Actinium
The first element of the actinide series. It was discovered in 1899 by Debierne. It is known to have two isotopes, whose masses are 227 and 228; they are both radio active and can be produced artificially, by bomcarding radium 226 with neutrons; radium 227 is obtained which by means of beta disintegration, given actinium 227. It occurs in minute quantities in uranium ores and has been used as a source of alpha particles. Symbol: Ac; m.p. 10500C; b.p. 32000C (calc); r.d. 10.07 (calc); p.n. 89; stablest isotope (half-life 21.6 years)
Actinochemistry
A branch of chemistry concerned with chemical reactions produced by light or other radiations.

Activated charcoal
A powdery form of charcoal

Activation
Treatment of a substance by heat, radiation, or activating reagent to produce a complete or rapid chemical or physical change.

Active mass
Effective concentration of a substance in a reaction expressed in mols per liter or for gases in partial pressures.