88 Lord Kelvin on the Age of the 



6 tons per English acre per annum or 1^ tons per square 

 metre per thousand years. At this rate it would take only 

 1533 years, and therefore in reality a much longer time 

 would almost certainly be required, to produce the 2*3 tons 

 of oxygen which we have at present resting on every square 

 metre of the earth's surface, land and sea*. But probably 

 quite a moderate number of hundred thousand years may 

 have sufficed. It is interesting at all events to remark that, 

 at any time, the total amount of combustible material on the 

 earth, in the form of living plants or their remains left dead, 

 must have been just so much that to burn it all would take 

 either the whole oxygen of the atmosphere, or the excess of 

 oxygen in the atmosphere at the time, above that, if any, 

 which there was in the beginning. This we can safely say, 

 because we almost certainly neglect nothing considerable in 

 comparison with what we assert when we say that the free 

 oxygen of the earth's atmosphere is augmented only by 

 vegetation liberating it from carbonic acid and water, in 

 virtue of the power of sunlight, and is diminished only by 

 virtual burning f of the vegetable matter thus produced. 

 But it seems improbable that the average of the whole 

 earth — dry land and sea-bottom — contains at present coal, 

 or wood, or oil, or fuel of any kind originating in vegeta- 

 tation, to so great an amount as "767 of a ton per square 

 metre of surface ; which is the amount at the rate of one ton 

 of fuel to three tons of oxygen, that would be required to 

 produce the 2'3 tons of oxygen per square metre of surface, 

 which our present atmosphere contains. Hence it seems 

 probable that the earth's primitive atmosphere must have 

 contained free oxygen. 



§ 43. Whatever may have been the true history of our 

 atmosphere it seems certain that if sunlight was ready, the 

 earth was ready, both for vegetable and animal life, if not 

 within a century, at all events within a few hundred cen- 

 turies after the rocky consolidation of its surface. But was 

 the sun ready ? The well founded dynamical theory of the 

 sun's heat carefully worked out and discussed by Helmholtz, 



* In our present atmosphere, in average conditions of barometer and 

 thermometer we have, resting on each square metre of the earth's surface, 

 ten tons total weight, of which 7-7 is nitrogen and 2*3 is oxygen. 



f This " virtual burning " includes ereuiacausis of decay of vegetable 

 matter, if there is any eremacausis of decay without the intervention of 

 microbes or other animals. It also includes the combination of a portion 

 of the food with inhaled oxygen in the regular animal economy of pro- 

 vision for heat and power. 



