206 ANITUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



By the first method, 70 of the chemical elements can readily be 

 detected and measured. The permanent gases, carbon, and a few 

 other nonmetallic elements produce few lines in the spectral regions 

 commonly photographed, and since almost all molecules are disso- 

 ciated in the arc into their constitutent atoms, organic materials are 

 burned away without appearing in the analysis. When an apple 

 or a cranberry is ashed in an electric arc, for example, the strongest 

 lines which appear in the spectrum are those due to such elements 

 as sodium and copper and iron. 



When one is interested in molecules, the second method of spectro- 

 graphic analysis can be used. Though its application is limited to 

 materials which are somewhat transparent, it has the advantage of 

 not affecting the material which is being analyzed, since this is merely 

 penetrated by a beam of light. 



The modern spectroscope is not the small brass tube standing on a 

 tripod so familiar to chemistry students, who have used it to observe 

 the colored lines produced by different salts when burned in a flame. 

 One of the research instruments used for precise work may fill a 

 room 40 feet square (as shown in pi. 1), and several laboratories 

 contain more than a dozen different types of spectrographs. For 

 industrial work three main types are in most common use. The 

 quartz prism instrument of medium size, which records on one plate 

 the spectrum between the limits 5000 angstroms and 2000 angstroms, 

 is useful for absorption spectrophotometry, since the absorption 

 bands of solids or liquids are usually so wide that great separation 

 of the waves of different lengths is valueless. This instrument can 

 also be used for emission analysis where the substances being studied 

 have fairly simple spectra, such as the alkalies and alkaline earths, 

 and in general those atoms which lie on the left-hand side of the 

 periodic table. Iron and most other atoms of the long periods have 

 such complex spectra that greater resolution of the lines is required, 

 and a large quartz instrument of the Littrow type is most commonly 

 used. This records in 3 settings the part of the spectrum most im- 

 portant for analytical purposes, lying between 2000 angstroms and 

 5000 angstroms, when a quartz prism is used, and a glass prism can 

 be substituted with which the visible spectrum can also be recorded. 



The concave diffraction grating spectrograph has long been con- 

 sidered one of the more convenient types of spectrograph in research 

 laboratories, and this instrument is now slowly making its way into 

 industrial use. It has the advantage that it can be used with light 

 waves of any length from the long infrared to the shortest ultra- 

 violet, a single instrument covering all the desired ranges without 

 change of optical parts. Again, the grating produces almost uniform 

 dispersion from the shortest to the longest wave lengths, whereas 

 the dispersion of a prism decreases rapidly toward longer wave 



