154 MEMORIAL OF JOSEPH HENRY. 



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

 age past the point at which 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 chimneys 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 path of a stroke of 

 lightning, that the mechanical effects which accompany the latter 

 are generally to be attributed. 



In connection with this series of investigations an experiment 

 was devised for exhibiting the screening effect, 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 phosphoro- 

 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 phosphorescence. 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 phosphorescence is produced by exposing the substance 

 to the electric discharge. Another fact was discovered by BECQUE- 

 REL, of the French Institute, that the agent exciting phosphores- 

 cence traverses with difficulty a plate of glass or mica, while it is 

 transmitted apparently without impediment through plates of black 

 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 different interposed media. It was found 



