Intelligence and Miscellaneous Articles. 321 



Below the pressure at which the conductivity is at its maximum, 

 this diminishes with the pressure. Thus under a pressure of O'l mil- 

 lim. the conductivity is still 



150 for atmospheric air. 

 ] 20 for hydrogen. 

 110 for carbonic acid. 

 110 for nitrogen. 

 86 for oxygen. 



M. Morren attaches, with justice, great importance to the con- 

 ductivities of nitrogen, oxygen, and atmospheric air; for it is pro- 

 bable that in the layer of greatest conductivity the phenomena of the 

 electric light take place of which the atmosphere is the theatre. 

 Now this layer, whose clastic force is 1 millim., ought, from Boyle 

 and Mariotte's law, to be at a height of 8000 metres, and to have 

 a thickness of 8000 metres also ; that would be the zone of the phe- 

 nomenon of the aurora borealis. 



M. Morren concludes his memoir by some remarks, especially on 

 the precautions to be taken to avoid the influence of moisture, and 

 on the necessity, when using glass tubes, of working in dry weather, 

 or at all events under the same hygrometric state. 



He also gives some details of the attempts made to determine the 

 conductivity of compound gases which the current decomposes. 

 Their conductivity is generally very small, and commences late. 

 Thus carbonic oxide commences to allow the current to pass only 

 under a pressure of 11 millims., carburetted hydrogen under a 

 pressure of 16 millims., and sulphurous acid under that of 5 mil- 

 lims. For this latter gas the greatest deflection is 54°; it takes 

 place under a pressure of 2 millims., and then descends rapidly. 

 Cyanogen allows thecurrent to pass under a pressure of 5 millims. ; 

 at 4 millims. the deflection is 7°, at 3 millims. 21°; it then rapidly 

 ascends to 38°, to 45°, and even to 49°. A powerful reaction then 

 takes place, accompanied by an appreciable modification in the tint ; 

 from the time when the tint is modified, the stratifications appear 

 with extreme delicacy, passing from the negative to the positive 

 electrode with a magnificent undulating motion. The luminous 

 phenomena which the passage of electricity in rarefied cyanogen 

 present are very brilliant, and deserve to be studied more closely. — 

 Annates de Chimie et dc Physique, vol. iv. p. 325 ; Bibliotheque Uni- 

 vcrselle, January 20, 1866. 



ST. ELMO'S FIRE. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 I have just received from Captain Briggs, the intelligent com- 

 mander of the steamer ' Talbot,' the following account of the occur- 

 rence of this rare electrical phenomenon in the Irish Channel on the 

 morning of the 7th inst. There has been of late a good deal of 

 electrical disturbance in the atmosphere, and I have ascertained that 



