There is not merely a transformation of so much human energy into the same amount of natural energy; but nature adds to the former a non-human energy of her own; as--to take a good illustration of Dr.Crozier's--obviously happens in the case of a charge of gunpowder, which, "when used for purposes of blasting, has," he observes, "in itself a thousand times the quantity of pure economic power that is bought in the work of the labourers who supply and mix the ingredients." That is to say, whenever human talent invents and produces a machine which adds to the productivity of any one who uses it with sufficient intelligence, the inventor has shut up in his machine some part of the forces of nature, as though it were an efreet whom a magician has shut up in a bottle, and whose services he can keep for himself, or hand over to others.

The efreets shut up in machinery will not work for human beings at all, unless there are human magicians who manage thus to imprison them.

They therefore belong to the men who, in virtue of their special capacities, are alone capable of the effort requisite to perform this feat; and it matters nothing to others, by whom the efreets' services are borrowed, whether the effort in question occupied a year or a day, or whether it took place yesterday or fifty years ago.
The borrowed efreet produces the same surplus in either case, and interest is a part of this surplus which goes, not to the efreet himself (for this is not possible), but to his master, just as a cab-fare is paid to the cabman and not his horse.
Machine-capital, then--or capital in its typical modern form--consists of productive forces which are usable by, and which indeed exist for, the human race at large, because, and only because, they have been captured and imprisoned in implements by the efforts of exceptional men, whose energy thus exercised is perpetuated, and can be lent to others; and what these men receive as interest from those by whom their energy is borrowed, is a something ultimately due to the energy of the lenders themselves--nor is this fact in any way altered by lapse of time.

Thus, so far as these special men are concerned, the alleged difference between earned income and unearned altogether disappears; and if one man lives in luxury for sixty years on the interest of an invention which it took him but a month to perfect, while another man every day has to toil for his daily bread, the difference between the two consists not in the fact that the one man works for his bread and the other man does nothing for it, but in the fact that the work of one produces more in a day than that of the other would do in a hundred lifetimes.
Here, however, we shall be met with two important objections.

In the first place, it will no doubt have occurred to many readers that throughout the foregoing discussion we have assumed that the persons who receive interest on machinery are in all cases the persons by whom the machinery was invented and produced.