Various Gases by the a Particles of Radium. 335 



If no other radioactive substances were present, the curve 

 would show a continuation o£ the portion BP down to the axis 

 of x. in some such manner as PC 



If the ionization curve were completed in this way, the 

 area ABPCD would represent the total ionization due to the 

 a. radiation from RaC. If now the air were removed, another 

 gas substituted for it, and the air re-measured, a comparison 

 of the values obtained would give the result which this 

 research aims at. We may call it the specific ionization of 

 the gas. But the complete determination of the boundaries 

 of the area is so long and complicated a process as to render 

 this procedure impracticable. It can, however, be shown that 

 the product of the coordinates of a certain point on the curve 

 may be taken as a measure of the area of the curve, provided 

 certain assumptions are made. The point is at the intersection 

 of the top portion of the curve representing the effects of RaA 

 with the side of that showing the effects of RaC. The co- 

 ordinates of this point are comparatively easy to obtain. 



Xow, it might appear that it would be better to measure 

 of one time the whole of the ionization produced by the 

 particle, rather than to determine the ionization point by 

 point along its path : since, if this were done, it would no 

 longer be necessarv either to find the exact form of the ioni- 

 zation curve, or to depend upon the validity of assumptions. 

 "We might spread a layer of radioactive material on the floor 

 of an ionization- chamber, and so arrange the temperature and 

 pressure of the gas in relation to the dimensions of the 

 chamber, that all the a particles completed their paths within 

 the gas. But the potential gradient required to separate and 

 collect the ions made by the « particle is generally very 

 great. For example, in ethyl chloride at 30 cm. pressure 

 and ordinary temperatures, about 1000 volts per cm. is de- 

 sirable, if saturation is to be certain. With such gas it would 

 be necessary to make the height of the chamber about 4 cm., 

 ill order to allow all the a. particles to complete their ranges ; 

 even if the radioactive material were uranium or polonium. 

 Thus, a potential of 4000 volts would be required, and such 

 large electromotive forces are out of the question. If the 

 pressure of the gas were lowered, less electric force would be 

 sufficient ; but the paths of the a particles would be longer, 

 the chamber would need to be higher, and the total potential 

 as great as ever. 



It is absolutely necessary to use a narrow ionization- 

 chamber if sufficient electric force is to be obtained without 

 the use of enormous battery-power. Clearly it would be no 

 gain to use such a chamber if the radioactive material were 



