2 . PHENOMENA, ATOMS, AND MOLECULES 



or choice was impossible. Others thought that cause and effect relations 

 were mere illusions. 



From the viewpoint of the early classical scientist the proper field for 

 science was unlimited. Given sufficient knowledge, all natural phenomena, 

 even human affairs, could be predicted with certainty. Ampere, for 

 example, stated that if he were given the positions and velocities of all the 

 atoms in the universe it should be possible theoretically to determine the 

 whole future history of the universe. Practically, of course, such predic- 

 tions would be impossible because we could never hope to get the necessary 

 knowledge nor the time to carry out such elaborate calculations. 



A little later scientists developed the kinetic theory of gases according 

 to which the molecules of a gas are moving with high velocity and are 

 continually colliding with one another. They found that the behavior of 

 gases could be understood only by considering the average motions of the 

 individual molecules. The particular motion of a single molecule was of 

 practically no importance. They were thus taught the value of statistical 

 methods, like those which the insurance company now uses to calculate 

 the probable number of its policy-holders that will die within a year. 



The theories or explanations which were developed in connection with 

 the natural laws usually involved a description in terms of some kind of 

 a model. In general, instead of thinking of the whole complex world we 

 select only a few elements which we think to be important and concentrate 

 our minds on these. Thus, the chemist developed the atomic theory accord- 

 ing to which matter was made up of atoms of as many different kinds as 

 there are chemical elements. These were thought of as small spheres, but 

 no thought was given as to the material of which they were made. When 

 later theories indicated that these atoms were built up of electrons and 

 positive nuclei this made very little difference to the chemist, for he had 

 not needed previously to consider that aspect of the model. 



High-school boys to-day are asked to build model airplanes. These must 

 be of such shape that the different types of airplanes can be recognized 

 when the profiles of the models are seen against a white background. It 

 naturally is not particularly important just what kind of material is used 

 in constructing them. Airplane designers build model airplanes to be 

 studied in wind tunnels, but these do not need to be provided with motors. 



Most of the models which the scientist uses are purely mental models. 

 Thus, when Maxwell developed the electromagnetic theory by which he 

 explained the properties of light he thought of a medium through which 

 these waves traveled. This was called the ether. It was supposed to have 

 properties like those of elastic solid bodies. The reason for this choice of 

 a model was that at that time the average scientist had been taught in great 

 detail the theory of elasticity of solid bodies. Thus the magnetic and elec- 



