6 4 



INFRA-RED REFLECTION SPECTRA. 



depends upon the metallic (basic) atom to which the group of atoms is 

 united to form a compound. 



It is a remarkable fact that nearly all the substances examined, espe- 

 cially the oxides, have a region of strong selective reflection from 8 to 1 2 ;i. 

 Drude 1 has shown that the optical properties {e.g., dispersion) lead to the 

 conclusion that each atom is a union of many independently vibrating 

 elementary masses or ions. In the ultra-violet the absorption band is due 

 to the sympathetic vibration of particles which have a charge and a mass 

 identical with the negative ion or "electron," while in the infra-red the 

 absorption and reflection bands are due to positive ions ("ponderable 

 atoms") which have a mass of the order of magnitude of the atom. From 

 this standpoint, one would expect to find a shift of the maximum toward 

 the long wave-lengths, as we increase the atomic weight of the element 

 which is attached to the radical. 



The latest theoretical discussion of this subject is due to Einstein, 2 who 

 shows how the theory of radiation, especially Planck's, leads to a modi- 

 fication of the molecular-kinetic theory of heat, and clears up points here- 

 tofore difficult. He also shows relations between the thermal and optical 



properties of solid bodies. 



Table II. 



From the value of the atomic heat of a substance, he computes the 

 maxima of the bands of infra-red selective reflection. The observed 

 reflection bands and the same as calculated by Einstein are given in table II. 

 From this table it will be seen that carbon (diamond) would have a large 

 reflection band at 11 //, which is possible, as will be noticed in the trans- 

 mission curve, fig. 30. 



This table shows that certain calcite maxima are due independently to 

 C at 1 1.4 [x and to O at 21 /t. In quartz the band at 20 /1 is due to Si at 

 20 n and O at 21 p.. From this line of reasoning, it would appear that the 

 large reflection band of carborundum at 12.5 [i is due to C, and that there 

 is another band at 20 \i due to Si. A similar computation for selenium 

 and iodine indicates a reflection band at about 42 /. 



1 Drude: Ann. der Phys. (4), 14, 677, 1904. 



2 Einstein: Ann. der Phys. (4), 22, p. 181, 1907. 



