FOOD OF PLANTS IN GENERAL. 471 



recent times this has been called Liebig's theory of the nutrition of 

 plants : but this is doubly wrong, for in the first place it is no mere 

 theory of the nutrition of plants ; and in the second place, it did not 

 originate with Liebig, but with Priestley, and has been gradually de- 

 veloped from his time by the most distinguished experimentalists. 

 Liebig has, indeed, in recent times dwelt upon the importance of its 

 universal recognition, and its relation to the development of a true 

 physiology of plants. He has also done great service by working out 

 the whole problem upon a new method, which was first introduced into 

 the natural sciences by Alexander von Humboldt.* This method con- 

 sists in disregarding at first individual and unarranged observations!, 

 and directing attention to the great mass of the phenomena of nature, 

 and where the deficiencies, on account of their great number, attain a 

 minimum of importance, to make calculations, which may be made the 

 basis of points of departure, alike in the estimate of the value of the 

 smallest as of the most isolated part. 



But on account of the great influence which leading maxims exert, com- 

 prehending, as they do, not only facts and groups of facts, but entire 

 circles of hypotheses, it is above all things necessary that they should be 

 placed on a secure footing, and be susceptible of the strongest possible 

 proof. To the most common examples belong the asserted constancy of 

 the composition of the atmosphere, which Liebig has frequently put for- 

 ward in the fore-ground. "Respiration and combustion consume an 

 immense mass of oxygen, yet the quantity of oxygen in the air re- 

 mains the same; consequently the vegetable world appropriates the 

 carbon of the generated carbonic acid, and again sets free the oxygen." J 

 If we need proof of this view, we have the following : A man in the 

 course of a year changes 225 Ibs. of carbon into carbonic acid, so that 

 a thousand millions of men would consume 2250 millions of centners ; 

 for all the animals || on the earth we may take double this quantity : thus, 

 in the whole, 6750 millions of centners of carbon are yearly burned, 

 which, during the process of burning, would consume 1800 millions of 

 centners of oxygen gas, to which may be added about 400 millions of 

 centners for the burning of coal. \. The remaining processes of combus- 

 tion would give 1500 millions of centners of carbon, which consume 4000 

 millions centners of oxygen. Hence we may take the consumption of 

 oxygen in the course of 300 years at 660 billions of pounds, or about 

 -^ths of the present contents of the atmosphere ; and this would fall within 

 the oscillations of our eudiometrical calculations, if we had observed 

 them as accurately 300 years ago as at present, f 



* The talent which forms an epoch in the history of the natural sciences consists not 

 in the discovery of individual facts or laws, but in the introduction of new ways, the 

 discovery of new methods. 



t To what absurdity and charlatanerie a dwelling upon individual facts, without a 

 comprehensive view, may lead, has been recently seen, in the most forcible manner, in 

 the work of C. H. Schultz, on the Discovery of the true Food of Plants. 



J According to Liebig, man consumes daily between 17 and 27 ounces of carbon. 



A centner is about 100 pounds. TRANS. 



|| Boussingault calculates the horse consumes 158 oz., and the cow 141 ioz. 

 daily. 



| According to Ure, 67 7| millions of centners of coal contain 71 per cent, of carbon, 

 which is equal to 481 millions of centners of carbon. 



H At my request, my colleague, Professor E. Schmid, had the goodness to calculate 



H H 4 



