Rectilinear Electrical Current on itself. 145 



Gore's sphere, which touches each of the two rails in almost a 

 mathematical point, the heat developed at the point of contact 

 may in fact be one of the causes of motion : if we were unaware 

 of the repulsion which ought to be developed on this point inde- 

 pendently of the heat, the cause of the phenomenon would have 

 to be attributed to the latter. In the case of the vibrating blocks 

 of Trevelyan this explanation becomes less probable, because the 

 weight of the upper block ought to render the two surfaces in 

 contact much greater. In fact if their surfaces are increased n 

 times, the heat developed, and the resultant sudden expansion, 

 ought to be r? times less. If now one of the two is a liquid 

 metal, like mercury, which assumes exactly the extent of surface 

 of the solid conductor immersed in it, and which ought therefore 

 to touch this in all points, the expansion produced by the heat 

 developed in these points of contact ought to be very small. We 

 nevertheless see that the repulsion takes place; and its effect is 

 readily distinguished from that produced by the heat which 

 causes the mercury and all the vertical parts of the conductor 

 to expand, by the fact that it ceases instantaneously with tha 

 rupture of the circuit, whereas heat requires a tolerably long 

 time to become dissipated. In observing, through the micro- 

 scope, the apparatus just described, as soon as the two cups are 

 connected with four or six Bunsen's elements arranged in a series 

 of elements of double surface, as indicated for the experiments 

 with the floater, we see the disc suddenly sink, and then the 

 moveable conductor rise about a tenth of a millimetre. This 

 motion is immediately succeeded by another in the same direc- 

 tion, but far slower, due to the expansion by heat. When the 

 circuit is broken, either after the lapse of a few seconds or after 

 ten minutes, the conductor is seen at the same moment to fall 

 again, exactly as it had been raised at the commencement, and 

 then sink with great slowness in consequence of cooling. 



This, Sir, is what we had to communicate to you. If our 

 experiments still do not seem to you entirely conclusive, you will 

 oblige us by saying so before the publication of this communica- 

 tion, so that we may, if possible, be able to modify them or am- 

 plify them according to your desires. 



lteceive, Sir, the assurances of our very distinguished con- 

 sideration*. 



J. G. S. van Breda. 



Teylerian Laboratory, Haarlem, 

 October 1861. 



* Though this letter is throughout in the plural number, it bears the 

 signature of M. van Breda alone. — Edit. 



Phil Man, S. 4, Vol. 23. No. 152. Feb. 186.2. L 



