[1]A microgram bears the same relationship to a 1000-pound steer as a
penny does to $4½ billion.
[2]The roentgen is a measure of ionizing radiation, as the foot-candle
is a measure of light. In simple terms the roentgen is that amount
of X or gamma radiation which produces one electrostatic unit (esu)
of electricity in one cubic centimeter (cc) of dry air at standard
conditions of temperature and pressure. This may sound like a
trivial amount of energy, but it amounts to more than two billion
ionizations in each cubic centimeter.
[3]From “Epistle To Be Left in the Earth.”