967 
§ 11. Further observations upon ferrous sulphate and ferric. sul- 
phate. After the conclusion of the investigation treated in Comm. 
N°. 1295, we turned to the determination of the water contained in 
the preparations ferrous sulphate I and ferrous sulphate III. 
Prof. van -Irarrir kindly investigated the preparations and found 
that they contained ferric as well as ferrous sulphate. They cannot 
therefore be taken as a reliable basis for calculations of the number 
of magnetons, and to make these possible the measurements will 
be repeated with purer preparations. 
The quantitative result arrived at in Comm. N°. 1296 concerning 
the appearance of disturbances of the first kind in Curm’s law and 
the possibility of finding the constant of Curte for these substances 
by means of a correction, still retains its value. 
As regards the ferric sulphate, which the measurements in § 4 of 
Comm. N°.1295 referred to, the admixture of water may be put at 
about '/, in first approximation. The molecular susceptibility of ferrous 
sulphate is therefore ‘/, smaller than that of ferric sulphate, so that 
valency shows its influence in this iron salt also; all this in contra- 
diction to what was observed in § 4. 
We must also remark, that the sign and the order of magnitude 
of the corrections which would be necessary to deduce the number 
of magnetons for the pure materials from the measurements of the 
ferrous sulphate I of our Comm. N°. 1295 and those of the crystal- 
lized ferrous sulphate of KAMERLINGH ONNES and Perrier in Comm. 
N°. 122a, make it seem possible that there is a double analogy between 
ferrous sulphate and manganese sulphate. Just as in manganese sul- 
phate the number of magnetons in the crystallized and in the anhy- 
drous substance is equal, the same would be found for crystallized 
and anhydrous ferrous sulphate (viz. 26) (if for the anhydrous sub- 
stance Curtk’s constant is calculated with the help of the correction 
by A’ = 31°), and in further analogy with manganese sulphate, this 
number with ferrous sulphate is also one less than in the solution, 
if for the latter one may take the number, that has been found by 
WILIs *). . 
Should the disappearance of A’ with the introduction of water 
molecules be ascribable to the increase of distance between the iron 
atoms which is caused thereby, then it would be possible that with 
different contents of water of crystallization A’ decreases with the 
increase of the number of molecules of water of crystallization. We 
intend therefore, to examine a salt in this respect, that crystallizes *) 
a P. Weiss. Journ. de physique 1911. p. 977. 
*) Compare the investigation of Mlle Feryris, G. R. 153, p. 668. 1911 on the 
63* 
