of pure water to pour slowly out of a funnel under an alcoholic solution of gamboge or mastic. An emulsion is formed where the layers meet which consists of a great number of minute spheres. When these particles are viewed in a strong light with a high power microscope, they all exhibit the characteristic Brownian movement, i. e., the particles dart to and fro in irregular and tumultuous fashion, and never appear to be at rest for more than a moment. The motions of these small particles under a microscope irresistibly convey the impression that they are hurled to and fro by the action of mysterious forces resident in the solution. Such a result is to be anticipated if the molecules of the liquid are themselves in rapid though invisible tumultuous motion of the kind outlined on the kinetic theory. The particle is very large compared with the molecule, and it is bombarded on all sides by great numbers of molecules. Occasionally the pressure due to the bombardment is for a moment greater on one side of the particle than on the other, and the particle is urged forward, until a new distribution of impacts hurls it in another direction. In fact, the movement of these particles has been found to conform exactly with that predicted by the molecular theory.
It would take too long to discuss the remarkable conclusions that Perrin has reached from a study of the distribution and motion of small particles. The particle which may be an agglomeration of many millions of molecules, behaves in many respects like the much smaller molecule. A great number of particles in a liquid do not distribute themselves uniformly under gravity, but the numbers decrease with height according to the same law as the gases in our atmosphere.
On the kinetic theory, we thus have strong evidence for believing that the atoms of matter, whether in the solid, liquid or gaseous form, are in continuous agitation and irregular motion. The velocity of agitation decreases with lowering of temperature, and at the lowest attainable temperature the motion has either ceased or become very small. It is well known that under suitable conditions, the same type of matter can exist in three distinct forms, solid, liquid and gas. If we take the ordinary air of the room, it can be turned into a clear liquid under certain conditions of temperature and pressure, and this liquid can be frozen solid by still further lowering of the temperature. The most refractory gas of all, helium, has only recently been shown to conform with the behavior of all other gases, and to pass into a liquid at a temperature only a few degrees removed from absolute zero. The remarkable changes in appearance and physical qualities of an element in passing from one state to another is a matter of common knowledge—but it is not for that reason very easy of explanation. These changes are believed to be connected with the average distance which separates one atom or molecule from the other and their rapidity of motion. In the gas or vapor form, the molecules are, on an average, so far apart
