BIOGKAPIIICAL MEMOIR OF JOSEPH HENRY. 163 



the clondts passed between two chimneys throngh 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 electricity. It consisted 

 ill coating the outside of a hollow glass globe with tinfoil and after- 

 ward 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 phosphorogenic 

 emanation from the sun. It had long been known that, when the dia- 

 mond 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 i^eculiar to the diamond, but is 

 possessed by a number of substances, of which the sulphuret of lime is 

 the most i^rominent. It is also well known that phosphorescence is 

 produced by exposing the substance to the electric discharge. Another 

 fact was discovered by Becquerel, of the French Institute, that the 

 agent exciting phosphorescence traverses with difficulty a i^late of glass 

 or mica, while it is transmitted ap]3arently 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 substances 

 which possess the capability of exhibiting iihosphorescence, as well as 

 the effects of different interposed media. It was found that, among a 

 large number of transparent solids, some were i)ermeable to the i^hos- 

 phorescing agent, and others impermeable, or imperfectly permeable. 

 Among the former were ice, quartz, common salt, alum. Among the 

 latter class, mica, tourmaliue, camphor, &c. Among liquid permeable 

 substances were water, solutions of alum, ammonia: while among the 

 unpermeable liquids were most of the acids, sulphate of zinc, sulx)hate 

 of lei>d, alcohol, «&c. 



It was found that the emanation took place from every point of the 

 line of the electric discharge, but with more intensity from the two ex- 

 ti'emities ; and also, that the emanation producing phosphorescence, what- 

 ever be its nature, when reflected from a mirror, obeys the laws of the 

 reflection of light, but no reflection was obtained from a surface of 

 polished glass. It is likewise refracted by a i>rism of rock salt, in ac- 

 cordance with the laws of the refraction of light. By transmitting the 

 rays from an electrical spark through a series of very thin plates of 

 mica, it was shown that the emanation was callable of polarization, and, 

 consequently, of double refraction. 



IX. The next series of investigations was on a method of determining 

 the velocity of projectiles. The plan proposed^ for this purpose con- 

 sisted in the application of the instantaneous transmission of elec- 

 trical action to determine the time of the passage of the ball between 

 two screens, placed at a short distance from each other in the path of 

 the projectile. For this purpose the observer is provided with a re- 

 volving cylinder moving by clock-work at a uniform rate, and of which 

 the convex surface is divided into equal parts indicating a fractional 

 part of a second. The passage of the ball through the screen breaks a 

 galvanic circuit, the time of which is indicated on the revolving cylinder 



