On the Theory of Manures. 13 



been brought forward as proof of the fact that plants get most of 

 their carbon from the air. These experiments, however, are 

 difficult to manage so as to prevent error ; and Schlieden asserts 

 that, in the average of recorded experiments, it has been found 

 that the enclosed air of the vessel has neither been altered in 

 its quantitive nor qualitive relations. Dumas expresses him- 

 self hesitatingly on the point, and, though he leans to the 

 supposition of the carbon being got principally from the air, 

 and undoubtedly, he thinks, from carbonic acid, yet notices 

 the great quantity of carbonic acid found by Boucherie to 

 issue from the trunks of divided trees when felled in full sap, 

 evidently derived, he says, from the roots. Professor Johnson 

 seems to lean to the supposition that the greater part is from 

 the air. Professor Sprengel and Dr. Madden seem to be of 

 opinion that the greater part of the carbon is got in the humic 

 acid absorbed with the ammonia : the latter allows that very 

 little of the carbon is got in the state of carbonic acid by the 

 roots, while many are of opinion that what is got by the roots 

 is principally in that form. I shall have an opportunity of 

 entering more at length into this subject in the article " Ve- 

 getable Physiology." While so much difference of opinion, 

 however, prevails among learned men on the subject, practical 

 men will do well to preserve and dejjosit in the soil the carbon, 

 as well as the other portions of the manure. Fortunately, 

 in the mixed manures generally applied, there are carbon, 

 nitrogen, and inorganic matter ; and in most soils, though not 

 all, as asserted by some, there is generally a reserve of humus to 

 assist when neglected. I doubt it would be found very dif- 

 ficult to grow plants luxuriantly in washed sand, with either 

 ammonia or saline matter, or both, in an ordinary atmosphere. 

 Such an experiment, however, might throw more light on the 

 subject than most of those tried. Solutions of nitrates, sulj)hates, 

 and phosphates of ammonia, potash, soda, lime, and magnesia, 

 with silicates of potash and soda, and a little common salt, 

 would furnish the nitrogen and inorganic substances wanted; 

 but, I fear, would not produce luxuriance of growth without 

 carbon. 



On the subject of inorganic manures, the experiments re- 

 corded this season are manifold, and, as might have been 

 expected, much at variance. Some applications, in certain 

 circumstances, appearing to have had little effect; others to 

 have done much good, and some to have done harm. The 

 tables furnished of the quantities of these substances found in 

 the different plants under cultivation, and the quantities 

 furnished by the different kinds of manure, will, when jDro- 

 perly regulated and corrected, show what are the wants of 

 plants as to constituents, and how far the food deposited is 



