176 THE DISSOCIATION THEORY. chap.
measuring the attraction suffered by the flask empty, and when filled with water, then from the value obtained with the solution, the part due to the dissolved salt can easily be calculated. The attraction is proportional to the quantity of dissolved salt. If the flask contains 1 gram-molecule of dissolved salt, the attraction is a direct measure of the molecular magnetism. In the same way the atomic magnetism of a gram-atom of iron can be determined. Wiedemann found that all the ferrous salts possess the same molecular magnetism ; thus he obtained for the sulphate, the nitrate, and the chloride, the relative numbers, 3900, 3861, and 3858 ; whilst for the ferric salts he obtained as mean value, 4800. The nitrate, sulphate, and chloride of nickel gave 1433, 1426, and 1400; manganous sulphate, nitrate, acetate, and chloride gave 4695, 4693, 4586, and 4700 ; and cupric nitrate, acetate, and chloride, 480, 489, and 477.
If we set the atomic magnetism of iron in ferric salts = 100, we obtain the following values for the magnetism of a gram-atom of the metal in the salts: in manganous salts, 100*4 ; in ferrous salts, 831 ; in cobaltoua salts, 67'2 ; in nickelous salts, 30*5 ; in didymium salts, 22*6 ; in cupric salts, 10*8 ; in eerie salts, 10*3 ; and in chromic salts, 41*9.
It is noteworthy that the temperature coefficient for the teinporary magnetism is almost the same for all salt solutions ; it is given by —
m, = m,(l - 000325 t)
where t is the temperature (centigrade), and m, and m^ the temporary magnetisms at i^ and 0*^.
Eecent and more exact determinations by du Bois and Liebknecht (8) have proved that the atomic magnetism is not strictly additive, but, on the other hand, the additivity mentioned is so general that it can be applied not only to solutions, but also to crystallised salts, in which the dis- sociation is very small if it takes place at all.
Natural Rotatory Power in Solution.— Some few organic compounds are capable of rotating the plane of polarisation of light passed through them. If, therefore, we
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