CEEW AND BASQUIN. — LUMINOSITY IN THE ELECTRIC ARC. 349 



very quickly used up after the arc was once started. Indeed, we always 

 find a slight whitish deposit of oxide on the electrodes of Zn and Mg 

 when the arc is started too early. 



And with pure zinc and magnesium poles we may feel fairly certain 

 that no ordinary or well known chemical action is going on. Observa- 

 tions were taken only after the stream of hydrogen had been running 

 through the hood for half an hour. After passing two tubes of phos- 

 phoric anhydride, it would seem wellnigh impossible that any moisture 

 should remain in the hydrogen. Figure 11 shows the hooded arc in 

 position. 



Under these conditions, neither the arc nor the luminous cloud above 

 the arc is to be seen. Though once in a long while a bit of yellow 

 cloud Hashes out for an instant. We have not been able satisfactorily to 

 account for this. 



The only light that remains, aside from that emitted by the red-hot 

 electrode, is a sort of blue haze, — a blue glow, that fills the whole hood, 

 and is there all the time. This light appears to be exactly the same for 

 all three metals, iron, zinc, and magnesium. It is too diffused and faint 

 to be easily examined in the spectroscope. Is this possibly a phospho- 

 rescence of hydrogen, or of the finely abraded particles of metal ? 



In reply then, to the main question, viz. can the characteristic spectra 

 of the metals be produced by heat alone, we can only say that on heating 

 metallic vapors to the temperature of the electric arc we are unable to 

 discover any characteristic spectrum after an interval of one thousandth 

 of a second. 



The form of electrode used by us is, however, very favorable to the 

 rapid cooling of the metallic vapor ; and we hope, therefore, soon to try 

 another form, already suggested by one of us, such that the heat will be 

 less rapidly diffused. 



Northwestern University, Evanston, III., 

 February 8, 1898. 



