Now, in the experiment, the electrometer readily measured a current of 10⁻³ electrostatic units. Taking the charge on an ion as 3·4 × 10⁻¹⁰ electrostatic units, this corresponds to a production in the testing vessel of about 3 × 10⁶ ions per sec., which would be produced by about 40 expelled α particles per second. Each radiating particle cannot expel less than one α particle and may expel more, but it is likely that the number expelled by an atom of the thorium emanation is not greatly different from that expelled by an atom of the radium emanation. In section 133 it has been shown that, according to the law of decay, λ_N_ particles change per second when _N_ are present. Thus, to produce 40 α particles, λ_N_ cannot be greater than 40. Since for the thorium emanation λ is ¹⁄₈₇, it follows that _N_ cannot be greater than 3500. The electrometer thus detected the presence of 3500 particles of the thorium emanation, and since in the static method the volume of the condensing spiral was about 15 c.c., this corresponded to a concentration of about 230 particles per c.c. An ordinary gas at atmospheric pressure and temperature probably contains about 3·6 × 10¹⁹ molecules per c.c. Thus the emanation would have been detected on the spiral if it had possessed a partial pressure of less than 10⁻¹⁷ of an atmosphere. It is not surprising then that the condensation point of the thorium emanation is not sharply defined. It is rather a matter of remark that condensation should occur so readily with so sparse a distribution of emanation particles in the gas; for, in order that condensation may take place, it is probable that the particles must approach within one another’s sphere of influence.