LAW VII.  When two or more things are jointly demanded, in proportions
   which cannot easily be varied, the tendency will be for an increase
   (or decrease) in the supply of one of them to increase (or
   decrease) the demand for the others.  These results will be more
   certain, and more marked, the more difficult it is to vary the
   proportions in which the things are used.

Similarly, when two or more things are jointly supplied, in proportions which cannot easily be varied, the tendency will be for an increase (or decrease) in the demand for one of them to increase (or decrease) the supply of the others.  These results again will be more certain and more marked, the more difficult it is to vary the proportions in which the things are supplied.

Sec.7. Composite Supply and Composite Demand.  Joint Demand and Joint Supply do not complete the list of relations between the demand and supply of different things.  Between tea and coffee, or beef and mutton there is a relation of a different kind.  These things are in large measure what we call “substitutes” for one another.  An increased supply, and a lower price of mutton, will probably induce us to consume less beef.  This relation it is convenient to describe as Composite Supply.  Beef and mutton make up a composite supply of meat; tea and coffee a composite supply of a certain type of beverage.  For any group of things, between which the relation of Composite Supply exists, we can say, with complete generality, that an increased supply of one of them will tend to diminish the demand for the others.  Parallel to the relation of Composite Supply is that of Composite Demand.  There are frequently several alternative uses in which a commodity or service can be employed; and these alternative uses make up a composite demand for the thing in question.  Thus railways, gasworks, private households and a great variety of industries contribute to a Composite Demand for coal.  It is worth noting that there is frequently an association in practice between Joint Demand and Composite Supply on the one hand; and between Joint Supply and Composite Demand on the other.  Wool and mutton, for instance, we have described as an instance of Joint Supply; but, in so far as the proportions of wool and mutton can be varied, we can regard these things as constituting a Composite Demand for sheep.  And this conception may help us to retain a clearer and more orderly picture of the problems we have discussed above.  We can regard the fact that wool and mutton are produced together as their Joint Supply aspect, and the fact that these proportions can be varied as their Composite Demand aspect; and the question as to whether an increased demand for mutton will increase the supply of wool turns upon whether the former aspect is more important than the latter.  Similarly labor and machinery, employed together for the same purpose, form an instance of Joint Demand; but in so far as they can be substituted for one another, they constitute a Composite Supply of alternative agents of production.