Absorption of Rontgens Bays by Solutions of Metallic Salts. 413 



likely to be adequately considered unless the sources of weakness in 

 the respective methods are clearly understood. 



Again, there has been an increasing tendency of late years,, in 

 most lines of physical research, to add figure to figure, and to aim 

 at higher accuracy, or at least the appearance of higher accu- 

 racy. If people were content with giving magnetic force to four 

 significant figures, the majority of the sources of uncertainty 

 specified in this paper need awaken little apprehension. But now-a 

 days hardly any one is content with less than five significant figures, 

 and one occasionally sees six. I am not prepared to say that the reten- 

 tion of six figures is indefensible in the case of open range magneto- 

 graphs, when it is clearly understood that differences only are concerned, 

 but I do think that with unifilars of the Kew pattern, as hitherto made, 

 the least probable error we can reasonably expect in an absolute 

 measurement is two or three units in the fifth significant figure. 



§ 49. We have seen that there is every reason to expect that the values 

 of the horizontal force given by a unifilar are in error to a different extent, 

 according to the temperature and the magnitude of the force. This 

 must introduce a source of uncertainty into the comparisons effected 

 between unifilars at distant observatories through the intermediary of 

 a travelling instrument. To adequately forewarn those engaged in 

 such comparisons may be to put them in a position to obtain more 

 satisfactory results. 



§ 50. For the opinions expressed in this paper, and for the accuracy 

 of its conclusions, I am alone responsible ; but it is only proper that I 

 should acknowledge the valuable assistance given me by Mr. T. W. 

 Baker, Chief Assistant at the Kew Observatory. For many years 

 Mr. Baker has taken the great majority of magnetic force observations 

 at Kew, and his experience in determining the " constants " of collima- 

 tor magnets is probably quite unique. Mr. Baker has always been 

 ready to place his great practical knowledge of the subject freely at my 

 disposal, and I have also to thank him for carefully carrying out a 

 variety of special experiments, made to elucidate doubtful points at 

 various stages of the inquiry. 



"The Absorption of Eontgen's Eays by Aqueous. Solutions of 

 Metallic Salts." By the Eight Honourable Lord Blyths- 

 wood, LL.D., and E. W. Marohant, D.Sc. Communicated by 

 Lord Kelvin, F.E.S. Eeceived March 11, — Eead June 15, 

 1899. 



The absorption of X rays by metallic salts is a subject that has not 

 received very much attention up to the present time, although it 

 appears to be of considerable importance. It seemed possible that if the 



