8 OF THE ASSIMILATION OF CARBON. 



Any great improvement in that most important of 

 all arts is inconceivable without a deeper and more 

 perfect acquaintance 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 

 humus of vegetable physiologists to be really en- 

 dowed with the properties recognised by chemists 

 in the brownish black deposits which they obtain 

 by precipitating an alkaline decoction of mould 

 or peat by means of acids, and which they name 

 humic acid. 



Humic acid, when first precipitated, is a floccu- 

 lent substance, is soluble in 2500 times its weight 

 of water, and combines with alkalies, lime and mag- 

 nesia, 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 

 of being absorbed by the roots of plants. But ac- 

 cording to the observation of chemists, humic acid 

 is soluble only when newly precipitated, and be- 

 comes completely insoluble when dried in the ah*, 

 or when exposed in the moist state to the freezing 

 temperature. (Sprengel.) 



Both the cold of winter and the heat of summer 



