U6 
with all commodities, a liability to appreciation in ex- 
changeable value. And it so happens that, because of the 
enormous stock of these metals in circulation as currency, 
their value in exchange can only be appreciated by a demand 
for them as currency beyond the limits of supply. An 
increased demand for them for consumption in the arts 
beyond the limits of the new production would always be 
supplied at the expense of the stock used as coin. We may 
therefore dismiss from con.sideration all the demand except 
for the purposes of currency, and say that only by the 
demand for them as currency outstripping the supply can 
the exchange value of the precious metals be appreciated. 
How then are we to prevent such appreciation ? 
In attempting to solve this problem the economists may 
again learn something from physical science. In constructing 
an unvarying measure of time the physicists have taken 
two metals of unequal contractility, and so arranged them 
in what is known as the gridiron pendulum, that a shorten- 
ing in one direction is always exactly balanced by a 
lengthening in another, so that the pendulum as a whole in 
any change of temperature remains the same length. In 
other words, they have adopted the principle of compensation, 
and compensation implies duality. 
As a general rule it may be stated that the probability of 
two commodities being appreciated simultaneously by the 
same arbitrary influences is much more remote than the 
probability of any one commodity being appreciated. Thus 
a poor wheat year is generally a good grass year. Hence if 
we take two commodities and say that a given quantity of 
either shall represent a given figure of account, we shall find 
that, as the debtor would always pay in the cheaper 
commodity, and the alternative increase of demand for one 
or the other for the special function of currency would tend 
to counteract the cheapening tendency, we must necessarily 
