weight: crystals and crystal forces 329 



constitution of matter the atom is considered to consist of an 

 electrically positive nucleus surrounded by negative electrons. 

 The fields surrounding the atoms in a crystal may be electro- 

 magnetic in nature. 



Influence of a crystal system of forces on other systems of forces, 

 especially light waves. Crystals exert a profound influence on 

 transmitted light waves. The effects which are thereby pro- 

 duced constitute the subject matter of crystal optics, in which 

 light waves are commonly treated from the viewpoint of the 

 electromagnetic theory. It is known that with change in pres- 

 sure or in temperature the distances between the component 

 atoms of a crystal are changed slightly and that these slight 

 changes induce corresponding changes in the optical constants 

 of the crystal. Under these conditions we are dealing with two 

 distinct systems of forces, the crystal system and the light wave 

 system; by measuring quantitatively the differential shifts which 

 these slight changes in the crystal system of forces produce in 

 the light wave system of forces we obtain a relation between the 

 differential changes in the two systems of forces; in short, a differ- 

 ential equation which, if we can integrate it and determine the 

 constants in terms of absolute elastic units, will enable us to 

 determine the form of the force-function of the crystal system 

 and thus to obtain a measure of atomic forces. This problem 

 is now under attack at the Geophysical Laboratory. Apparatus 

 has either been built or is under construction for measuring accu- 

 rately the changes in the crystallographical and optical con- 

 stants of crystals for temperatures ranging from —190° to 

 + 1600°C. and for hydrostatic pressures ranging from 1 to 2000 

 atmospheres. 



The fact that in a crystal each of the component atoms is 

 restricted largely to minute translational oscillations about a 

 point, in other words is limited in its degrees of freedom, is 

 exceedingly important from a thermodynamical standpoint. 

 Thermodynamics is a general system of statistical mechanics 

 applied to the energy relations involved in heat and work; because 

 of its generality thermodynamics is applicable to a great variety 

 of problems, but the mode of its application requires nice dis- 



