4 PROCEEDINGS OF THE AMERICAN ACADEMY. 



has been made on the impurities which are present in the natural min- 

 eral. The first of these investigations ^ was made on the gases con- 

 tained in fluorite, and the results of this research are wholly negative 

 as far as the question of the source of luminescence is concerned. 

 Nothing other than the ordinary gases was found in any case, and no 

 relation between the occluded gases and the emission of light under 

 excitation was discovered. 



At the present time, careful chemical analyses of a series of fluorites 

 from many parts of the world are being carried out, in the hope of find- 

 ing a clue to the source of the light-emission. The results of these 

 analyses, as far as they have gone, are most interesting. Many fluorites 

 are found to contain quite evident amounts of rare earths,* and from 

 one specimen, at least, enough neodymium and praseodymium have 

 been separated to give a quite measurable absorption spectrum. The 

 author intends to report the results of these investigations to the 

 American Academy as soon as possible. 



II. The spectra of a large number of fluorites, excited by kathode 

 rays, have been examined and photographed. Of this large number, 

 seven will be described in this paper. The crystals examined were : 



1. Fluorite from Amelia Court-House, Virginia. This region is a 

 famous one because of the occurrence of this fluorite, which has re- 

 markable properties, and also for many other minerals containing rare 

 earths. Very large microlite crystals were found near the fluorite de- 

 posits. The crystals of fluorite from this region are what are called 

 "chlorophanes," par excelleiice. They are very sensitive to heat, emit- 

 ting light strongly at the temperature of boiling water, and so strongly 

 at 300° as to be bright objects even in a well-lighted room. The fluor- 

 ites occur in colors varying from dark brown and dark purple to light 

 green. All show the same thermo-luminescence spectrum, and the 

 same kathodo-luminescence spectrum. The spectrum of thermo-lumi- 

 nescence of this variety has been given at length in a previous paper.^ 

 The details of the kathodo-luminescence spectrum are given in Table I, 

 and the appearance of this spectrum is seen in Figure 1, Plate o. 



2. Fluorite from Trumbull, Conn. This is also a brilliant "chloro- 

 phane," which shows the same thermo-luminescence spectrum as the 

 Virginia crystals, and a kathodo-luminescence spectrum which is very 

 closely related to that of the other mineral. Details of the latter 

 spectrum are given in Table II, and the appearance of the spectrum is 

 seen in Figure 2 of the plate. 



3 Studies on Fluorite, III. These Proceerlings, 41, 001 (Mar. 1906). 

 * See also llumplircys, Astroi>hysieal Journal, 20, 260 (1004). 

 5 Studies on Fluorite, II. These Proceedings, 41, 593 (Mar. 1906). 



