Effect of Pressure on Temperature of Crater of Electric Arc. 379 



the absorption bands nearly disappeared, so that the heating could 

 not have been the cause of the apparently enormous production of 

 N0 2 at high pressure. 



We next tried whether oxygen blown into the arc would burn up 

 the carbons, but found it did not do so to any serious extent, and so 

 tried the arc in a compressed atmosphere of this gas. 



The arc burned very nicely indeed in the oxygen, the carbons 

 keeping a good shape, and a very steady crater. The oxygen was, 

 however, so contaminated with nitrogen that at high pressure enor- 

 mous quantities of NO 2 were again formed, so that we could not 

 proceed further with the radiation experiments. The arc was a 

 bright blue bead, about the size of a pea, and the spectrum was a 

 beautiful banded one. 



From these results we concluded that the reduction of radiation, 

 and red-hot appearance of the crater in the former experiments in 

 nitrogen, were due to its being contaminated with oxygen and to the 

 large quantities of N0 2 , which were formed by the arc when under 

 pressure. 



We next tried the arc in hydrogen. The gas was obtained as pure, 

 but contained hydrocarbons as an impurity, possibly from having been 

 compressed into a cylinder which had previously been charged with 

 coal-gas. 



The arc in hydrogen at atmospheric pressures was a long, thin 

 flame, that moved as far up the carbons as possible ; especially on the 

 negative carbon it walked up a cm. along the cone. It went so far 

 that it fuzed the copper ring that held the negative carbon, and we 

 had to replace it by an iron wire lashing. It was very unsteady, and 

 trees of soot and a deposit of hard graphitic carbon formed on this 

 positive carbon as if there were electrolysis of the hydrocarbon, and 

 carbon were electro -negative compared with hydrogen. This growth 

 took place all round the crater, while there was no tendency for any- 

 thing to grow on the negative carbon. 



The arc was only 5 6 mm. wide, and sometimes over 2 cm. long. 

 There was a green outer flame, with a bright red line not a mm. wide 

 down the middle of it. Where it impinged on the negative carbon 

 there was a bright red flame from the middle of the bright spot on 

 the carbon. The outer greenish part seemed to give much the same 

 spectrum as the green cone in a Bunsen burner, while the red flame 

 and line was undoubtedly glowing hydrogen. As we saw the C and F 

 hydrogen lines very distinctly, the red C line being dazzlingly bright 

 and not nearly so wide as in a coil spark at atmospheric pressure 

 whenever the image of the red part of the arc was thrown on the 

 slit of the spectroscope, the appearance was quite like that of a solar 

 prominence. 



The end of the positive carbon was pitted into a number of craters 



2 G 2 



