Slight variations in composition often produce marked differences. The introduction of the least ferment into sugar (C₆H₁₂O₆) would break it up into two deadly poisons, alcohol (2C₂H₆O) and carbonic acid gas (2CO₂—two molecules of each). A slight addition of oxygen spoils all the sweetness of the preserves. The rhubarb manufactures oxalic acid, the grape tartaric acid, the apple malic acid, the lemon citric acid, the oak tannic acid, from carbon, hydrogen and oxygen, by simply varying the number of their atoms. If we add one atom of oxygen (C₁₀H₁₆O) to the constituents of the volatile oils previously mentioned, we form a new group comprising camphor, wintergreen, spearmint, cinnamon, bitter almonds, and many others. Notwithstanding the great uniformity in the composition of various vegetable products, it is now well understood that one crop may restore to land what another has removed, hence the modern agricultural doctrine of ROTATION OF CROPS. In southeastern Virginia you find many pine forests in which may be traced the ridges of the corn rows. These fields were planted with corn continuously, until the soil became so impoverished that it would not yield a crop. They were then abandoned and allowed to grow up to pines. A better system would have secured perpetual fertility. The soil of England produces far more than formerly, even after the cultivation of a thousand years. China furnishes a still more remarkable example of productiveness. The amount of the earth’s crust which is concerned in the support of life is exceeding small. The natural tendency is constantly to diminish this. Rains and rivers bear away the best of the soil and deposit it in the lakes and seas. Some inhabitant of our earth in the far future, may secure the benefit of these stores, when the beds of the present seas and oceans shall have risen above the waters and become the continent. Too much pressed by the demands of the present to even think of this, the wise farmer endeavors to return to his soil what it has lost. Growing crops are plowed under, fertilizers from the thronging cities are spread upon his fields, the seaweed cast up by the waves yields its potash, phosphorus, salt and iodine. The islands of the Pacific contribute their vast stores of ammonia accumulated for ages in guano beds; marl deposited in the estuaries of ancient geologic seas feeds the cereals; and the limestone deposits are made to give verdure to the grasses of a thousand meadows. In the meantime, nature has her own processes of restoration. The crumbling of the rocks by frost, their abrasion by water, the accumulation of humus by decay, and various chemical influences conspire to convert the unproductive rocks into fertile soil. It would seem that this intelligent forethought, united with the beneficent processes of nature may secure perpetual productiveness, to the end that the earth may continue to yield its increase for the sustenance of the animal, for, as the Scriptures say,