422 Lord Blythswood and Dr. E. W. Marehant. 



that in the calcium, strontium and barium group the amount of 

 ^absorption does not increase rapidly with the increase in atomic weight. 

 It may therefore be generally stated that the absorptive power of 

 salts having different bases, but the same acid radicle, increases with 

 the atomic weight of the metal forming the base, though not accord- 

 ing to any definite law. 



This agrees with the results obtained by Dr. Gladstone and Mr. 

 Hibbert. 



Effect of Bays of different Penetrative Powers. 



As stated above, the penetrative power of the X rays used did not 

 liave much effect on the apparent relative absorptive powers of the 

 different solutions, but as a still further check on the power of the rays 

 used, the time taken for the developer to bring the strip on the plate 

 photographed, through 20 mm. of water, to sufficient density (allow- 

 ance being made for the temperature) was always observed, and if this 

 differed much from that necessary on a normal plate, the experiment 

 was repeated. Measurements made on these rejected plates, however, 

 showed that within limits of experimental error, there was no differ- 

 ence between absorptions thus obtained and those calculated from mea- 

 surements made on fully exposed negatives. Measurements were also 

 made using rays of different penetrative power, and developing for the 

 same time; but the plates obtained admitted of only such rough 

 measurement that here again no differences were detected, the strips 

 being either too dense, or the differences in intensity at the two ends 

 corresponding to the thick and thin ends of the wedges being too 

 slight, to admit of accurate photometry. It may be stated generally 

 therefore that the rays of different penetrative powers do not appear 

 to have any effect on the relative absorptive powers of the different 

 solutions. 



Effect of Strength of Solution. 



In order to determine the effect produced by using solutions of dif- 

 ferent strengths, a number of measurements were made on various 

 salts : (1) Lead acetate, (2) potassium bromide, (3) iodine in potas- 

 sium iodide, (4) nickel chloride, (5) sodium thiosulphate (strong solu- 

 tion). 



Curves are plotted below showing the relation between the amount 

 of salt in solution and the thickness necessary to produce the same 

 amount of absorption as 20 mm. of water. From these numbers it is 

 clear that the amount of absorption is not proportional to the number 

 of molecules of the salt traversed by the ray, and does not appear to 

 follow any simple law. The connection appears to be logarithmic from 

 the shape of the curves. 



