The Wealth of Nations by Part 2 Chapter 1 Page 3

First, it may be employed in raising, manufacturing, or purchasing goods, and selling them again with a profit. The capital employed in this manner yields no revenue or profit to its employer, while it either remains in his possession, or continues in the same shape. The goods of the merchant yield him no revenue or profit till he sells them for money, and the money yields him as little till it is again exchanged for goods. His capital is continually going from him in one shape, and returning to him in another, and it is only by means of such circulation, or successive exchanges, that it can yield him any profit.

Such capitals, therefore, may very properly be called circulating capitals.

Secondly, it may be employed in the improvement of land, in the purchase of useful machines and