IO6 INFRA-RED REFLECTION SPECTRA. 



while serpentine contains a large band at 3 p., which is in common with 

 substances containing hydroxyl groups. 



The reflecting power of the metals Zn, Co, Al, Cd, Pd, and Ir is high, 

 and the observed values at 12 ^ are in close agreement with those com- 

 puted from their electrical conductivity. It is a remarkable fact that 

 the region of the first occurrence of selective reflection of the majority 

 of substances examined (other than silicates) lies between the wave- 

 lengths 8 to 10 /JL. This is probably to be expected. Drude 1 has shown 

 that the infra-red free vibrations (Eigenschwingungen) depend upon 

 the ponderable mass of the molecule, and, from this, that one can obtain 

 some idea of the molecular weight of the substance. 



The study of the reflecting power of solids in solution is not suffi- 

 ciently extended to draw general conclusions, but the data shows that 

 the method is feasible that the reflecting power is proportional to the 

 concentration and that the maxima may or may not agree with those 

 of the substance in the solid state, depending probably upon the solvent, 

 just as is true of the visible spectrum. A notable example is iodine, in 

 CS 2 and in C 2 H 5 OH. The experiment also shows that a single band 

 of a mineral in a solid state may appear as several bands when in solu- 

 tion. This is an interesting field that deserves further investigation. 

 The difficulties involved are not greater than for absorption spectra, 

 while the intensity of the energy is still quite large in this region of the 

 spectrum. In the visible spectrum it is known that different maxima 

 appear, depending upon the solvent and upon the addition of acids to 

 the solvent. But the visible spectrum is so narrow in comparison with 

 the absorption bands that the infra-red is far better adapted to the study 

 of this phase of the problem. 



As mentioned in the text, the chief difficulty in this research was in 

 obtaining minerals suitable to illustrate the questions involved. This 

 has placed the writer under deep obligations to the late Prof. S. L. 

 Penfield, of Yale University, who donated a number of rare minerals not 

 obtainable from dealers, and to the officials of the U. S. National 

 Museum, who also supplied a large number of specimens. The Director 

 of the Bureau of Standards generously supplied apparatus and material. 

 I am also under obligations to Dr. J. C. Blake, whose advice on the 

 mineralogical side of this question was frequently sought. 



WASHINGTON, D. C., June 20, /pod. 



1 Drude : "Optische Eigenschaften and Elektronentheorie," Ann. d. Phys., 14, 

 pp. 677 and 936, 1904. 



