stance, or system, in which the force at work is not gravity, but chemical affinity, and the substance, or system, may, under certain peculiar conditions, become chemically unstable.
When a substance is chemically unstable, it means that the slightest impulse of any kind may determine a chemical change, just as in the case of the egg the slightest impulse from without occcasioned a mechanical displacement.
In fine, a substance, or system, chemically unstable bears a relation to chemical affinity somewhat similar to that which a mechanically unstable system bears to gravity. Gunpowder is a familiar instance of a chemically unstable substance. Here the slightest spark may prove the precursor of a sudden chemical change, accompanied by the instantaneous and violent generation of a vast volume of heated gas. The various explosive compounds, such as gun-cotton, nitro-glycerine, the fulminates, and many more, are all instances of structures which are chemically unstable.
Machines are of two kinds.
216. When we speak of a structure, or a machine, or a system, we simply mean a number of individual particles associated together in producing some definite result. Thus, the solar system, a timepiece, a rifle, are examples of inanimate machines; while an animal, a human being, an army, are examples of animated struc-
