CHAPTER I. 



INTRODUCTION. 



In the various determinations of the physical properties of matter, 

 such as electrical and thermal conductivity, specific inductive capacity, 

 coefficients of expansion and refraction, etc., the spectroscopic study of 

 the transparency of substances to radiant energy, and in particular to 

 heat-waves, has been left in the background, although this data is just 

 as necessary in meteorology, geology, and the allied sciences as is our 

 knowledge of any of the other so-called constants of nature. 



The following research was undertaken to ascertain the cause of 

 absorption, and the connection between absorption and the structure of 

 crystals. In this and in a previous research it has been shown that cer- 

 tain absorption bands in the infra-red are due to particular groups of 

 atoms. The relation of these results to the question of the structure of 

 crystals will be obvious to the reader. For, if the crystal is composed 

 of molecules of, say, water and calcium sulphate, which separately have 

 characteristic absorption bands, then, if these molecules or certain 

 groups of atoms in them undergo no physical change when they com- 

 bine to form a crystal (of selenite in this case), one would naturally 

 infer that the absorption spectrum of the product will be the composite 

 of the absorption bands of the two constituents. 



This phenomenon is different from the one in which Julius 1 showed 

 that the absorption spectrum of a chemical compound is not the com- 

 posite of the bands of the constituent elements. Here the physical 

 molecule has been changed. 



Just how these specific groups of atoms are placed with respect to the 

 crystallographic axes it is impossible to determine. 



A practical question in optics, viz, the possibility of finding material 

 suitable for prisms, in certain regions of the spectrum in which our 

 present prism material is opaque or in which its dispersion is small, was 

 also kept in view. 



The reflecting power of metals was included to fill a gap left in the 

 work of Hagen and Rubens done at the Physikalische Technische 

 Reichsanstalt. The reflection from minerals composing the earth's crust 

 has unexpectedly furnished us with data which will be useful to the 

 meteorologist. It may even aid in clearing up such an obscure question 



1 Julius : Verhandl. Konikl. Akad. Amsterdam, Deel I, No. I, 1892. 



