have been evolved from the solidi- 

 fying magma ; these, with the residual gases from the initial atmosphere, 

 formed the new atmosphere which, as the Earth was then relatively cool, 

 could not escape. 



It has been recognised for more than a century that the presence of 

 free oxygen in the atmosphere of the Earth, which we are apt to take for 

 granted without a thought, needs explanation. Oxygen is an element 

 that is chemically active and processes are in continual operation that are 

 depleting the store of oxygen in the atmosphere. One of the principal 

 sources of depletion arises from the weathering of the igneous rocks to 

 form sedimentary deposits — sand, clay and mud. The iron contained 

 in the igneous rocks is not completely oxidised. The greyish hue of these 

 rocks results from the iron being present mainly in the form of ferrous 

 oxide. During the process of weathering, much of the ferrous oxide is 

 oxidised into ferric oxide, which gives the red or brown tints to the 

 weathered deposits. The amount of oxygen that is withdrawn from the 

 atmosphere by this process is very considerable and it has been estimated 

 that during geological times the amount of oxygen thus depleted from the 

 atmosphere is about twice the quantity now present. It is clear that some 

 process must be in operation which replenishes the oxygen in the atmo- 

 sphere. The vegetation over the Earth's surface provides the means for 

 this replenishment. The green plant absorbs carbon dioxide from the 

 air and uses energy from sunlight to decompose it, the energy-transformer 

 being the green colouring matter, called chlorophyll, contained in the 

 plant cells. The carbon is used to build up the complex organic sub- 

 stances found in living plants, the oxygen being returned to the atmosphere 

 as a by-product. 



The supply of carbon dioxide is in turn renewed by the decay of 

 vegetable matter and other organic materials. During the decay of such 

 matter, oxygen is absorbed and carbon dioxide is liberated. This 

 carbon dioxide is again available for building up new plant cells. When-- 

 ever organic matter is buried, as in coal measures and oil deposits, so 

 that it cannot become oxidised and decay, there is a net gain of oxygen 



