The explanation of this curious economic phenomenon is to be found in the assumption and positive assertion on the part of not a few distinguished economists that the truly scientific and only correct use of the term "rent" is its application to the "income derived from things of all kinds of which the supply is limited, and can not be increased by man's action."[17] As a rule, economists who accept this definition confine its application to the hire of land alone, although it professes to include other things, "of all kinds," to which the same description applies--namely, that they can not be increased in quantity by any human action. There are, however, no such other things specified, and in any literal sense there are no such other things existing, unless water and the atmosphere be intended. Now, although it is indisputably true that man by his action can not increase the absolute or total quantity of land, any more than of water and air, appertaining to the whole globe on which we live, there is practically no limitation to the degree of value which man's action can impart to land, and which is the only thing for which land is wanted, bought, or sold, and which, as already shown, can be truly made the subject of taxation. The tracts of land on the earth's surface which are of no present marketable value are its deserts, its wildernesses, the sides and summits of its mountains, and its continually frozen zones, where no results of labor are embodied in or reflected upon it; while, on the other hand, its tracts of greatest value are in the large cities and marts of trade and commerce, as in the vicinity of the Bank of England, or in Wall Street, where the results of labor are so concentrated and reflected upon land that it is necessary to cover it with gold in order to acquire by purchase a title to it and a right to its exclusive use. The difference between land at twenty-five dollars an acre and twenty-five dollars a square foot is simply that the latter is or may be in the near future covered or surrounded by capital and business, while the former is remote from these sources of value. One of the greatest possible, perhaps probable, outcomes of the modern progress of chemistry is that through the utilization of microbic organizations the value of land as an instrumentality for the production of food may be increased to an extent that at the present time is hardly possible of conception. Again, in the case of air and water, although their total absolute quantity can not be increased, their available and useful quantity in any place, as before shown, can be by the agency of man, and their use made subject to hire or rent.