The fact that the literal correctness of a given hypothesis is not demonstrable except by results, in no wise militates against its value in the domain to which it belongs. Indeed, it would cease to be a hypothesis the moment it were demonstrated. Newton's theorem is undemonstrable except from its results. Its correspondence, however, with every known fact, the facility with which astronomical calculations can be made, and the precision with which every result can be predicted, constitute a sufficient demonstration of its substantial correctness to inspire the absolute confidence of the scientific world. No one would hesitate to act in the most important concerns of life--nay, to stake his very existence--upon calculations based upon Newton's hypothesis. Yet there are not found wanting men who deny or doubt its abstract correctness. Volumes have been written to disprove it. But as no one has yet discovered a fact or witnessed a phenomenon outside of its domain, the world refuses to surrender its convictions. When such a fact is discovered, then, and not till then, will there arise a necessity for revising the "Principia." It is a trite and true saying that one antagonistic fact will destroy the value of the finest theory ever evolved. It is equally impossible to demonstrate the abstract correctness of the atomic theory. An appeal to the evidence found in uniform results is all that is possible to one who would give a reason for the faith that is in him. No one ever saw, felt, tasted, or smelled an atom. It is beyond the reach of the senses; nor is it at all probable that science or skill will ever be able to furnish instrumental aids capable of enabling man to take cognizance of the ultimate unit of matter. It exists for man only in hypothesis. Nevertheless, the fact remains, that in all the wide range of human investigation there is not a more magnificent generalization, nor one more useful to mankind in its practical results, than the atomic theory. Yet there are those who doubt its abstract correctness, and labor to disprove the existence of the atom. If the ultimate object of chemical science were to demonstrate the existence of the atom, or to seize it and harness it to the uses of mankind, it might be worth while to set the chemical fraternity right by demonstrating its non-existence. If the practice of chemistry on the basis of the theory were defective in its practical results, or failed in universal application, it would then be the duty of scientists to discard it entirely, and to seek a better working hypothesis.