10. Thus the hypotheses which we accept ought to explain phenomena which we have observed. But they {86} ought to do more than this: our hypotheses ought to _foretel_ phenomena which have not yet been observed; at least all phenomena of the same kind as those which the hypothesis was invented to explain. For our assent to the hypothesis implies that it is held to be true of all particular instances. That these cases belong to past or to future times, that they have or have not already occurred, makes no difference in the applicability of the rule to them. Because the rule prevails, it includes all cases; and will determine them all, if we can only calculate its real consequences. Hence it will predict the results of new combinations, as well as explain the appearances which have occurred in old ones. And that it does this with certainty and correctness, is one mode in which the hypothesis is to be verified as right and useful. The scientific doctrines which have at various periods been established have been verified in this manner. For example, the _Epicyclical Theory_ of the heavens was confirmed by its _predicting_ truly eclipses of the sun and moon, configurations of the planets, and other celestial phenomena; and by its leading to the construction of Tables by which the places of the heavenly bodies were given at every moment of time. The truth and accuracy of these predictions were a proof that the hypothesis was valuable, and, at least to a great extent, true; although, as was afterwards found, it involved a false representation of the structure of the heavens. In like manner, the discovery of the _Laws of Refraction_ enabled mathematicians to _predict_, by calculation, what would be the effect of any new form or combination of transparent lenses. Newton's hypothesis of _Fits of Easy Transmission and Easy Reflection_ in the particles of light, although not confirmed by other kinds of facts, involved a true statement of the law of the phenomena which it was framed to include, and served to _predict_ the forms and colours of thin plates for a wide range of given cases. The hypothesis that Light operates by _Undulations_ and _Interferences_, afforded the means of _predicting_ results under a still larger extent of conditions. In like manner in the {87} progress of chemical knowledge, the doctrine of _Phlogiston_ supplied the means of _foreseeing_ the consequence of many combinations of elements, even before they were tried; but the _Oxygen Theory_, besides affording predictions, at least equally exact, with regard to the general results of chemical operations, included all the facts concerning the relations of weight of the elements and their compounds, and enabled chemists to _foresee_ such facts in untried cases. And the Theory of _Electromagnetic Forces_, as soon as it was rightly understood, enabled those who had mastered it to _predict_ motions such as had not been before observed, which were accordingly found to take place.