CONSTITUTION OF MATTER — EUTHERFORD. 169 



spheres of the size required can be produced by a variety of methods. 

 One of the simplest used by Perrin is to allow a solution of pure water 

 to pour slowly out of a funnel under an alcoholic solution of gam- 

 boge 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 ap- 

 pear to be at rest for more than a moment. The motions of these 

 small particles under a microscope irresistibly convey the impres- 

 sion 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 de- 

 crease 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 ve- 

 locity of agitation decreases with lowering of temperature, and 

 at the lowest attainable temperature the motion has either ceased or 

 become very small. 



It is w^ell 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 re- 

 moved from absolute zero. The remarkable changes in appearance 



