Which was first, Matter or Force? If we think on this question, we shall find that we are unable to conceive of matter without force, or of force without matter. When God created the elements of which the earth is composed, He created certain wondrous forces, which are set free, and become evident when matter acts on matter. All these forces, with many differences, have much in common, and if one is set free, it will immediately endeavour to free its companions. Thus, heat will enable us to eliminate light, electricity, magnetism, and chemical action; chemical action will educe light, electricity, and heat. In this way we find that all the forces in nature tend to form mutually dependent systems; and as the motion of one star affects another, so force in action liberates and renders evident forces previously tranquil. The present publication presents one or two points of interest. In the first place, the Lectures were especially intended for young persons, and are therefore as free as possible from technicalities; and in the second place, they are printed as they were spoken, verbatim et literatim. A careful and skilful reporter took them down; and the manuscript, as deciphered from his notes, was subsequently most carefully corrected by the Editor as regards any scientific points which were not clear to the short-hand writer; hence all that is different arises solely from the impossibility, alas! of conveying the manner as well as the matter of the Lecturer. May the readers of these Lectures derive one-tenth of the pleasure and instruction from their perusal which they gave to those who had the happiness of hearing them! CONTENTS: LECTURE I. THE FORCE OF GRAVITATION. 7 LECTURE II. GRAVITATION-COHESION. 27 LECTURE III. COHESION-CHEMICAL AFFINITY. 45 LECTURE IV. CHEMICAL AFFINITY-HEAT. 62 LECTURE V. MAGNETISM-ELECTRICITY. 77 LECTURE VI. THE CORRELATION OF THE PHYSICAL FORCES. 95 LECTURE ON LIGHT-HOUSE ILLUMINATION-THE ELECTRIC LIGHT