When the potential energy residing in so much food has been converted into the energy of so much human labour, and when this is so directed that a productive machine results from it, there is in the machine, as Dr. Crozier puts it, an indefinitely larger amount of "pure economic power," than that which has been expended in the work of the labourers' muscles. While the energy of the labourers has merely resulted in a bottle, or a cage, we may say, of sufficient strength, the genius of the man who directed them has captured and imprisoned an elemental slave in it, who, so long as the cage confines him, will supplement the efforts of human muscle with his own. But when the energy latent in food is converted into such efforts as dancing, the result produced is the equivalent of the human effort only. Thus in the modern world of scientific enterprise and invention, to invest capital in machinery and then live on the interest from it, means to press into the service of mankind an indefinite number of non-human auxiliaries, and year by year to live on a part of the products which these deathless captives are never tired of producing. To spend capital as income on securing immediate pleasures means either to forgo the chance of adding any new auxiliaries to those that we possess already, or else to let those who are at our service already, one after one, escape us--or, in other words, to make the productive force now at the disposal of any prosperous modern country decline towards that zero of efficiency from which industrial progress starts, and which marks off helpless savagery from the first beginnings of civilisation. It is no doubt inconceivable, in the case of any modern nation, that a climax of the kind just indicated could never reach its completion. If all the capitalists, for example, of Great Britain or America, were suddenly determined to live on their capital itself, they could do so only by continuing for a considerable time to employ a great deal of it precisely as it is employed at present. Indeed, so long as they continued to demand the luxuries which machines produce, it might seem that it was hardly possible for them to get rid of their capital at all. But what would really happen may be briefly explained thus:--