172 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Of the other lines, those which are reduced more in intensity than the 

 series Hnes, must be less intimately related to the electrical or thermal 

 causes of radiation than are the series lines. 



Let us agree that the average intensity of the spectrum of the arc in 

 hydrogen is only one fifth of its intensity in air, and let us agree that 

 the electrical cause of radiation remains practically constant with constant 

 current and voltage although the general intensity of the arc is greatly 

 reduced by the hydrogen atmosphere, then it follows that of the total 

 radiation, that fraction which must be attributed to the electrical cause 

 alone, is relatively five times as great in hydrogen as it is in air. Any 

 line, therefore, which is a function of the electrical cause alone, should 

 have in hydrogen five times the relative intensity that it has in air. It 

 seems quite likely that this may account for the appearance in hydrogen 

 of numerous strong spark lines, not found in the arc in air. 



The appearance of the spark lines in hydrogen is not confined to the 

 rotating arc ; the magnesium spark line at 4481 appears clearly in the 

 stationary metallic arc in hydrogen but not in air. The above explana- 

 tion for the appearance of these lines makes it probable that the electri- 

 cal cause of radiatiou is not zero in either atmosphere. 



In the rotating arc the current is interrupted about twenty-five times 

 per second when the rotating electrode is a rod, instead of a disc, of 

 metal, and this spark at the breaking of the current may account, in part, 

 for the appearance of these spark lines in hydrogen. But we may in- 

 quire why this spark should partake any more of the nature of tlie true 

 spark in hydroo'en than in air. The reduction of the chemical action in 

 the arc reduces the temperature and conductivity of the gases between the 

 poles in hydrogen, and it occurred to me that this action may affect the 

 appearance of the spark lines in either of two ways : — 



1. It may be that a gas which is in the hot condition of the arc in 

 air cannot give off spark lines; the arc spectrum may be characteristic 

 of this condition of the gas and may have nothing to do with electrical 

 action, and so, in this state, would give off only arc lines if a spark were 

 passed through it. 



2. It may be that the conductivity of the gases in air is reduced so 

 slowly at the breaking of the current in the rotating arc that the voltage 

 of break never rises high enough to make a true spark. 



In either of these cases, in hydrogen, the hot gases are largely absent, 

 owing to reduction of chemical action, and give opportunity for the spark 

 to appear. 



In order to test the first suggestion I arranged an electrical circuit as 



