918 

 TABLE XIII. 



shows some ii-regnlarities, wliich are greater tliaii we should have 

 expected considering the degree of accuracy of the experiments. 



§ 17. Ferric ammoniumsidphate {iron alum). 



This substance will be seen to follow Curie's law throughout the 

 whole range of temperature that was examined. This, according to 

 the theory developed by Oosterhuis in Suppl. No. 31, would be 

 owing to the great moment of inertia which a molecule of this 

 substance ujidoubtedly possesses. 



It may also be explained by the theory which FoËx gives fol- 

 lowing Weiss (C. R. T. 157, p. 1145. 1913). In fact iron alum is a 

 substance crystallizing in the regular system and according to FoËx, 



for such substances the line — ^/'(T) will be a straight line at all 



temperatures. 



This property of ferric alum may also be regarded from another 

 point of view if we consider the latest results by Kamerlingh Onnes and 

 Perrier (Comm. N". 139J). In § 3 we pointed out that interposition 

 of water molecules between the molecules of ferrous sulphate, as 

 occurs when this salt crystallizes with water of crystallisation, causes 

 the deviation from Curie's law to disappear, and thus diminishes A. 



