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



coal fires. From investigations that have recently been made 

 in France, it has been stated that traces of sulphate of soda are 

 often present in the air : it may be objected that under the in- 

 tense heat of the hydrogen-flame, it is this body which has been 

 decomposed and burnt, and thus the sulphur of the compound 

 mistaken for the free element. This conclusion is incorrect ; for 

 sulphate of soda does not apparently decompose in this way, no 

 blue coloration having ever been produced by contact of a hydro- 

 gen-flame with this salt. On the other hand, the various speci- 

 mens of alum which were tried, both on old and new surfaces, 

 showed a vivid blue figure of the flame. On the sulphides 

 generally, and on some of the sulphates, the blue was well seen, 

 owing to their decomposition by the burning hydrogen. The sin- 

 gular persistence of the blue appearance on sulphuric acid points 

 to the probability that the liquid was instantaneously decomposed 

 by the contact of the flame*. 



The explanation of the capricious appearance of the blue colo- 

 ration seen on the surface of many bodies is now easy. Those 

 which showed the luminosity had either been long exposed to 

 the air, or had in some way come into contact with sulphur. 

 The latter will account for the blueness seen on the surface of a 

 block of ice. In order to convey the gas from its reservoir to 

 the jet from which it was burnt, a piece of vulcanized india- 

 rubber tubing had been used, in handling which the fingers had 

 become coated with a film of sulphur. This was readily proved 

 to be the case by exposing the fingers to the burning hydrogen ; 

 on the fore finger and thumb the blue section of the flame was 

 seen to perfection, which was not the case after the hands had 

 been thoroughly washed. Having grasped the ice with unwashed 

 hands, the sulphur on the fingers had been partially transferred 

 to its surface. By the liquefaction of the ice the particles of 

 sulphur were carried down to the base of the block ; hence, as 

 already noticed, the blueness was always seen brightest around 

 the lower part of the ice. But the fingers had not thus come into 

 contact with the water on which the blueness occasionally flashed 

 out. Here the coloration was found to be due to particles of 

 sulphur carried over, by the current of hydrogen, from the inte- 

 rior of the tube, on to the surface of the water : there they were 

 burnt ; and when observed through a lens, their presence could 

 be verified by the discontinuity of the blue colour. If absent 

 before, a slight shake of the tubing was generally sufficient to 

 cause the appearance of the blueness on water or other liquids f. 



* Subsequent experiments render it probable that the sulphur detected 

 in dust is chiefly derived from a decomposition of sulphate of ammonia. 



f This is in harmony with a fact observed by Professor Tyndall in his 

 researches on radiant heat : the passage of a gas through a new piece of 



