196 Mr Herschel on the Mechanical Effects of 



effect is one purely dynamical. It is wonderful that Professor 

 Erman should not have apprehended this distinction, and the 

 fallacy of his explanation, as he has noticed the very violent 

 circulation which takes place in the drop — a circulation total- 

 ly incompatible with the state of statical equilibrium his theory 

 supposes. Yet, so satisfied does he rest of the truth of his 

 explanation, that, having found the same extension of the drop 

 not to take place on a solid metal, he observes, that, " there- 

 fore, the curvature of the two surfaces, mutually altered by 

 their increased attraction of cohesion, is the fundamental prin- 

 ciple of the phenomenon, whence all the remaining detail 

 flows." 



Mr Erman then describes an experiment in which mer- 

 cury and water being introduced into a capillary tube, and 

 electrified, the column of mercury advanced by starts towards 

 the negative pole. This motion he regards as, indeed, capa- 

 ble of rigorous explanation by the augmentation of the capil- 

 lary action of the water on the mercury ; but having also ob- 

 served that a drop of mercury electrified under water exhibits 

 motions precisely similar, though less marked, he hence con- 

 cludes, that attraction at sensible distances has a share in these 

 phenomena. 



Whoever repeats the experiment described in my paper, 

 where a drop of mercury is placed under sulphuric acid be- 

 tween the two poles, even many inches asunder, and has wit- 

 nessed the extraordinary activity with which it darts to the 

 negative pole, like a ball of iron to a powerful magnet, will 

 undoubtedly believe, as I myself did when I first observed an 

 effect so surprising, that a more evident case of attraction and 

 repulsion at a distance was never exhibited, and that a new 

 species of magnetism was here produced. Yet the analysis 

 given of this phenomenon in my paper is sufficient, I presume, 

 to convince any one of the absence of all traces of such attrac- 

 tions and repulsions, and to demonstrate the justice of the ex- 

 planation there given of it, viz. the reaction of the fluid and 

 the bottom of the vessel on the mercurial currents, which ra- 

 diate in all directions from the point in the globule opposite 

 to the negative pole, along its surface, and return along its 



