154 MEMORIAL OP JOSEPH HENRY. 



immediately preceded by a negative charge ; the two in tlieir pass- 

 age past the point at wliich the spark was drawn giving rise to its 

 duplex character. It was also shown by a series of experiments in 

 transmitting a powerful discharge through a portion of air, that 

 the latter, along the path of discharge, was endowed for a moment 

 with an intense repulsive energy. So great is this that in one 

 instance, when an electrical discharge from the clouds passed 

 between two cinnmeys through the cockloft of a house, the whole 

 roof was lifted from the walls. It is to this repulsive energy, or 

 tendency in air to expand at right angles to the })ath of a stroke of 

 lightning, that the mechanical eifecta which accompany the latter 

 are generally to be attributed. 



In connection with this series of investigations an experiment 

 was devised for exhibiting the screening efiect, within a space 

 inclosed with a metallic envelope, of an exterior discharge of elec- 

 tricity. It consisted in coating the outside of a hollow glass globe 

 with tinfoil, and afterward inserting, through a small hole in the 

 side, a delicate gold leaf electrometer. The latter, being observed 

 through a small opening in the tinfoil, was found to be unaffected 

 by a discharge of electricity passed over the outside coating. 



VIII. Another series of investigations was on the j)hosphoro- 

 genic emanation from the sun. It had long been known that when 

 the diamond is exposed to the direct rays of the sun, and then 

 removed to a dark place, it emits a pale blue light, which has 

 received the name of i)hos|)horescence. This effect is not peculiar 

 to the diamond, but is possessed by a number of substances, of 

 which the sulphuret of lime is the most prominent. It is also well 

 known that phosphoreswnco is produced by exposing the substance 

 to the oloclrio disehargxi. Another fact was discovereil by Beoque- 

 REL, of the French Institute, that the agent exciting ])hosphores- 

 cenco traverses with diffuiulty a ])late of glass or mica, while it is 

 (ruu!«m(((tHl »p|kuvu(ly \\i(hiMi( iiu)H>diiuot)r lhi\)ugh pla(i<c$ ofblaok 

 quartz impervious to light. 



My experiments consisted, in the first place, in the reproduction 

 of these results, and afterward in the extension of the list of sub- 

 stances which possess the capability of exhibiting phosphorescence, 

 as well as the effects of difl'erent interposed media. It was found 



