34 OF THE ASSIMILATION OF CARBON. 



hitherto rendered it impossible for the true theory 

 of the nutritive process in vegetables to become 

 known, and has thus deprived us of our best guide 

 to a rational practice in agriculture. Any great im- 

 provement in that most important of all arts is in- 

 conceivable, without a deeper and more perfect ac- 

 quaintance with the substances which nourish plants, 

 and with the sources whence they are derived ; and 

 no other cause can be discovered to account for the 

 fluctuating and uncertain state of our knowledge on 

 this subject up to the present time, than that modern 

 physiology has not kept pace with the rapid progress 

 of chemistry. 



In the following inquiry, we shall suppose the hu- 

 mus of vegetable physiologists to be really endowed 

 with the properties recognised by chemists in the 

 brownish black deposits, which they obtain by pre- 

 cipitating an alkaline decoction of mould or peat by 

 means of acids, and which they name humic acid,^ 



Humic acid, when first precipitated, is a flocculent 

 substance, is soluble in 2500 times its weight of wa- 

 ter, and combines with alkalies, lime and magnesia, 

 forming compounds of the same degree of solubility. 

 (Sprengel.) 



Vegetable physiologists agree in the supposition 

 that by the aid of water humus is rendered capable . 



* The extract obtained by Berzelius from black brownish soils has 

 been designated as humic extract, in some cases with a substance called 

 glairin. The glairin is described by Thomson as a peculiar substance 

 which has been observed in certain sulphureous mineral waters, and 

 was first noticed by Vauquelin {Jinn, de Chim. XXXIX. 173), who de- 

 scribed several of its properties and considered it analogous to gelatin. 

 An account of it was drawn up by M. Anglada, of Montpellier, and 

 communicated to the Royal Academy of Medicine of Paris, in 1827. It 

 gelatinizes with water when sufficiently concentrated. Sometimes it is 

 white, and at others of a red color; when dried it shrinks to ^gth of its 

 bulk when moist. It saturates ammonia, and decomposes several me- 

 tallic salts. It is destitute of smell and taste. It does not glue sub- 

 stances together like gelatin and albumen. It yields animonia by de- 

 composition, and is capable of putrefaction like animal bodies. The 

 general opinion is, that it is of vegetable origin, and allied to the genus 

 tremella, though its existence in mineral waters has not been account- 

 ed for. Thomson's Organic Chemistry, 694. I found it very abun- 

 dant about the hot sulphureous waters of the island of St. Michael, 

 Azores. — IV. 



