by the contact of a Hydrogen-flame with various Bodies. 325 



temperature of the hydrogen-flame, but may probably have 

 some connexion with the chemical nature of the gas. 



But how came the sulphur on the surface of those bodies 

 which had not been purposely placed in contact with that ele- 

 ment ? It was extremely unlikely that some sulphurous sub- 

 stance had accidentally touched all the different solids on which 

 the blueness was distinctly seen. Could the sulphur proceed 

 from the atmospheric dust deposited on the surface of these 

 solids ? This explanation was established by the following ex- 

 periment. An old block of granite was taken and examined 

 by the hydrogen-flame ; it showed the blue coloration vividly 

 on every part of its surface. The block was now split in 

 halves and again searched by the flame : on the newly fractured 

 surfaces not the faintest trace of colour was seen even in a 

 darkened room. The two fragments were now placed on an 

 upper shelf in a store-room adjoining the laboratory, in which 

 no fire was ever made. Over one of the pieces a glass vessel 

 was inverted, dust being thus excluded from it, whilst the other 

 fragment was left uncovered ; both had their fractured surfaces 

 turned uppermost. After three days they were again tried 

 with the hydrogen-flame. The upper side of the portion which 

 had been under the glass showed, as before, no blueness; 

 whereas the other exposed fragment was now found to give a 

 well-marked blue colour on its upper and previously inert sur- 

 face. A piece of white paper had been placed by the side of 

 the granite, and from this it was seen that dust had settled on 

 the exposed fragment. Two plates of chemically clean platinum 

 similarly exposed gave corresponding results ; the same was true 

 of marble and other substances; and similar effects were ob- 

 tained when the bodies were exposed in other places away from 

 the laboratory, in private dwellings, or in the open air. An 

 instructive result was given by the examination of a flask which, 

 having remained in the laboratory undisturbed for some time, 

 had become thickly coated with dust on its upturned parts. After 

 trying the under surface of the flask, which showed no blueness 

 whatever, the hydrogen-flame was caused to pass slowly up the 

 surface of the flask. As the upper portions were approached, a 

 slight coloration made its appearance; and when the flame came 

 into contact with the dust en the upturned surface, an extremely 

 bright but momentary blueness was seen on the spot examined. 

 The duration of the colour could easily be prolonged by moving 

 the flame to other dusty portions of the flask. 



These experiments, as far as they go, show, I believe, that 

 among the other foreign ingredients of our atmosphere, sulphur, 

 either free or in a state of combination, must be regarded as occu- 

 pying not a very unimportant place — derived, it may be, from our 



