Chemical Physiology and Pathology. 527 



tables is carbon ; the great mass of vegetable matter is formed 

 of compounds, which contain carbon and the elements of 

 water : the latter either in the same condition in which they 

 exist in water, (woody fibre, starch, sugar and gum :) or in that 

 of carbon with the elements of water and a certain quantity 

 of oxygen, (most vegetable acids :) or in that of carbon and 

 hydrogen with little or no oxygen (volatile and fatty oils, 

 wax and resins.) The vegetable acids are constituents of 

 all the fluids of plants, and are generally united to in- 

 organic bases, metallic oxides : these last are never wanting, 

 and after incineration are found in the ashes. Nitrogen 

 which is not entirely absent in any plant, (being present 

 in at least some of the fluids,) but which forms the small- 

 est portion of its constituents, is especially the constituent 

 of vegetable albumen, gum, &c. It also at times occurs in 

 the form of acids, of indifferent substances, and of peculiar 

 combinations possessing the properties of metallic oxides 

 (organic bases.) The development of a plant is therefore 

 dependent on the presence of a combination of carbon, 

 which presents to it carbon in an assimilable state, and one 

 of nitrogen, which supplies nitrogen in a like condition : it 

 further requires water and its elements, and also a soil 

 which supplies inorganic materials. The assimilation of 

 carbon takes place by a decomposition of the carbonic acid 

 of the air, of the decaying humus, or of water, by means of 

 which carbon enters into the constitution of the plant, and 

 oxygen is exhaled. This process, which probably goes on 

 in all parts of the vegetable, takes place only under the 

 influence of light, especially the light of the sun, and is inde- 

 pendent of a high degree of temperature. Hydrogen is taken 

 up, when the water is decomposed, by its oxygen being set free. 

 Nitrogen, without which no plant reaches its full develop- 

 ment, much less flowers and fructifies, is supplied to it only 

 in the form of ammonia, (in rain water and in manures.) The 

 inorganic substances, of which it used to be believed that the 



