Secrets in manufactures are capable of being longer kept than secrets in trade. A dyer who has found the means of producing a particular colour with materials which cost only half the price of those commonly made use of, may, with good management, enjoy the advantage of his discovery as long as he lives, and even leave it as a legacy to his posterity. His extraordinary gains arise from the high price which is paid for his private labour. They properly consist in the high wages of that labour. But as they are repeated upon every part of his stock, and as their whole amount bears, upon that account, a regular proportion to it, they are commonly considered as extraordinary profits of stock. Such enhancements of the market price are evidently the effects of particular accidents, of which, however, the operation may sometimes last for many years together. Some natural productions require such a singularity of soil and situation, that all the land in a great country, which is fit for producing them, may not be sufficient to supply the effectual demand. The whole quantity brought to market, therefore, may be disposed of to those who are willing to give more than what is sufficient to pay the rent of the land which produced them, together with the wages of the labour and the profits of the stock which were employed in preparing and bringing them to market, according to their natural rates. Such commodities may continue for whole centuries together to be sold at this high price; and that part of it which resolves itself into the rent of land, is in this case the part which is generally paid above its natural rate. The rent of the land which affords such singular and esteemed productions, like the rent of some vineyards in France of a peculiarly happy soil and situation, bears no regular proportion to the rent of other equally fertile and equally well cultivated land in its neighbourhood. The wages of the labour, and the profits of the stock employed in bringing such commodities to market, on the contrary, are seldom out of their natural proportion to those of the other employments of labour and stock in their neighbourhood. Such enhancements of the market price are evidently the effect of natural causes, which may hinder the effectual demand from ever being fully supplied, and which may continue, therefore, to operate for ever. A monopoly granted either to an individual or to a trading company, has the same effect as a secret in trade or manufactures. The monopolists, by keeping the market constantly understocked by never fully supplying the effectual demand, sell their commodities much above the natural price, and raise their emoluments, whether they consist in wages or profit, greatly above their natural rate.