The Wealth of Nations by Part 4 Chapter 1 Page 43

from the sale of the returns. But when they are sent abroad merely to pay a debt, he gets no returns, and consequently no profit. He naturally, therefore, exerts his invention to find out a way of paying his foreign debts rather by the exportation of commodities than by that of gold and silver.

The great quantity of British goods exported during the course of the late war, without bringing back any returns, is accordingly remarked by the author of The Present State of the Nation.

Besides the three sorts of gold and silver above mentioned, there is in all great commercial countries a good deal of bullion alternately imported and exported for the purposes of foreign trade. This bullion, as it circulates among different commercial countries in the same manner as the national coin circulates