1864.] Dynamical Theory of the Electromagnetic Field, 531 



ducts of the action of heat on the ethylphosphates, may aid in determining 

 this question. Meanwhile the observation, already recorded, as to the 

 stability of ordinary ethylphosphoric acid and its salts in the presence of 

 permanganate of potassium, requires a word or two of further comment. 

 When argentic diethylphosphate is acted upon with iodine, the silver and 

 one atom of ethyl may be removed, and after treatment with finely divided 

 silver and a little oxide of silver to remove any iodine and hydriodic acid, 

 and then with excess of carbonate of barium, an ethylphosphate of barium 

 is obtained, which, unlike the ordinary salt, immediately reduces perman- 

 ganate of potassium ; perhaps the ethyl in this salt exists in a different and 

 less intimate form of combination. I am inclined to think that the per- 

 manganates will afford, in some cases, criteria for the detection of slight 

 differences in isomeric compounds, although it would be premature at 

 present to hazard an exact interpretation of the phenomena to which they 

 give rise. I may add that treatment of an ethylphosphate with strong 

 nitric acid fails to decompose the ethylphosphoric acid ; so that phosphoric 

 acid cannot thus be separated from this remarkably stable body. 



II. " A Dynamical Theory of the Electromagnetic Field.^^ By Pro- 

 fessor J. Clerk Maxv^ell, P.E.S. Received October 27, 1864;. 



(Abstract.) 



The proposed Theory seeks for the origin of electromagnetic effects in 

 the medium surrounding the electric or magnetic bodies, and assumes that 

 they act on each other not immediately at a distance, but through the 

 intervention of this medium. 



The existence of the medium is assumed as probable, since the investi- 

 gations of Optics have led philosophers to believe that in such a medium 

 the propagation of light takes place. 



The properties attributed to the medium in order to explain the propa- 

 gation of light are — 



1st. That the motion of one part communicates motion to the parts in 

 its neighbourhood. 



2nd. That this communication is not instantaneous but progressive, and 

 depends on the elasticity of the medium as compared with its density. 



The kind of motion attributed to the medium when transmitting light is 

 that called transverse vibration. 



An elastic medium capable of such motions must be also capable of a 

 vast variety of other motions, and its elasticity may be called into play in 

 other ways, some of which may be discoverable by their effects. 



One phenomenon which seems to indicate the existence of other motions 

 than those of light in the medium, is that discovered by Faraday, in which 

 the plane of polarization of a ray of light is caused to rotate by the action 



