§ 2. The Brownian Movement.
The Existence of the Brownian Movement.—The simplest way of judging of the working activity of matter is to observe it when the liberty of the particles is not interfered with by the action of the neighbouring particles. We approximate to this condition when we watch, through the microscope, grains of dust suspended in a liquid, or globules of oil suspended in water. Now what we see is well known to all microscopists. If the granulations are sufficiently small, they seem to be never at rest. They are animated by a kind of incessant tremor; we see the phenomena called the “Brownian movement.” This movement has struck all observers since the invention of the magnifying glass or simple microscope. But the English botanist, Brown, in 1827, made it the object of special research and gave it his name. The exact explanation of it remained for a long time obscure. It was given in 1894 by M. Gouy, the learned physicist of the Faculty of Lyons.
The observer who for the first time looks through the microscope at a drop of water from the river, from the sea, or from any ordinary source—that is to say, water not specially purified—is struck with surprise and admiration at the motion revealed to him. Infusoria, microscopic articulata, and various micro-organisms people the microscopic field, and animate it by their movements; but at the same time all sorts of particles are also agitated, particles which cannot be considered as living beings, and which are, in fact, nothing but organic detritus, mineral dust, and debris of every description. Very often the singular movements of these granulations, which simulate up to a certain point those of living beings, have perplexed the observer or led him to erroneous conclusions, and the bodies have been taken for animalcules or for bacteria.
Characters of this Movement.—But it is as a rule quite easy to avoid this confusion. The Brownian movement is a kind of oscillation, a stationary, dancing to-and-fro movement. It is a Saint Vitus’s dance on one and the same spot, and is thus distinguished from the movements of displacement customary with animate beings. Each particle has its own special dance. Each one acts on its own account, independently of its neighbour. There is, however, in the execution of these individual oscillations a kind of common and regular character which arises from the fact that their amplitudes differ little from each other. The largest particles are the slowest; when above four thousandths of a millimetre in diameter, they almost cease to be mobile. The smallest are the most active. When so small as to be barely visible in the microscope, the movement is extremely rapid, and can only occasionally be perceived. It is probable that it would be still more accelerated in smaller objects; but the latter will always escape our observation.
Its Independence of the Nature of the Bodies and of the Environment.—M. Gouy remarked that the movement depends neither on the nature nor on the form of the particles. Even the nature of the liquid has but little effect. Its degree of viscosity alone comes into play. The movements are, indeed, more lively in alcohol or ether, which are very mobile liquids; they are slow in sulphuric acid and in glycerine. In water, a grain one two-thousandth of a millimetre in diameter traverses, in a second, ten or twelve times its own length.
The fact that the Brownian movement is seen in liquors which have been boiled, in acids and in concentrated alkalies, in toxic solutions of all degrees of temperature, shows conclusively that the phenomenon has no vital significance; that it is in no way connected with vital activity so called.
Its Indefinite Duration.—The most remarkable character of this phenomenon is its permanence, its indefinite duration. The movement never ceases, the particles never attain repose and equilibrium. Granitic rocks contain quartz crystals which, at the moment of their formation, include within a closed cavity a drop of water containing a bubble of gas. These bubbles, contemporary with the Plutonian age of the globe, have never since their formation ceased to manifest the Brownian movement.
Its Independence of External Conditions.—What is the cause of this eternal oscillation? Is it a tremor of the earth? No! M. Gouy saw the Brownian movement far away from cities, where the mercurial mirror of a seismoscope showed no subterranean vibration. It does not increase when the vibrations occur and become quite appreciable. Neither is it changed by variation in light, magnetism, or electric influences; in a word, by any external occurrences. The result of observation is to place before us the paradox of a phenomenon which is kept up and indefinitely perpetuated in the interior of a body without known external cause.
The Brownian Movement must be the First Stage of Molecular Motion.—When we take in our hands a sheet of quartz containing a gaseous inclusion, we seem to be holding a perfectly inert object. When we have placed it upon the stage of the microscope, and have seen the agitation of the bubble, we are convinced that this seeming inertia is merely an illusion.
Repose exists only because of our limited vision. We see the objects as we see from afar a crowd of people. We perceive them only as a whole, without being able to discern the individuals or their movements. A visible object is, in the same way, a mass of particles. It is a molecular crowd. It gives us the impression of an indivisible mass, of a block in repose.