But Exchange is so separate an action that students of Economics have agreed to treat it as a sort of chapter by itself, and we will do so here. The characteristic of Exchange is that you take a thing from a place where it has less value to a place where it has more value, thus adding an economic value to the thing moved and so creating wealth. In the same transaction you bring back something else against it, which has more value in your own place than it had in the place from which you took it, that is again adding an economic value and therefore creating wealth. We saw how this was in the case of the salt and the meat, and so it is with thousands upon thousands of exchanges going on all over the world. For instance, we in England have grown fond of drinking tea in the last 200 years. But our climate will not allow us to grow tea. Tea can only grow in a very hot country. Now in very hot countries specially heavy labour upon metal work is not to be expected. Men are not fit for it. But in this cool climate men are fit for it, and also men here have through long practice become very skilful at working metal: smelting iron, for instance, and making it up into machines. Therefore, there is a double advantage to us and to the people who live in the hot countries where tea is grown if we _exchange_. We send them metal things that we have made and which are useful to them, and which they could hardly make themselves, or only with very great difficulty (and, therefore, at a great expense of energy), and we get from them tea, which we could not grow here except in hot houses: that is, at much more expense of energy than is needed in the countries where tea grows naturally out of doors. When there are present two or more objects of this kind, such that the exchange of them between two places will benefit both parties, we may speak of “_a potential of exchange_,” stronger or weaker according to the amount of mutual advantage derived. This word “potential” you will not find yet in many books, but it is coming in, for it is a very useful word. It is taken by way of metaphor from Physical Science. When there is a head of water over a dam, or a current of electricity of such and such an intensity, we talk of the “_potential_” and measure it. For instance, we say this electrical current is double the potential of that, or the head of water working such and such turbines is at double the potential of another head of water in the neighbourhood. In the same way we talk of a “potential” of exchange, meaning a tendency for exchange to arise between two places or people because it is of mutual benefit to both. Potentials of exchange come into existence not only through difference of climate or differences of habit, but also through what is called the _Differentiation of Employment_, which is also called Division of Labour.