THE CONSTITUTION OF MATTER in 



Now that we have a method of determining the arrangement and 

 distances apart of the atoms in a crystal, the next step will be to 

 examine the intensity and type of forces which are brought into play 

 to keep the atoms in equilibrium and relatively fixed in their places. 

 It is to be expected that the atoms are able to move to and fro about 

 their position of equilibrium, and this is indicated by the effect of 

 lowering the temperature of the crystal; for the intensity of the dif- 

 fraction spectra increases as the amplitude of motion of the atom 

 diminishes. The sharpness of the diffraction spectra suggests that the 

 atoms are not only arranged at definite distances from one another 

 but that each atom is orientated in a definite position with regard to 

 its neighbor. 



While varieties of crystals are kDown of all degrees of hardness, the 

 work of Lehmann has brought to light the unexpected existence of 

 crystalline arrangement in some liquids. These liquid crystals are best 

 shown in certain complex organic substances at a temperature slightly 

 above their melting point, and they are only observable in the liquid 

 by the patterns and colors developed when polarized light passes through 

 them. These crystals are mobile like a drop of oil in a solution and 

 can be squeezed into a variety of patterns. Such results would indicate 

 that the molecules of the liquid have a tendency to arrange themselves in 

 ordered patterns, although it is difficult to understand how the freedom 

 of relative motion that is supposed to characterize a liquid can con- 

 temporaneously exist with an ordered arrangement of some of the con- 

 stituent molecules. 



Light Spectra 



We will now direct our attention to another type of phenomenon 

 which ultimately promises to throw much light on the detailed structure 

 of the atom. When the light from an incandescent vapor or gas is 

 passed through a prism or reflected from a grating, it is resolved and 

 gives a characteristic spectrum consisting of a number of bright lines. 

 By suitable methods, the wave-length of these radiations can be deter- 

 mined with great accuracy. -Each of these lines represents a definite 

 and characteristic mode of vibration of the atom, and from the exceed- 

 ing complexity of the spectra of many of the heavy elements, we are 

 forced to conclude that an atom can vibrate in a great variety of ways. 

 When the meaning of the dark lines in the solar spectrum was correctly 

 interpreted, we were enabled at one stride to extend our methods of ob- 

 servation to the sun and the furthest fixed stars. It was soon recognized 

 that atoms of the same element always vibrated the same way under all 

 conditions. It was found, for example, that hydrogen atoms in the 

 earth vibrated in exactly the same way as the same atoms in a distant 

 star. The important bearing of this result on the structure of atoms 

 was pointed out by Clerk Maxwell in his well-known address on ee Atoms 



