In just the same way energy can be measured as a quantity in terms of a fundamental unit. The discovery that this was so was made by Joule and others towards the middle of the nineteenth century, and lit the road for further advance as a dark street is lit by the sudden turning-up of the lamps. All modern industry rests on this principle. We are now so accustomed to the idea that energy is a quantity that we can hardly realize a time when it was merely a vague term. If we want an illustration of how thoroughly we have grasped this idea let us remember that when we pay our electric-light bill we pay so much money for so many units of energy supplied; for so much energy, let us note, not for so much electricity, since we take into account not only the actual amount of electricity driven through our house wires, but also the magnitude of the force which is there to drive it. Energy exists in many forms: energy of motion, heat, gravitational energy, chemical energy, radiation, and so on. In the transformations of energy which are continually occurring in all natural processes, there is never any change in the total amount of energy. This is the famous principle of the Conservation of Energy. Sometimes it is stated in the form 'Perpetual motion is impossible'. One of the most important forms of energy is radiation. The constant outpouring by the sun of energy in this form is vital to us. The fact was obvious long ago and that is one of the reasons why light and heat have interested students of science in all ages. There exist then three main subjects of study--matter, electricity, and energy. These themselves and their mutual relations have been, and are, the principal objects of interest to the scientific student, and from our strivings to understand them we have learnt most of what we know. All three are quantities and all are expressible in terms of units. Now there is one point which I have thought would especially interest you. A very remarkable tendency of modern discovery shows more and more clearly that not only are these things quantities which we can express in units of our own choosing, but that Nature herself has already chosen units for them. The natural unit does not, of course, bear any exact connexion with our own. This being so, it must be of the utmost importance that we should know what these natural units are and so be able to understand what Nature is ready to tell us. Nature has chosen to speak in a certain language; we must get to know that language.