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POPULAE SCIENCE REVIEW. 
even injurious forms those substances, such as ferric oxide, 
which are in a suitable condition for assimilation. The in- 
jurious effects of decaying and putrescent organic matter in 
excessive abundance are more especially injurious when the 
soil is undrained and badly cultivated ; for then the oxygen of 
the air and that dissolved in water reach these decaying sub- 
stances in small quantities only and beneficially affect them but 
slowly. This point, concerning the true source of the carbon 
in plants being carbonic acid, and carbonic acid only, is further 
elucidated by two well known phenomena. The rich organic 
mud of ponds, saturated as it is with manurial matters, is yet 
found hurtful till it has been sweetened, that is to say oxidised, 
by exposure to the air. Then look at the effects of applying 
lime to such a soil as that to which we have referred. It neutral- 
ises the acids of the peat or humus, but it does more ; it favours 
the oxidation of these bodies, and thus restores in a rapid and 
marked manner fertility to the soil. All that we have said on 
this subject of carbon (and we might have added many more 
experimental proofs of our assertion) is to illustrate these two 
points : that carbonic acid is the only form in which plants can 
take up carbon, and that carbonaceous matters during their 
decay are sources of carbon only as their carbon gradually 
oxidises into carbonic acid. These carbonaceous matters have 
indeed quite another function to fulfil in the soil, but with this 
we are not now concerned. And carbonic acid itself acts also 
in many other ways than as a plant food ; for its chemical and 
physical actions upon the soil are of the very highest importance. 
We cannot here find space to do more than enumerate the 
sources of carbonic acid : such as the respiration of animals, 
the combustion of coal and other fuel, and the products of the 
volcanic and other chemical actions going on within the crust of 
the earth and upon its surface. The carbonic acid from all 
these sources does not accumulate in the atmosphere beyond the 
proportion of 4 parts in 10,000 of air, for it is continually with- 
drawn by the action of vegetation. But in the interstitial air of 
the soil itself a much higher proportion of carbonic acid is reached; 
often more than 1 in 100 ; while oxygen is proportionately defi- 
cient. So also the water penetratiug through a soil, as well as 
the atmospheric moisture falling upon it, is rich in this gas. 
We need not linger in speaking of the sources of hydrogen. 
In rain, dew, and mist, and in the water always present, dis- 
solved in the atmosphere, we have the chief if not the only 
source of this element. Not only is water the form in which 
the element hydrogen must be presented to the plant, but water 
itself and the elements of water, in the proportion in which they 
combine to form water, enter into the constitution of most plant 
products. Starch, for example, not only contains hydrogen and 
