632 Prof. W. M. Thornton on Thunderbolts. 



compounds yellow *. In the electric wind no yellow coloration 

 is to be seen. If a stream of ozone produced electrically in 

 a Siemens tube from oxygen is passed over a metal plate 

 attached to an electroscope charged with positive electrifi- 

 cation, the leaves collapse, and the rate of decay is pro- 

 portional to the speed at which the gas is passed through the 

 ozonizer. The discharge is accelerated by the influence of a 

 negatively charged plate, showing clearly that the fresh ozone 

 carries a negative charge. 



This suggests an explanation of the origin of the energy 

 in ball lightning. On the occurrence of a flash of lightning 

 from a charged cloud there is an immediate readjustment 

 of the surface electrical conditions, and in certain cases 

 there is the so-called return flash, closely following the 

 first, caused by such a readjustment of the distribution 

 of charge on cloud and earth. If at any projecting part 

 of a negatively charged cloud the stress is nearly but not 

 quite sufficient for a second flash, there will be for a time 

 ionization on a great scale with the formation of ozone which, 

 when sufficiently local in production, gathers into a ball, 

 is repelled and falls. The volume produced depends on the 

 energy immediately available. The process is of the same 

 nature as the point discharge ; but whereas under the stress 

 possible in a laboratory the space in which the glow occurs 

 has a radius of about half a millimetre, under the colossal 

 stress in thunder-clouds it may quite well occur simultaneously 

 in a space a yard in diameter, that of the largest fireball known. 

 At the mast-heads and yards of ships at sea in tropical 

 thunderstorms a blue light is frequently seen — St. Elmo's fire 

 — a foot or more in radius. 



All records agree that a thunderbolt is somewhat heavier 

 than air. Nitrogen is lighter than air, and no allotropic 

 form of it is known, though oxides of nitrogen are produced 

 under the influence of streams of electric sparks. Oxygen 

 is slightly heavier than air, ozone is nearly 70 per cent, 

 heavier. The gravitating force on a sphere of ozone a metre 

 diameter in air is 430 grammes — nearly a pound weight. 

 Such a sphere would descend at a rate quick enough to 

 be called a fall. On one of half this size the force would be 

 54 grammes. For such quantities not to fall but to travel 

 horizontally there must be electrostatic repulsion from the 

 earth requiring, since ozone carries a negative charge, a 

 similar charge on its surface, which is known to generally 

 exist. It is improbable that the cloud and earth below it 



* Fischer & Braemer, Ber. vol. xxxviii. No. 3, p. 2633 (1905) ; " On 

 the Production of Ozone bv Ultra-violet Light." 



