In the general line of progress it is natural to find that there are certain broad roads along which the main advance has been directed. Students of physics and chemistry and the subjects which are allied to them find that they are in general considering either matter, or electricity, or energy. I make this classification, not from any philosophical point of view, but simply for present convenience. The first important principle to which I would like to draw your attention is that each of these things can be measured quantitatively. If we accept the weight of a substance as an indirect measure of the amount of matter present, then we all know we can express the amount of matter in any given body in terms of a fundamental unit, like a pound or a gramme; and the idea has been put to immemorial use. In later years we have learnt that electricity itself is also a quantity and that the amount of electricity which stands on an electrified body, or flows past a given point in an electric conductor, as for example the wire connected to an electric light, can be expressed arithmetically in terms of some unit. Instruments are made for the purpose of measuring quantities of electricity in terms of the legal standard. It is one of the functions of a Government Institution, like the National Physical Laboratory, to test such instruments and report on their accuracy. International conferences have been held for the purpose of reducing these units to as small a number as possible so that people may be able to trade less wastefully and more conveniently, so that also the barriers between peoples may be broken down and the interchange of ideas as well as of materials may be made more easily. Without an arrangement of this kind it would be impossible to carry on industrial life in which use is made of electricity. It would be as difficult as to hold a market without the use of weights and scales, more difficult, in fact, since anyone can estimate the size of a piece of cloth or the amount of corn in a sack, but no one has a natural sense by which he can estimate an amount of electricity.