POPULAR SCIENCE 633 



difficulties to the investigator, who aimed at elucidating its 

 molecular mechanism, than was presented by the gaseous or 

 even by the liquid state. The gaseous state was indeed rela- 

 tively well understood at a time when practically nothing was 

 known about the solid except the external crystalline form. 

 Within the last decade, however, very great progress has been 

 made in visualising the processes operative in solids, so that 

 at the present time it is safe to say that we know more about 

 solids than we do about liquids. This progress has been made 

 along two distinct lines : the first in the domain of atomic 

 heat and energy content, as a result of Einstein's (n) applica- 

 tion of the quantum theory to the energy content of physical 

 resonators or vibrating atoms, the second in the domain of the 

 inner structure of solids of which the crystalline form is the 

 outward and visible sign, chiefly as a result of the investiga- 

 tions of W. H. and W. L. Bragg, with the aid of the X-ray 

 spectrometer (12). The first line of attack has given us con- 

 siderable information regarding the mode of motion of atoms 

 in simple solids ; the second chiefly concerns itself with the 

 problem of the mode of spatial distribution of atoms in a 

 crystal. 



The existence of crystalline form indicates that, whatever 

 the mode of motion of the ultimate particles may be — and 

 motion of some kind must be present in order to account for 

 the temperature of the substance — the existence of crystalline 

 form indicates, that the particles do not undergo translational 

 movement from one part of the solid to another as is the case 

 with gases and liquids. In the simplest case, namely, that of 

 a solid consisting of atoms all of one kind, a piece of copper 

 for example, the most likely mode of motion is that of vibra- 

 tion of the atoms in three dimensions in space over a fixed 

 centre of gravity. At absolute zero the atoms are quiescent. 

 As temperature rises, the recent work of Einstein, Nernst (13), 

 and others has shown, that for a certain range, there is prac- 

 tically no motion. As the temperature is raised still further 

 a certain number of the atoms are vibrating in the manner 

 referred to, and the amplitude of such vibrations goes on in- 

 creasing until, at a high temperature, which we identify as the 

 melting point, the amplitude of vibration becomes equal to 

 the mean distance of the atoms apart. As a result of this 

 condition the crystalline form can no longer be maintained. 



