the Cause and Nature of Radioactivity. 373 



a hundredfold. An average value for the capacity of the 

 electrometer and connexions is 3 x 10~ 5 microfarads ; and 

 when this is charged up to 10~ 2 volts, a quantity of elec- 

 tricity corresponding to 3 x 10 ~ 13 coulombs is stored up. 

 Now in the electrolysis of water one gram of hydrogen 

 carries a charge of 10 5 coulombs. Assuming, for the sake of 

 example, that the conduction of electricity in gases is 

 analogous to that in liquids, this amount of electricity corre- 

 sponds to the transport of a mass of 3 x 10 -1S grams of 

 hydrogen ; that is, a quantity of the order of 10 ~ 12 times that 

 detected by the balance. For a more delicate instrument, 

 this amount would produce a large effect. 



The examples of radium in pitchblende and of the thorium- 

 excited radioactivity make it certain that comparativelv large 

 ionization effects are produced by quantities of matter beyond 

 the range of the balance or spectroscope. 



The electrometer also affords the means of recognizing; and 

 differentiating between the emanations and radiations of 

 different chemical substances. By the rate of decay the 

 emanation from thorium, for example, can be instantly 

 distinguished from that produced by radium; and although a 

 difference in the rate of decay does not of itself argue a 

 fundamental difference of nature, the identity of the rate of 

 decay furnishes at least strong presumption of identity of 

 nature. 



Radiations, on the other hand, can be compared by means 

 of their penetration powers (Rutherford, Phil. Mag. 1899. 

 vol. xlvii. p. 122). If the rays from various radioactive 

 substances are made to pass through successive layers of 

 aluminium-foil, each additional layer of foil cuts down the 

 radiation to a fraction of its former value, and a curve can 

 be plotted with the thickness of metal penetrated as abscissae, 

 and the intensity of the rays after penetration as ordinates, 

 expressing at a glance the penetration power of the rays 

 under examination. The curves so obtained are quite different 

 for different radioactive substances. The radiations from 

 uranium, radium, thorium, each give distinct and character- 

 istic curves, whilst that of the last-named again is quite 

 different from that given by the excited radioactivity pro- 

 duced by the thorium emanation. It has been recentlv 

 found (Rutherford and Grier, Phys. Zeit. 1902, p. 385) that 

 thorium compounds, in addition to a type of easily absorbed 

 Rontgen-rays, non-deviable in the magnetic field, emit also 

 rays of a very penetrating character deviable in the magnetic 

 held. The latter are therefore similar to cathode-rays, which 

 are known to consist of material particles travelling with a 



