322 Mr, W. F. Barrett on some Physical Effects produced 



the gas was made in the usual way, and was washed before pass- 

 ing into the reservoir which held it. The blueness was not due 

 to any foreign element in the gas ; for when chemically pure hy- 

 drogen was used, the colour appeared as bright as with the 

 impure gas. 



I now examined a number of bodies taken at random from 

 various places in the laboratory, and found the blueness was to 

 be seen on most of them ; it appeared with extreme brightness 

 on some pieces of granite and glass tubing ; but was not at all 

 confined to non-metallic bodies, as the majority of the metals 

 presented the same appearance. Still there were several sub- 

 stances which showed no blueness whatever, even when subjected 

 to the most careful and repeated examinations ; and there were 

 others on which the blueness was only faintly and momentarily 

 seen. This latter occurred with many objects taken from the 

 cases of the mineralogical collection in the museum of the Insti- 

 tution. A classification of the substances experimented upon, 

 according to the effects they gave, yielded no information as to 

 the cause to which the phenomenon might be ascribed. The 

 results obtained were in many cases so utterly anomalous, and 

 the blueness appeared so capriciously, it at once became evident 

 that only by varying the experiments in every way could I hope 

 to discover the origin of the colour. Nevertheless, from the 

 nature of the appearance, I imagined it might probably have to 

 be attributed to a new kind of fluorescence, differing from that 

 investigated by Professor Stokes in requiring contact of the 

 source of rays, and in its disappearance after a short exposure to 

 the flame — therefore in some respects resembling phosphores- 

 cence, yet quite distinct from it in the sharpness of its outline, 

 the rapidity of its appearance, and its non-persistence when the 

 source was removed even to the smallest distance. 



The examination of the various bodies demonstrated one fact 

 of some importance — namely, that in every case where the blue- 

 ness appeared it was always of the same colour and shape, the 

 area over which it extended being, as already mentioned, strictly 

 determined by the size of the hydrogen-flame. Heating the 

 solid to redness, by any means, sufficed to destroy its power of 

 showing the colour : for example, a piece of granite was taken 

 which gave the blueness in every part ; one spot on it was then 

 exposed to the oxyhydrogen-flame : the granite when cold was 

 again examined, when the blue appeared distinctly on every part 

 except that portion which had been heated ; here the blueness 

 was suddenly extinguished. This spot, which by heating had 

 been deprived of its power to give the blue colour, and which it 

 is evident did not recover the property on cooling, had its power 

 restored after a few days' exposure in the laboratory, the blue- 



