Thus as a result of the disintegration of the thorium atom, a series of chemical substances is produced, each of which has distinctive chemical properties. Each of these products is radio-active, and loses its activity according to a definite law. Since thorium has an atomic weight of 237, and the weight of the α particle is about 2, it is evident that, if only _one_ α particle is expelled at each change, the process of disintegration could pass through a number of successive stages and yet leave behind, at the end of the process, a mass comparable with that of the parent atom. It will be shown later that a process of disintegration, very similar to that already described for thorium, must be supposed to take place also in uranium, actinium and radium. The full discussion of this subject cannot be given with advantage until two of the most important products of the three substances thorium, radium and actinium, viz. the radio-active emanations and the matter which causes excited activity, have been considered in detail. =137. Magnitude of the changes.= It can be calculated by several independent methods (see section 246) that, in order to account for the radio-activity observed in thorium, about 3 × 10⁴ atoms in each gram of thorium suffer disintegration per second. It is well known (section 39) that 1 cubic centimetre of hydrogen at atmospheric pressure and temperature contains about 3·6 × 10¹⁹ molecules. From this it follows that one gram of thorium contains 3·6 × 10²¹ atoms. The fraction which breaks up per second is thus about 10¹⁷. This is an extremely small ratio, and it is evident that the process could continue for long intervals of time, before the amount of matter changed would be capable of detection by the spectroscope or by the balance. With the electroscope it is possible to detect the radiation from 10⁻⁵ gram of thorium, _i.e._ the electroscope is capable of detecting the ionization which accompanies the disintegration of a single thorium atom per second. The electroscope is thus an extraordinarily delicate means for detection of minute changes in matter, which are accompanied, as in the case of the radio-elements, by the expulsion of charged particles with great velocity. It is possible to detect by its radiation the amount of Th X produced in a second from 1 gram of thorium, although the process would probably need to continue thousands of years before it could be detected by the balance or the spectroscope. It is thus evident that the changes occurring in thorium are of an order of magnitude quite different from that of ordinary chemical changes, and it is not surprising that they have never been observed by direct chemical methods. Footnote 225: Crookes, _Proc. Roy. Soc._ 66, p. 409, 1900. Footnote 226: Becquerel, _C. R._ 131, p. 137, 1900; 133, p. 977, 1901. Footnote 227: Rutherford and Soddy, _Phil. Mag._ Sept. and Nov. 1902. _Trans. Chem. Soc._ 81, pp. 321 and 837, 1902. Footnote 228: