In these processes it would seem that phosphoric acid plays a most important, and, as it would seem, an essential part. To this I shall return: at present I merely consider man’s influence on the soil or earth he lives on, what he derives from it, and what he returns to it, and in what form it is and ought to be returned. If it be true that without trees there would be no underwood, no corn, and no crops,--for trees attract the fertilizing rain, and cause the springs perpetually to flow which diffuse prosperity and comfort,--then assuredly man ought to be most careful in interfering with nature. It is the remark, I think of the illustrious Humboldt, that when the white man took possession of certain districts of North America, vast forests prevailed everywhere. On taking possession, experience showed that agues prevailed, and that wheat might be grown successfully. The forests have been now destroyed, and agues have disappeared; but phthisis pulmonalis prevails, and wheat no longer grows to maturity. We interfere with the soil as nature made it when we force it to produce from one acre the natural produce of ten; we interfere with the processes of nature when we load the air with the products of thousands of furnaces, manufactories, and the poison exhaled from poisonous rivers and brooks; and we interfere with nature when we alter the constitution of those streams and rivers from a natural to an artificial state, loading them with the refuse of our artificially-drained fields, &c. Let us listen to Liebig on a matter to which he has given the utmost possible attention:--