The alternation of crops seems destined to effect a great change in agriculture. A French chemist informs us that the roots of plants imbibe matter of every kind from the soil, and thus necessarily abstract a number of substances, which are not adapted to the purposes of nutrition, and that they are ultimately expelled by the excretory vessels, and return to the soil as excrement. The excrementitious portion of the food also returns to the soil. Now, as excrement cannot be assimilated by the same animal or plant that ejected it, without danger to the organs of digestion or eliminations, it follows that the more vegetable excrement the soil contains, the more unfitted must it be for plants of the same species; yet these excrementitious matters may, however, still be capable of assimilation by another kind of plant, which would absorb them from the soil, and render it again fertile for the first. In connection with this, it has been observed that several plants will flourish when growing beside each other; but it is not good policy to sow two kinds of seed together: on the other hand, some plants mutually prevent each other's development. The same happens if young cattle are suffered to graze and sleep in the barn together; the one lives at the expense of the other, which soon shows evidences of disease. The injurious effects of permitting young children to sleep with aged relatives are known to many of our readers; yet some parents see their children sicken and die without knowing the why or wherefore. From such facts as these,--which we might multiply to an indefinite extent, were it necessary,--we learn that nature's laws are immutable and uncompromising; and woe be to the man that transgresses them: they are a part of the divine law, which cannot be set at nought with impunity. Ignorance on these important subjects has existed too long: yet we perceive in the distant horizon a ray of intellectual light, streaming through our schools and agricultural societies. The result will be, that succeeding generations will be better acquainted with nature's laws, from which shall flow untold blessings. Chemistry teaches us that animals and vegetables are composed of a vast number of different compounds, which are nearly all produced by the same elementary principles. Vegetables consist of carbon, hydrogen, and oxygen; and the same substances, with the addition of nitrogen, are the principal constituents of the animal economy. In a word, all the constituents of animal creation have actually been discovered in vegetables: this has, we presume, led to the conclusion that "all flesh is grass."