538 



Geological Society :- 



the loss due to reflexion by the walls of the bulb was 

 eliminated. 



The effect of absorption will be to cause a decrease in the 

 intensity of the fluorescence with increasing vapour-density. 

 The effect is somewhat complicated by the circumstance that 

 the green portion of the spectrum is more strongly absorbed 

 than the red. This will cause a change in the colour of the 

 fluorescence, apart from the cause already mentioned, namely 

 collisions with other molecules. The intensity curve con- 

 sequently falls more rapidly than it would if absorption were 

 absent. It is not very easy to correct for absorption, since 

 the light from each element of the column of vapour illumi- 

 nated is obliged to traverse a different thickness of vapour. 



The chief cause of the diminution of intensity is the mutual 

 -action between the molecules. There is no trace whatever 

 ■of superficial fluorescence, or a glowing of a thin layer of the 

 vapour in contact with the wall. This would be practically 

 uninfluenced by absorption. It is present in a very marked 

 degree with mercury vapour, both for the visible fluorescence, 

 ■obtained by illuminating the dense vapour contained in a 

 heated quartz bulb with the light of the spark, and the ultra- 

 violet resonance radiation, stimulated at pressures below 1 mm. 

 by the 2536 line of the mercury arc. as has been shown 

 recently by one of the writers. 



LXIII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 224.] 



January 7th, 1914. — Dr. Aubrey Strahan, F.R.S., President, 

 in the Chair. 



r PHE following communications were read : — 



1. ' The Ordovician and Silurian Rocks of the Lough Nafooey 

 Area (County Galway).' By Charles Irving Gardiner, M.A., 

 F.G.S., and Sidney Hugh Reynolds, M.A., F.G.S., Professor of 

 Geology in the University of Bristol. 



The Lough Nafooey area is a direct continuation of the Kilbride 

 area (described in 1912), from which it is separated by the Finny 

 River. It forms a ridge about 4 miles long, which reaches its 

 highest point (1678 feet) at Curraghrevagh Mountain, and slopes 

 steeply down to Lough Nafooey on the north, and more gradually 

 to Glen Trague on the south. 



