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ROYAL HORTICULTURAL SOCIETY. 



the humic particles dissolved in the water, the humic extracts, and 

 humates, may be traced through every step, which is quite im- 

 possible according to the carbonic-acid theory. 



The leading fact is, that the materials of the sap are imbibed 

 from the soil with the water, but that carbonic acid and carbonate 

 of ammonia are not found in the water, and that these substances 

 are not found in the raw sap. Nothing is more natural than 

 that we must find in the imbibed raw sap those materials which, 

 after the Ingenhousz-Saussure theory, are the chief nutriment of 

 plants. Let them come whence they will, whether from the air 

 or from the ground ; they must be present in tbe raw sap, if they 

 are really the nutritive matter of plants. But since they are 

 not found there at all, they cannot belong to this category. It 

 is, moreover, quite impossible to explain the origin of the matters 

 which really exist in raw (sap as gum, sugar, tartaric acid, gallic 

 acid, malic acid) from carbonic acid and carbonate of ammonia, 

 which, however, must be the case if that theory is true. On the 

 contrary, the origin of all the component parts of the sap from 

 the constituents of the water which we have enumerated, is easily 

 and fully explicable, since the altered constituents of humus really 

 exist in the sap of plants. 



The near relation of humous extract to grape-sugar was 

 pointed out by Saussure, though possibly without knowing that 

 this was a constituent of sap, and in ignorance of the presence 

 of dextrine and its relation to humous extract. These relations, 

 in the perplexity of the views which have arisen from the carbonic- 

 acid theory, are never mentioned. Sprengel, who was the first, 

 after Saussure, to examine humus completely and to exhibit its 

 different salts, set out from the notion that humic acid alone, in 

 the form of humate of lime, was the prime nutriment. Liebig 

 contended against this view ; and it is easy to prove that neutral 

 humate of liine is not the only or the chief nutriment of plants, 

 since, independently of their difficulty of solution, neutral salts 

 cannot be directly assimilated, as I have proved. 



Sprengel, as well as his opponents, had overlooked the im- 

 portance of humous extract and perhumates. Liebig was exas- 

 perated against the notion of the reception of humous extract by 

 plants, since this is brown, while the juice of plants is mostly 

 colourless. It was overlooked, however, that very weak solutions 

 of humous extract, as those in river-water or surface-water, are 

 often quite colourless, though when concentrated by inspissation 

 they become brown. The clearest waters of mountain-streams 



