PRESIDENTIAL ADDRESSES. 549 



my mind that the difference of quality between 

 vitreous and resinous electricity, conventionally 

 called positive and negative, essentially ignored as 

 it is in the mathematical theories of electricity and 

 magnetism with which I was then much occupied 

 (and in the whole science of magnetic waves as we 

 have it now), must be studied if we are to learn 

 anything of the nature of electricity and its place 

 among the properties of matter. This distinction, 

 essential and fundamental as it is in frictional 

 electricity, electro-chemistry, thermo-electricity, 

 pyro-electricity of crystals, and piezo-electricity of 

 crystals, had been long observed in the old known 

 beautiful appearances of electric glow and brushes 

 and sparks from points and corners on the con- 

 ductors of ordinary electric machines and in 

 exhausted receivers of air pumps with electricity 

 passed through them. It was also known, probably 

 as many as fifty years ago, in the vast difference of 

 behaviour of the positive and negative electrodes 

 of the electric arc lamp. Faraday gave great 

 attention to it 1 in experiments and observations 

 1 Experimenta, Researches, Series 12 and 13, Jan. and Feb., 1838. 



