The highest test of truth is verified prediction; if we calculate beforehand that an eclipse of the Sun will occur at a certain hour and minute, and that eclipse occurs accurately at the predicted moment, we may rest assured that our knowledge upon that point is true and complete. If we say that a piece of clean iron, immersed in a solution of blue vitriol, will become covered with a layer of metallic copper, and we find upon trial that this result invariably occurs when we fulfil those conditions, we may be certain that our knowledge of this phenomenon and its conditions is also of a definite and certain character. Similarly, when we become able to predict with certainty the conditions of the Sun's surface, we shall probably also be able to predict severe winters, famines, &c., and therefore be prepared to suggest precautions to be taken against them. Even now the new truth necessary for this purpose is beginning to be evolved by means of scientific research.[20] Amongst the great axioms and principles of science, possessing great certainty, and which enable us to predict, are, _1st._ the general truth known as the Principle of Causation, that every effect has a cause; that the same cause, acting under the same conditions, always produces the same effect; and that causation acts through all time and all space:--_2nd._ the great truth, that every phenomenon requires time; and every substance occupies space:--_3rd._ the Principles of Conservation and Persistency of Matter {159} and Energy; that out of nothing, nothing comes; and out of everything, everything proceeds; that all the future states of the Universe are implicitly contained in and will be evolved out of the present state of the Universe; that we have no experience and possess no verified knowledge either of creation or annihilation of Matter or Energy; that we cannot absolutely create or destroy even an idea;[21] and that Matter and Energy appear to be eternal:--_4th._ the Principle of Convertibility and Equivalency of the different forms of Energy, according to which the various forces known as mechanical power, heat, light, electricity, magnetism, chemical action, &c., being modes of motion, are convertible into each other in equivalent quantities and without addition or loss. These and other great principles constitute the basis of physical and chemical science, by obeying which, we have been enabled to evolve all the wonderful practical realities of science of the present day.[22] To these great principles may be added the more concrete truth called the "Law of Progress," the essential idea of which is time, a time-rate; which regulates the speed of increase of civilization, and is evidently connected with the great truth that every phenomenon occupies time.