TABLES 395-397. -RADIOACTIVITY. 337 



Radioactivity is a property of certain elements of high atomic weight. It is an additive 

 property of the atom, dependent only on it and not on the chemical compound formed nor 

 affected by physical conditions controlling ordinary reactions, viz : temperature, whether solid or 

 liquid or gaseous, etc. 



With the exception of actinium, radioactive bodies emit a, /3, or y rays, a rays are easily ab- 

 sorbed by thin metal foil or a few cms. of air and are positively charged atoms of helium emitted 

 with about 1/15 the velocity of light. They are deflected but very slightly by intense electric or 

 magnetic fields. The rays are on the average more penetrating, are negatively charged particles 

 projected with nearly the velocity of light, easily deflected by electric or magnetic fields and 

 identical in type with the cathode rays of a vacuum tube. The y rays are extremely penetrating 

 and non-deviable, analogous in many respects to the very penetrating Rontgen rays. These rays 

 produce ionization of gases, act on the photographic plate, excite phosphorescence, produce certain 

 chemical reactions such as the formation of ozone or the decomposition of water. All radio- 

 active compounds are luminous even at the temperature of liquid air. 



Table 398 is based very greatly on Rutherford's Radioactive Substances and their radiations 

 (Oct. 1912). To this and to Landolt-Bornstein Physikalisch-chemische Tabellen the reader is re- 

 ferred for references. In the three radioactive series each successive product (except Ur. Y, and 

 Ra. C 2 ) results from the transformation of the preceding product and in turn produces the follow- 

 ing. When the change is accompanied by the ejection of an o particle (helium, atomic weight = 4.0) 

 the atomic weight decreases by 4. The italicized atomic weights are thus computed. Each pro- 

 duct with its radiation decays by an exponential law ; the product and its radiation consequently 

 depend on the same law. I = loe-^t where IQ = radioactivity when t = O, I that at the time t, 

 and \ the transformation constant. Radioactive equilibrium of a body with its products exists 

 when that body is of such long period that its radiation may be considered constant and the 

 decay and growth of its products are balanced. 



International radium standard : As many radioactivity measures depend upon the purity of the 

 radium used, in 1912 a committee appointed by the Congress of Radioactivity and Electricity, 

 Brussels, 1910, compared a standard of 21.99 m g- of P ure Ra - chloride sealed in a thin glass tube 

 and prepared by Mme. Curie with similar standards by Honigschmid and belonging to The 

 Academy of Sciences of Vienna. The comparison showed an agreement of I in 300. Mme. 

 Curie's standard was accepted and is preserved in the Bureau international des poids et mesures 

 at Sevres, near Paris. Arrangements have been made for the preparation of duplicate standards 

 for governments requiring them. 



TABLE 395. Relative Phosphorescence Excited by Radium. 



(Becquerel, C. R. 129, p. 912, 1899.) 



The screen of black paper absorbed most of the a rays to which the phosphorescence was greatly due. For the last 

 column the intensity without screen was taken as unity. The y rays have very little effect. 



TABLE 396. The Production of a Particles (Helium). 



(Geiger and Rutherford, Philosophical Magazine, 20, p. 691, 1910.) 



TABLE 397. Heating Effect of Radium and Its Emanation. 



(Rutherford and Robinson, Philosophical Magazine, 25, p. 312, 1913.) 



Other determinations : Hess, Wien. Ber. 121, p. i, 1912, Radium (alone) 25.2 cal. per hour per gram. Meyer and 

 Hess, Wien. Ber. 121, p. 603, 1912, Radium in equilibrium, 132.3 gram. cal. per o hour per gram. See also, Callendar, 

 Phys. Soc. Proceed. 23, p. i, 1910; Schweidler and Hess, Ion. i, p. 161, 1909; Angstrom, Phys. ZS. 6, 685, 1905, etc. 

 SMITHSONIAN TABLES. 



