barter into two acts,
in this way--

#Sale.# #Purchase.#
_____/\______ ____/\____
/ \ / \
#Cart.# #Money.# #Plough.#

No doubt it turns one act of exchange into two; but the two are far more
easy to manage than one, because they need not be made with the same
person.

#78. Money as a Measure of Value.# When money is used in exchange, he
who receives money is said #to sell goods#, and he who pays money is
said #to buy or to purchase#. In each case there is an act of exchange,
and sales and purchases are not really different in nature from acts of
barter, except that one of the commodities given or received is employed
for the purpose of arranging the exchange. Thus money may be called
#current commodity#, because it is merchandise chosen #to run# about as
a medium of exchange. Now, in every purchase or sale there must be some
proportion between the quantity of the money, and the quantity of the
other commodity. This proportion expresses the value of the one
commodity as compared with the other. Value in exchange means nothing
but this proportion, as was before explained (section 72). Now when
money is used, the quantity of money given or received for a certain
quantity of goods is called #the price of that goods#, so that the price
is the value of goods stated in money. But as money when once introduced
is used in almost every act of exchange, a further great advantage
arises. We are able to compare the value of any commodity with that of
any other commodity. If we know how much copper may be had for so much
lead; how much iron for so much steel; and so on with zinc and brass,
bricks and timber, and so forth, it would not be possible to compare the
value of copper with zinc, or iron with timber. But if we know that for
one ounce of gold we can get 950 ounces of tin, 1,700 ounces of copper,
6,400 ounces of lead, and 16,000 ounces of wrought iron, then we learn
without any trouble that for 1,700 ounces of copper we can get 16,000
ounces of iron, and so on. Thus gold or any other substance used as
money serves as a #common measure of value#; it measures the value of
every other commodity, and thus enables us to compare the value of each
commodity with that of every other.

This is an immense convenience. It leads every one to think and speak of
the values of things in terms of a money known to everybody. All lists
of values of goods are given as lists of pri