MORSE. — STUDIES ON FLUORITE. 601 



is more difficult of decision in the latter case. A spectro-photometric 

 study of the rate of decay of the lines and bands is already under way. 



All attempts to place the sharp lines to the credit of any known 

 elements have been so far unsuccessful. There are of course many 

 coincidences with lines of well-known substances, especially since the 

 dispersion and the corresponding accuracy of measurement are so small ; 

 but the other strong lines of the substance are absent in every case, and 

 one would demand the coincidence of an entire series of lines of an 

 element before admitting that a metal or a gas could show a line 

 spectrum under the conditions of the experiments. The very exact 

 coincidence of the line at A 5893 with the mean of two sodium lines 

 suggests the possibility that this may be that pair unresolved ; but it 

 would seem that resolution into the two constituent lines is the very 

 least that could be demanded before deciding that the sodium pair do 

 actually appear under these circumstances, and any decision on this 

 point should be reserved until this resolution has been accomplished. 



There can be no doubt that these thermo-luminescence spectra are 

 composites, and that the sources, whatever they may be, of the parts of 

 the composite, are distinct. One of these sources gives rise to the diffuse 

 band part of the spectrum in each case, and this part is like the usually 

 observed spectra of fluorescence and phosphorescence. It is evidently 

 different in the two fluorites. The other source is capable of giving 

 a spectrum in most respects similar to the sharp-line spectra usually 

 associated with incandescent gases. This source is common to the two 

 fluorites. 



It seems imperative that as minute an examination as possible of the 

 impurities present in fluorites should be made, to include not only a 

 chemical analysis but also an examination of the gaseous and liquid 

 inclusions in the mineral. 



A generous grant from the Kumford Fund of the American Academy 

 of Arts and Sciences has been of much aid in this work, and a con- 

 tinuance of the investigation in several directions is already under 

 way. 



III. Gaseous and Liquid Inclusions in Fluorspar. 



Many facts connected with the various phenomena of luminescence in 

 the mineral fluorspar point to the possible influence of impurities of a 

 gaseous or liquid nature, which are usually present as inclusions, and 

 which are driven out or decomposed when the mineral is heated. There 

 is a very general impression that these impurities exercise a determining 



