34 THE EIGHT HON. LORD KELVIN^ G.C.V.O.^ ON 



oxygen as that of a Hessian forest, as estimated by Liebig** 

 50 years ago, or of a cultivated English hayfield of the 

 present day, a very improbable supposition, and if there 

 were no decay {eremacausU, or gradual rec^ombination with 

 oxygen) of the plants or of portions such as leaves falling 

 from plants, the rate of evolution of oxygen, reckoned as 

 three times the weight of the wood or the dry hay produced, 

 would be only about 6 tons per English acre per annum orl-^ 

 tons per square metre per thousand years. At this rate it 

 would take only 1533 years, and therefore in reality a much 

 longer time Avould almost certainly be required, to produce 

 the 2-3 tons of oxygen Avhich we have at jiresent resting 

 on every square metre of the earth's surface, land and sea.t 

 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 Ave say that the free oxygen of the 

 earth's atmosphere is augmented only by vegetation liber- 

 ating it from carbonic acid and v/ater, in virtue of the 

 power of sunlight, and is diminished only by virtual burn- 

 ing! 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 Avood, or oil, 

 or fuel of any kind originating in A^egetation, to so great an 

 amount as -767 of a ton per square metre of surface; Avhich 

 is the amount at the rate of one ton of fuel to three tons of 

 oxygen, that Avould be required to produce the 2*3 tons ot 



* Liebig, "Chemistry in its application to Agriculture and Physiology," 

 English, 2nd ed., edited by Playfair, 1842. 



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

 thermometer we have, resting on each square meti-e of the earth's surface, 

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



\ This " virtual burning " includes eremacausis 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- 

 A-ision for heat and power. 



