I 



254 Prof. IT. A. Rowland on Magnetic Attractions, 



the important distinction, indicated in the present communi- 

 cation, between spontaneous motion and motion produced 

 under arbitrary conditions. The above assertion applies only 

 to spontaneous motion. The recognition of this distinction 

 suffices to meet completely Professor Stokes's objections. 



At the end of a communication to the Philosophical Maga- 

 zine for June 1880, I stated that I did not expect to have 

 occasion to discuss any additional questions in Theoretical 

 Physics. The importance and novelty of the present com- 

 munication respecting spontaneous fluid-motion, the views 

 respecting which have only very recently occurred to me, 

 may, I think, be considered sufficient to justify my recurring 

 once more to the subject of the Analytical Principles of Hydro- 

 dynamics. 



Cambridge, February 21, 1881. 



XXXV. On the new Theory of Magnetic Attractions, and the 

 Magnetic Rotation of Polarized Light. By H. A. Rowland*. 



N a note published in the ' American Journal of Mathe- 

 matics,' and also in the Philosophical Magazine for 

 June 1880, I showed that the new action of magnetism on an 

 electric current, recently discovered by Mr. Hall in my labora- 

 tory, was essentially of a rotational character, and I showed 

 also that it was probably of the same nature as the rotation of 

 the plane of polarization of light. I have since published a 

 paper in the ' American Journal of Mathematics ' — " On the 

 General Equations of Electro-magnetic Action, with Appli- 

 cations to a new Theory of Magnetic Attractions, and to the 

 Theory of the Magnetic Rotation of the Plane of Polarization 

 of Light " — in which the subject is treated in full, and Max- 

 well's formula for the magnetic rotation of the plane of polariza- 

 tion deduced from the newly discovered action of magnetism. 



Mr. Hopkinson has recently drawn attention f to the fact 

 that Maxwell has inserted a certain " Rotatory Coefficient " 

 in his equations of resistance (' Electricity,' art. 303). Max- 

 well further states that the coefficient should be found, if 

 anywhere, in magnets, or, of course, in any magnetic field, as 

 it has now been found. But 1 believe Maxwell nowhere con- 

 nects this quantity with the rotation of the plane of polariza- 

 tion of light ; and hence I think it may be well to give here an 

 abstract of this portion of my paper. 



In the c Note ' before referred to, I thought that it would 

 be necessary to prove that the lines of electrostatic induction 



* Communicated bv the Author, 

 t Phil. Mag. Dec. i860, p. 430. 



