358 On the Electricity of Drops. 



the charge will be accompanied by an increase in the potential 

 energy. Thus there will, on account of the property which 

 may be expressed as a specific attraction of the atom for 

 electricity, be a tendency for the atom to acquire the definite 

 charge CQ. If, however, the atom has a definite charge it 

 has a definite valency. We must remember, however, that 

 when chemical changes are going on, the charges gained by 

 one atom are lost by another ; and it is the total change 

 in the potential energy which determines whether the change 

 shall go forwards or backwards. Thus in some cases we may 

 have an atom charged to an extent which involved an increase 

 in its potential energy, because the abnormal charging of 

 this atom may have involved such a decrease in the potential 

 energy of some other atom as to more than compensate for the 

 increase in that of the atom under consideration. In this 

 case the atom would not have its normal valency. 



It would appear that if the atoms possess this specific 

 attraction for the two electricities, a large rotating body 

 ought to produce a magnetic field. For consider a substance 

 the atoms in whose molecules attract the two electricities 

 with different intensities, and suppose, for example, that 

 the electro-negative component is the more energetic of 

 the two : then, in consequence of the uncompensated at- 

 traction of this atom for a negative charge, there will be 

 in the neighbourhood of the atom an electric intensity in 

 the same direction as that which would exist if the atom 

 had a positive charge. This effect may be inappreciable 

 at finite distance from the molecule and produce no ex- 

 ternal electrostatic effects. When, however, the molecule is 

 set in motion, the specific attraction of the molecule would 

 make the positive electric tubes move in a different way to 

 the negative ones. This differential motion of the tubes would 

 produce a magnetic field of the same general character as 

 that due to a positive charge moving in the direction of the 

 molecule. In the case of a rotating sphere the maximum 

 magnetic force at the surface would be proportional to ©a 2 , 

 where co is the angular velocity of rotation, and a the radius 

 of the sphere. If we take the earth's magnetic force as an 

 index of the superior limit of the magnetic force due to a 

 rotating sphere of the size of the earth, we find that the mag- 

 netic force due to a sphere 1 foot in radius rotating 100 times 

 a second would not exceed more than about one hundred 

 millionth part of the earth's magnetic force. 



I have much pleasure in thanking my assistant, Mr. F. 

 Everett, for the assistance he has given me in these expe- 

 riments. 



