324 Prof. Brass: and Mr. Kleeman 



Oft 



on 



air at ordinary pressures and temperatures. A bundle of 

 copper tubes is placed in a vertical position over the hori- 

 zontal layer, so that only such a particles as move in a 

 direction nearly vertical are allowed to proceed upwards. 

 The bundle used at present contains about 100 tubes made of 

 thin copper, each 2 mm. in diameter and 1 cm. long. The 

 ionization-chamber consists of an upper horizontal plate of 

 :>lu minium, and a lower plate of brass gauze ; the distance 

 between the plates in most of the experiments described in 

 this paper was set at two millimetres. The ordinates repre- 

 sent the distances from the radium plate to the gauze ; and 

 the abscissae the leaks on the quadrant electrometer. The 

 absolute value of these leaks was not accurately measured, as 

 there was no object in knowing more than their relative 

 values; but it was calculated that the greatest leak measured 

 was about 10~ n ampere. 



Curve A, in fig. 1, shows the ionization due to the a, rays 

 when the dish was first prepared. It had been kept at a 

 bright red heat for some minutes after preparation. The 

 shape of the curve shows that all radioactive products except 

 the radium itself had been completely removed. On com- 

 parison with the similar curve on p. 730 of the paper already 

 cited, it will be seen that the new radium layer is thinner 

 than that from which our first results were obtained ; for the 

 maximum value of the ionization occurs at 2*85 cms. instead 

 of at 2'5. The greatest range is about 34 as before. This 

 means that the a particles from the bottom of the new layer 

 lose, in coming up through the material of the layer, a range 

 of 5*5 mm. measured in air : the corresponding amount for 

 the older layers was 9 mm. The new layer is nevertheless 

 nearly twenty times stronger than the old. The term layer 

 is, in fact, rather inappropriate. Actually the radium is not 

 distributed over the dish in a thin uniform sheet but is studded 

 over the surface in the form of small crystals which can be 

 easily seen under the microscope. Consequently a weaker 

 solution of radium bromide does not give, when evaporated, 

 a thinner layer; but simply a smaller number of crystals. The 

 various bundles, and in particular those from Ra, Emanation, 

 and IiaA do not therefore stand out any better from each 

 other when a weaker solution is evaporated, as we found on 

 making the experiment. But the greater purity of the new 

 layer gives the result which we formerly sought to obtain by 

 evaporating very weak solutions. Presumably the crystals 

 are now more regularly arranged, and are not overlaid by other 

 matter which acts as a screen. 



Since the radiation of the new layer is so strong, the 



