Question of the Homogeneity of y-Rai/s. 751 



subtracted from the gross leak obtained in the usual way the 

 curve was exponential right up to 20 cm. The amount of 

 this constant radiation bore about the same ratio to the 

 amount obtained in the first experiment as the ratio of the 

 quantities used now and before, and it seemed quite possible 

 that it was due to a primary radiation more penetrating than 

 7-rays, which could only be detected when the latter were 

 very much reduced in intensity. This, however, was disproved 

 by the next experiments. The two small windows of the 

 electroscope were blocked up by 0'3 cm. of sheet lead as 

 thoroughly as could be done without interfering with the 

 microscope or 'completely shutting off the light from the 

 lamp. This constant radiation was then greatly reduced. By 

 removing everything from the table except the apparatus 

 used, and by covering with sheet lead the microscope and 

 also the rubber cork by which the leaf system and the 

 charging rod were held in position, the constant radiation 

 was entirely eliminated. Three sets of readings were obtained 

 on different days for the absorption over the range of 10 to 

 20 cm., and in all the three cases the rays from the radium 

 were absorbed exponentially (\= '050 as before), the leaks 

 actually obtained being corrected only for the natural leak 

 of the instrument, obtained by removing the radium out of 

 the laboratory, and keeping everything else as it had been 

 during the series of measurements. When it is considered 

 that for the greatest thicknesses of lead employed the cor- 

 rected leaks vary from about 0*1 to 1*0 division per minute, 

 the natural leak being constant at 3*60 divisions per minute, 

 the necessity of shutting off every secondary effect may be 

 realised. An experiment was indeed made by exposing a 

 corner of one of the windows, and the corrected leak was so 

 doubled. As previous experiments indicate, brass or other 

 metals would be useless for this work. Lead alone can be used 

 with confidence in such measurements. Fig. 16 (PL XII.) 

 represents the absorption of the 7-rays by lead over the whole 

 range explored. From 2 to 12 cm. total thickness of lead 

 traversed, 6*7 mg. of radium bromide was used as source. 

 From 10 to 22 cm., 31 mg. were used in two series of 

 measurements and 45 mg. in a third. By making use of the 

 values of the rates of leaks at two points common to two 

 curves the composite absorption curve shown in fig. 10 is 

 obtained. The second half of the curve is the better of the 

 two series of readings obtained with the 31 mg. source. It is 

 plotted as obtained. The first half is that obtained with the 

 6*7 mg. source, each value of which has been multiplied by a 

 constant factor. In curve A (PI. XII. fig. 16), obtained with 

 31 mg., it may be noticed that there is a decided irregularity 



