11 



is absorbed by soils in preference to most bases, and if in the 

 compound both acid and base were necessary to vegetation, as 

 in phosphate of potash, we would find that the soil absorbed 

 them both in proportion to their degree of necessity. 



On the atmosphere, the absorptive power of the soil is also 

 freely exerted, taking therefrom air,' carbonic acid, ammonia, 

 water. 



These physical conditions of plant-life being present, let us 

 consider now the normal or natural system of plant growth. 



If we take a plant and burn it we find that the greater 

 portion is dissipated in the atmosphere, and there remains an 

 ush, which we find, by analysis, to contain some of the elements 

 which compose the surface of our earth. If we continue our 

 experiment, we will find that though the relative amounts of 

 these various ash-constituents vary in different plants, and even 

 in different organs of the same plant, yet the greater portion of 

 them are invariably present. If we now plant seed in a soil so 

 prepared that these ash constituents are absent, the plant will 

 not grow ; they are, therefore, necessary for the growth of the 

 plant and must exist in the soil in such a condition as to be 

 capable of assimilation. 



This is accomplished, as we have seen, by the atmospheric 

 (meteorological) phenomena which gradually decompose the 

 silicates and other insoluble salts existing in the soil, and sets 

 free these ash-constituents in a state capable of being dissolved 

 by water and absorbed by the plant. 



These ash constituents, therefore, form one important part of 

 the food necessary for plants and must be supplied by the soil ; 

 they have, therefore, been called " ash-food." 



The other portion of the plant which was dissipated by burn- 

 ing, we find by analysis to be composed of carbon, hydrogen, 

 oxygen, and nitrogen. These are equally important and essen- 

 tial, and, as the atmosphere is their source, we call them " air- 

 food." 



The carbonic acid of the atmosphere is under the influence of 

 light decomposed by the leaves of plants, the carbon being 

 assimilated, while the oxygen is returned for the use of animals. 

 Plants are in fact the world's scavengers, and render the earth 

 inhabitable by animals. Animals breathe in oxygen from the 

 atmosphere and exhale carbonic acid. This gas is poisonous and 

 if suffered to accumulate in the atmosphere, would soon cause 

 the death of all warm blooded animals ; but the plants seize it, 

 and, taking what is necessary for themselves, return the pure 

 oxygen for the use of the animals. 



If we turn to Geology we see this beautifully exemplified. 

 At one period the atmospnere,from igneous and other causes, was 

 reeking with moisture, and so filled with carbonic acid that 

 animal life, with the exception of some of the lower orders of 

 cold blooded reptiles, was impossible. During this period, called 



