An analogy offered by certain well-known chemical processes may help to make comprehensible this periodic nature of musical evolution. Chemists have a term, “critical point,” by which they name a stage in the behavior of a substance, under some systematic treatment, at which it suddenly undergoes some striking change, some catastrophic transformation. Put, for example, a lump of ice in a crucible and apply an even heat by which its temperature is raised, say, one degree each minute. Here is a systematic treatment of the ice, a steady influence exerted upon it. Yet, curiously enough, this ice which is being so equably acted upon will not change its form in the equable, regular fashion we might expect. It will seem to undergo little or no change until, at a given moment, suddenly, it passes into water, a liquid wholly different in appearance from the original solid. It has reached a “critical point.” Continue the heating, and presently another critical point will be reached, at which, with equal suddenness, the liquid will be transformed into a vapor--steam. These catastrophes, in which the physical properties of the substance suddenly change, are conditioned, of course, by its chemical nature. They take place in the midst of a systematic treatment which we might expect to produce only gradual, inconspicuous effects, but which, as a matter of fact, produces a series of events as strikingly differentiated one from another as the acts of a drama.
It is in a similar way that, in the history of music, the tonal material used, under the systematic treatment of man’s æsthetic faculty, has been constrained by its nature to undergo sudden changes, to recrystallize in novel ways, to take on unwonted aspects which initiate new periods. When the possibilities of one sort of tone-combination are nearly or quite exhausted, the keener minds of a generation, led by groping but unerring instinct, grasp an unused principle of organization, latent in the material, and inaugurate a new style. This in turn runs its course, develops its resources, reaches its perfection, and is succeeded by another, which, after due time, is also superseded. All these periods are but moments in one vast evolution, successive blossomings from the one root of human feeling expressible in music; yet each has its individual qualities, its peculiar style, its special masters. It is possible both to trace certain general tendencies through them all, and to define other special qualities in which each is peculiar; and it will be worth while, before passing on to our proposed study of the particular period of Beethoven, to describe thus in general terms the salient features of the evolution as a whole, and to characterize, however briefly, the individual periods we can discriminate in it.