of the ol Particles from Radium C and Thorium C. 543 



taken. In this way the whole of the range of the a 

 particle from about 1 cm. from the source to the end 

 was covered in a series of experiments, particular care 

 being taken with the last half centimetre of the range. 



The strengths of the different sources used were not equal 

 however, and it was impracticable to measure their effective 

 a ray activities directly. By taking as a starling point a 

 well-distributed series of observations it was possible to find 

 by trial and error a constant factor by which to multiply 

 each of the observations of another series, so as to make the 

 overlapping portions of the two curves coincide. Working 

 in this way, a factor was found for each series of observations 

 so that the whole of the observations were reduced to the 

 same scale. Numerous opportunities for checking the values 

 of these factors were afforded by the overlapping of different 

 carves. The whole of the observations were thus found to 

 fit well on one smooth curve. 



Lack of space prevents the giving of the results obtained 

 in tabular form, but they are shown graphically in figs. 2-6 

 (PL XVII.) where the circles represent observational values. 

 The abscissae represent distances from the source in cms., and 

 the ordinates ionization currents in arbitrary units, chosen to 

 make the maxima of the curves unity. Figures 2 and 3 

 refer to radium C, fig. 2 showing the general form of the 

 ionization curve, and fio-. 3 showing the curve near the end 

 of the range on larger scales. The complete ionization curve 

 of the a particles from thorium C is shown in fig. 4,, and the 

 curve near the end of the range of thorium C 2 is shown on a 

 larger scale in fig. 5. The upper full-line curve of fig. 6 

 shows the results observed near the end of the range of 

 thorium b and the dotted curve is the extrapolated curve of 

 the ionization due to the a particles of thorium C 2 . The 

 differences between tbe ordinates of the two curves are 

 plotted as the ordinates of the lower curve of fig. 6, which 

 represents the ionization curve due to thorium Q x alone. 



§ 4. The General Form of tlie Ionization Curves. 



An important point to be noticed in these curves is that 

 the observational points near the end of the range of each 

 type of a particle fall very closely on a straight, line. This 

 is brought out in figs. 3, 5, and 6, where the respective lines 

 of closest fit are shown as broken lines. 



For radium (3 l the equation of the line is 



I ==3-53 (6-592-*), (1) 



I being the ionization current in the arbitrary scale of the 

 figure and x the distance along the ranee in cms. 



