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 adjustment and screening of, 95 special key for, 97 application of, to measurements of radio-activity, 97 et seq. measurement of current by, 100 capacity of, 101 use with steady deflection, 103 use with quartz piezo-électrique, 105

Electron definition of, 56 production of, under different conditions, 76 et seq. identity of β rays with electrons, 120 et seq. variation of apparent mass of electron with velocity, 127 et seq. evidence that mass of electron is electro-magnetic, 129 et seq. diameter of, 131

Electroscope description of, used by Curie, 85 construction of, for accurate measurements, 86 use of, in measurements of minute currents, 86 of C. T. R. Wilson, 89 use of, in measuring conductivity of air in closed vessels, 531 et seq. use of, for determining radio-activity of ordinary matter, 537

Elster and Geitel radio-active lead, 27 effect of magnetic field on conductivity produced in air by β rays, 113 scintillations produced by active substances, 158 action of radium rays on spark, 208 photo-electric action of body, coloured by radium rays, 214 radio-active matter in earth, 494 discovery of excited activity in atmosphere, 501 emanations from the earth, 507 radio-activity of air in caves, 507 radio-activity of the soil, 515 radio-activity of fango, 516 variation of radio-activity in atmosphere with meteorological conditions, 517 effect of temperature and pressure on atmospheric radio-activity, 518

Emanation of thorium, discovery and properties of, 238 methods of measurement of, 240 decay of activity of, 241 effect of thickness of layer on amount of, 243 increase of, with time to a maximum, 245 of radium, 246 decay of activity of, 247 of actinium, properties of, 249 of radium, phosphorescence produced by, 251 rate of emission of, 254 effect of conditions on rate of emission of, 255 regeneration of emanating power, 256 continuous rate of production of, 257 source of thorium emanation, 261 source of radium and actinium emanation, 263 radiations from, 263 effect of pressure on production of, 265 chemical nature of, 267 experiments to illustrate gaseous nature of, 268 rate of diffusion of radium emanation, 269 rate of diffusion of thorium emanation, 275 diffusion of, into liquids, 276 condensation of, 277 temperature of condensation of, 280 volume of, from one gram of radium and thorium, 288 measurement of volume of, from radium, 289 diminution of volume of, 290 spectrum of emanation, 292 connection between emanation and excited activity, 298 effect of removal of, on activity of radium, 371 et seq. fraction of activity of radium due to, 374 effect of rate of escape of, on activity of radium, 374 heat emission of, 420, 431 variation of heat emission with time, 421 et seq. enormous emission of energy from emanation, 431 radio-activity of atmosphere due to emanations, 504 sucked up from the earth, 507 in caves, 507 et seq. rate of decay of activity of, from the earth, 508 condensation of, from the atmosphere, 510 in well water and springs, 510 et seq. from "fango," 516 effect of meteorological conditions on amount of, in atmosphere, 517 et seq.