Page:Popular Science Monthly Volume 45.djvu/64

52 will serve as an adequate test of progress. Any branch of knowledge becomes a science only when the relation between cause and effect is rigidly established, and the capricious and accidental are as rigidly eliminated. Comte found his test of science in the power of prediction. There is no science, unless under certain given conditions we can say precisely what will happen. But this, I take it, is only another way of saying the same thing: we can predict only when we have perceived the causal relations.

The most common affairs of life have not yet been reduced in practice to a science. Bread-making, for example, is still a black art. You put flour and water and yeast and salt and lard together, and do certain things to it, and then trust to the gods to make it into bread. Sometimes they do and sometimes they don't. Sometimes you have good bread and more often you don't. Yet I once met a man, an ex-college professor, who said that he always had good bread. His recipe was simple: he made the conditions invariable and the results were likewise invariable. We have all heard of the lady who, when her servant was out, put wood and paper and coal together, applied a match, and then went upstairs and prayed that she might have a fire.

Practically we do not disapprove of this condition of affairs. For the most part, it amuses us.

But the less domestic sciences afford better illustrations of the realization of the principle. In the hands of Kepler, for instance, astronomy failed to be a science. With wonderful skill he applied his knowledge of conic sections to the motions of the planets. Yet he could offer no better explanation of these motions than the suggestion that each planet was the chariot of an indwelling, guiding spirit. We could predict nothing of these imaginary charioteers, for the laws which might be presumed to govern them were quite beyond the limits of investigation. But with the introduction of the conception of universal gravitation, the study of astronomy took rank as a recognized science, and its observed phenomena were reducible to an orderly sequence of cause and effect. It is true that gravitation itself remains as profound a mystery as the charioteers of Kepler, and in substituting the one for the other we have not explained the universe. But we never hoped to do that. The superiority of gravitation lies in this, that it is the cause of uniform and measurable effects. Under Kepler's conception of things, the perturbations of Uranus might be ascribed to a little caprice on the part of the charioteer. Under Newton's conception such a disposition of the irregularities would be impossible. They could result only from the attraction of a definite amount of matter acting at a definite distance. When Adams and Leverrier had completed their calculations. Dr. Galle