THE CONSTITUTION OF MATTER 107 



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 par- 

 ticles 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 bombard- 

 ment 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 mil- 

 lions 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 at- 

 tainable 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 plrysical 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 



