Additional Phenomena of the Radiometer. 377 



of the vanes takes place — the reason being, according to the 

 present theory, that no electricity can pass, air of ordinary 

 density being a non-conductor. On producing exhaustion 

 the rotation commences and increases up to a certain point, 

 because rarefied air is a conductor of electricity. After a 

 high degree of exhaustion is attained, the rate of rotation 

 diminishes, because vacuum is a non-conductor of electricity, 

 and an approach to that condition has a retarding effect. 

 As the exhaustion proceeds, the effect of the friction at the 

 pivot becomes more prominent, which, however, appears from 

 the experimental results to be, after all, a very minute quan- 

 tity. A degree of exhaustion Avhich actually stopped the 

 vanes was not reached. I can see no a priori reason why 

 the stoppage should occur with the same degree of exhaustion 

 as that which would prevent the passage of a galvanic spark 

 under particular conditions. The vanes continued to move 

 when the degree of exhaustion was considerably greater than 

 that at which the spark ceased to pass under the particular 

 conditions arranged for the comparison, 



On further trial I failed to verify the statement made in 

 the May JSumber, that the presence of a magnet in the neigh- 

 bourhood of a radiometer affected the rate of its motion. On 

 the contrary, I proved by more careful experiments that the 

 streams of a magnet had no perceptible effect on the move- 

 ment of the vanes. This result might have been anticipated 

 from the circumstance that the electric streams of the theory 

 traverse the vanes transversely, so that the addition of mag- 

 netic streams (with which, according to hydrodynamics, the 

 electric streams may coexist) produces no variation ofaetherial 

 density at the vanes, and therefore no motive force tending to 

 give them motion. The case would be different for a set of 

 streams symmetrical with respect to an axis, inasmuch as addi- 

 tional streams resolved transversely to the axis would, on one 

 side of it, coincide in direction with the original streams re- 

 solved in the same direction, and on the other side be opposed 

 to them, so that at the axis there would be a gradation of 

 setherial density tending to impress motion. 



I think it right to explain here that the Theory of the Ra- 

 diometer I have proposed is wholly founded on those a priori 

 principles (stated at the beginning of the article in the Sep- 

 tember Number) which form the basis of the theories of the 

 different physical forces which I have for a long time been 

 engaged in discussing and verifying, and that consequently 

 it comes into no kind of competition with any empirical theory 

 which the experimentalist may be able to certify by the means 

 at his command. An established empirical theory might, how- 



