New Lines in the Spark- Spectrum of Aluminium. 289 



formed the kathode. No difference in definition could be 

 noticed ; there was, however, a great actinic difference. 

 Under the oscillatory nature of the leyden-jar discharge the 

 electrodes become alternately positive and negative. Possibly 

 some of the want of definition noticed in Rontgen photographs 

 taken even with the aid of electrical machines may be due to 

 the fact that the oscillatory discharge does not always emanate 

 from the same point on the anode surface. A small anode 

 should therefore give sharper images than one of a large 

 surface. 



What is supposed to be a resistance in the case of the 

 voltaic arc, and in the modification of this as seen in dis- 

 charges from high-tension transformers and in powerful 

 electric sparks, and presumably in lightning- discharges, is a 

 polarization which produces a variable difference of potential 

 at the spark-terminals. 



The inconstancy of spark potentials has been shown by 

 Jaumann*. In working with a revolving mirror it is found 

 that the spark-terminals have to be brightened in order to 

 preserve the same spark-length. 1 was interested also to 

 observe the effect of the surrounding medium upon the spark 

 potentials. Platinum terminals in sodium vapour showed 

 the polarizable condition ; but there did not appear to be an 

 appreciable change in resistance apart from this polarization. 

 The same was true when bromine vapour surrounded the 

 spark-terminals. In the case of Crookes tubes it is customary 

 to apply heat if the discharge will not pass through the tube : 

 and some makers provide a connecting receptacle filled with 

 a substance which, on being volatilized, modifies the internal 

 conditions of the tube. This modification is often spoken of 

 as a diminution of resistance of the tube. It should be more 

 properly termed a method of modifying the state of polarization 

 of the electrodes. 



Jefferson Physical Laboratory, 



Harvard University, 



Cambridge, Mass., U.S. 



XXXVIIT. On some Neio Lines in the Spark-Spectrum of 

 Aluminium. By Gustav A. HemsalechI . 



WHEN doing some spectroscopic work a short time ago 

 I observed some lines in the red, due to aluminium, 

 which I believe have not been recorded before. With a small 



* Wied. Ann. 1895, lv. p. 656. 



f Communicated by Arthur Schuster, F.R.S. 



