Dr Seller on the Nutrition of Plants. 57 



But, it will be asked, does the annual expenditure of the 

 carbon of organic compounds rise at present so high as 600 

 millions of tons ? 



If the daily waste of cai'bon by the human race, in the act 

 of respiration alone, be estimated at no more than five ounces 

 for each individual, the annual consumption will amount to 

 one-twelfth part of the above quantity, or to 50 millions of 

 tons. 



Domesticated animals, horses, and other beasts of burden, 

 dogs, oxen, sheep, hogs, and poultry, as coming to an early 

 maturity, consume at least twice this quantity, or 100 mil- 

 lions of tons. 



The rest of the animal creation cannot dispense with less 

 than the sum of these two quantities, or 150 millions of tons. 

 And the other numerous sources of waste above mentioned, 

 including combustion, cannot be over-rated at as much again, 

 or 300 millions of tons — or, in all, 600 millions of tons. All 

 the above estimates are designedly under-rated.* 



No doubt the final extinction of the two living kingdoms 

 of nature would be somewhat protracted beyond the period 

 assigned under either of the above suppositions, because a 

 gradual diminution in the sources of expenditure could not 

 but at last commence, while under the diminution of plants 

 and animals successive additions would be made to the soil. 

 But it seems pi'obable that an exact calculation would shew 

 that all the carbon of living plants and living animals would 

 not supply the present rate of expenditure much above 300 

 years ; while, on the other hand, there is good reason to be- 

 lieve that the rate of expenditure is still rapidly increasing, 

 and that it must hereafter greatly exceed its present limits 

 before any decline is possible. 



The exhaustion of the fertility of land by successive grain 

 crops gives some countenance to the belief of organic matter 

 in the soil being the chief food of plants ; but this is sus- 

 ceptible of a different explanation, while other facts lead to 

 the contrary inference ; for instance, the manifest increase 

 in quantity of organic matter in the soil of a field after some 



* See Appendix. 



