SCOPE OF PRESENT INVESTIGATION. 15 



The minerals were obtained from various dealers and from the U. S. 

 National Museum. They were selected according to composition. Of 

 the total number thus selected only about 40 per cent were obtainable 

 and these were oftentimes the least desirable to illustrate the points in 

 question. The expense involved is no small item, and I am very grate- 

 ful to Director Stratton for his generosity in meeting that question. 



SCOPE OF PRESENT INVESTIGATION. 



The principal object in this research was to gain information in 

 regard to the molecular structure of minerals containing oxygen and 

 hydrogen. To this end minerals were selected containing these con- 

 stituents. Previous work has shown that in organic compounds certain 

 radicals have characteristic absorption bands. One would therefore 

 naturally expect to find similar relations in inorganic compounds, 

 where more important radicals are to be found. Hence, the list was 

 extended so as to contain a series of minerals having the radicals CO 2 , 

 PO 4 , SO 4 and OH. The results obtained serve the purpose very well 

 for demonstrating the rashness of attempting to establish "laws" from 

 present data for future investigators to refute, just as is being done at 

 present in more thoroughly explored fields, such as, for example, the 

 absorption of solids in solution. 



In the study of the carbon compounds in organic chemistry, their 

 constitution has been established by the replacement of certain con- 

 stituents by organic radicals, by the preparation of a series of deriva- 

 tives, by vapor density determinations, or by studying their physical 

 properties in solution. Mineralogy is essentially the chemistry of 

 silicon compounds, to which, as yet, it has not been possible to apply 

 any of the above methods, so that the constitution of many minerals 

 has been derived from analogies with other compounds which are 

 better understood. Hence, in an investigation like the present, it is not 

 surprising to find exceptions to the tentative rules for classifying these 

 minerals. 



The transmission curves are given without correction for reflection 

 from the surfaces. In most cases, as will be noticed from the reflection 

 curves, the correction is negligible. In one case, viz, stibnite (Sb 2 S 3 ), 

 such a correction is worthy of notice. The transmission shows a uni- 

 form value of 45 per cent. To the writer this seemed a very transparent 

 substance until the greater surprise of a uniform reflecting power of 

 about 36 per cent was observed, which indicates that after correcting 

 for reflection this mineral (for the thickness examined) is practically 

 transparent. 



