THE HALF-LIFE PERIOD OF RADIUM; A CORRECTION. 185XIX.-T!ke HaljX;fe P e r i o d of Radium ; a, Correction.By ROBERT WHYTLAW GRAY and SIR WILLIAM RAMSAY, K.C.B.IN making the calculation of the half-life period of radium in thepaper which has recently appeared (Trans., 1909, 95, 1083), anerror, which Mr. Soddy was so good as t o point out to us, wasmade inadvertently. Half the reciprocal of the disintegration-constantof radium, equal to half the average life-period, was given as theperiod of half-decay, namely, 1258 years. The true half-life periodshould have been calculated from the equation : R = R,,e-*t, where Ris the initial amount of radium, R, the quantity remaining after halfhas decayed, e the base of natural logarithms, A the disintegration-constant for radium (the reciprocal of which is the average life-period),and t the time in years.To find A from the equilibrium-quantity, ATo, of emanation per gramof radium, the volume of which was 0.601 cub.mm., the amount ofemanation given off per second was calculated by means of the0.601 equation : qo=AN,= - = 1-25 x cubic millimetres per48 1,000second; the fraction 1/481,000 is the value of A, the disintegrationcoustant for the emanation, calculated from the half-life period of 3.86days. This volume is equal to the diminution in volume of 1 gram ofradium per second, supposing it to be a monatomic gas.Hence the fraction of the total quantity of radium changed persecond is 1.25 x of the volume of 1 gram of gaseous radium.This is equal to 1'25 226, 226 being the accepted atomic22.400 x 100186 BARNETT AND SMILES : THE INTRAMOLECULARweight for radium ; the expression is equivalent to 1/2516 per year,and is A, or the disintegration-constant for radium. I n decimals, itis 3.98 x 10-4 gram per year ; hence the half-life period for radium is1744 years, or, in other words, 1 gram of radium will have beenreduced to 0.5 gram after 1744 years,UNIVERSITY COLLEGE,LONDON