The supreme and all-pervading law of nature, the true and only cosmological law, is, in my opinion, the law of substance; its discovery and establishment is the greatest intellectual triumph of the nineteenth century, in the sense that all other known laws of nature are subordinate to it. Under the name of “law of substance” we embrace two supreme laws of different origin and age—the older is the chemical law of the “conservation of matter,” and the younger is the physical law of the “conservation of energy.” It will be self-evident to many readers, and it is acknowledged by most of the scientific men of the day, that these two great laws are essentially inseparable. This fundamental thesis, however, is still much contested in some quarters, and we must proceed to furnish the proof of it. But we must first devote a few words to each of the two laws.