754 Mr. & Mrs. Soddy and Mr. A. S. Russell on the 



It is obvious from the results set forth in the Table that 

 for the initial part of the range the absorption curves may 

 be straight, or may depart in either direction from the 

 exponential form according to the conditions of experiment 

 and the absorbing metal used. Thus, if in the original investi- 

 gations on y-rays, zinc or aluminium instead of lead had been 

 used as the absorbing metal, the curve obtained over the first 

 part of the range would have been found to be exponential. 



For disposition 1, the value of X rises in the case of aluminium 

 to about the normal value (X/d = 0*040), but for zinc, and still 

 more for tin, it rises beyond the normal value, while for lead 

 it diminishes, but does not reach the normal value. In dis- 

 position 3, which gives exponential curves for aluminium, 

 zinc, and tin, the value of X is from 1/5 to twice the normal. 



A further result may be mentioned which has been obtained 

 by a new disposition, in which a narrow cone of rays and a 

 shorter cylindrical ionization chamber connected to a separate 

 electroscope have been employed. The curves for aluminium 

 and lead are concave and convex respectively, while those 

 for zinc and tin are straight over the whole range. X for zinc 

 is 0'268, nearly the normal value, but for tin it is 0*355, 

 about 26 per cent, too great. Contrasting this result for zinc 

 with those given in Part II. with truncated hemispheres, we 

 see that lead is normal in the hemispherical and abnormal in 

 the cylindrical ionization chamber, while with zinc the converse 

 is true. Zinc, in the disposition last described, is the only case 

 so far found of a metal obeying the simple exponential law 

 with the normal value for X, from the thickness sufficient to 

 absorb /3-rays up to the greatest thickness tried (6 cm.). 



Finally, a number of experiments may be referred to on 

 the variation of the absorption coefficient X with variation of 

 the different components of the disposition employed, over 

 ranges of thickness greater than the equivalent of 1 cm. of 

 lead. The chief disposition used (denoted by A) was to place 

 the absorbing screens directly over the source of y-radiation 

 | at a distance of about 14 cm. below the electroscope, though 

 some experiments have been carried out by clamping up the 

 absorbing screens to act as base and leaving the ionizino- 

 source bare (B). It was found in general that X for any 

 substance varies within certain limits with practically every 

 important change in the disposition. The substances are 

 divided into classes according to density. Class I. denotes 

 lead and mercury, Class II. comprises substances in density 

 from copper to magnesia brick, and Class III. from sulphur 

 to pine-wood. For an electroscope made entirely of one 

 material, if disposition A be employed with a radium source, 

 the absorption coefficients vary slightly with the thickness of 



