J. Trowbridge — Oscillations of Lightning discharges. 195 



It is to be seen that these mixed salts correspond in compo- 

 sition to the bromide. The analogous mode of formation of 

 the iodide, the identical relation of the lead to the extra halo- 

 gen in the iodide and the other products, as well as the exist- 

 ence of these mixed salts, make it appear certain that the 

 analyzed iodide was invariably impure and that the pure com- 

 pound should be considered as analogous to the other salts. 

 This view has been confirmed by a crystallographic examina- 

 tion of the iodide and the red bromo-iodide, which Prof. S. L. 

 Penfield has kindly undertaken. He has found that both 

 these salts crystallize in prisms of the tetragonal system. 

 Unfortunately the crystals of the iodide were without termina- 

 tions, so that a more detailed comparison of the two salts 

 could not be made. 



The nature of these peculiar salts is not clear. If they are, 

 strictly, hydrous lc double salts," such higher halides as Pb 2 I 5 or 

 K 3 I 4 must be assumed. If they are formed from such com- 

 pounds as Pbl 4 or KI 3 , they must be considered as hydrous 

 triple salts. 



Sheffield Scientific School, April. 1893. 



Akt. XXIX. — On the Oscillations of Lightning discharges 

 and of the Aurora Borealis ; by John Tr6wbridge. 

 (With Plate IV.) 



It is well known that when air is subjected to a sudden 

 strain at the moment of an electrical discharge it acts like 

 glass or a similar elastic solid and is cracked in zigzag fissures : 

 indeed the resemblance between the ramifications of lightning 

 and the seams produced in plates of glass by pressure has been 

 commented upon by various observers. Photographs of power- 

 ful electric sparks lead one to conclude that a discharge of 

 lightning makes way for its oscillations by first breaking down 

 the resistance of the air by means of a disruptive pilot spark ; 

 through the hole thus made in the air the subsequent surgings 

 or oscillations take place. 



In examining the early photographs of electric sparks by 

 Feddersen one perceives indications that the electric oscillations 

 tend to follow for at least some hundred thousandths of a 

 second the path made by the pilot spark ; and there are ob- 

 servers who believe that by rapidly moving a camera they 

 have obtained evidence that successive discharges of lightning 

 follow the same path. Professor Lodge has protested, with 

 reason, against the conclusions drawn from the method of 

 " waggling " the head or camera : for the movement of the 



