172 Mr. Herschel on certain Motions produced in Fluid [Sept. 



8. To this end, if we operate on a considerable mass of mer- 

 cury, and, instead of covering it with the acid, merely moisten 

 it and the containing vessel, making the circuit as before, only 

 by the medium of the thin film of acid which adheres, the cir- 

 culation of the mercury will be not less violent ; but it will 

 then be evident that the origin of the motion is in the mercury 

 itself, the acid film being (so far as mechanical impulse is con- 

 cerned) merely passive, and dragged along by its adherence to 

 the mercury, coating it frequently with a stratum so thin as to 

 reflect iridescent colours over its whole surface, and render the 

 phsenouienon extremely beautiful. The motion of the mercury 

 consists in a continual radiation of its superficial molecules 

 from the point nearest to the negative pole, by which it is kept 

 in a constant state of circulation, each particle being urged 

 along the surface from the negative to the positive pole and 

 returning along the axis. Were the mercury insulated from 

 contact with the bottom of the sustaining vessel, and devoid of 

 adhesion to the liquid, the momentum of the portions going 

 and returning would be equal, and the centre of gravity of the 

 whole mass would remain at rest; but by reason of the friction 

 and adhesion of the fluid metal to the vessel and liquid, these 

 re-act on the globule in a direction contrary to that of the super- 

 ficial currents, and the centre of gravity accordingly advances 

 in that direction, or towards the negative pole. When this 

 motion cannot take place, the internal current, having all one 

 uniform direction, forces its way outwards to the negative pole, 

 distorting and elongating the figure of the mercury in propor- 

 tion to its energy. If the metal be oxidated, so as to give a 

 certain tenacity to the superficial film, the radiating currents 

 pursue their course under it; and the supernatant fluid, being 

 thus defended from their action, remains at rest. In this case 

 the only indication of their existence is the protuberance pro- 

 duced by the resultant interior streams. 



9. A number of singular appearances are explained by this 

 internal current. In some cases the mercury throws out pro- 

 jections or probosces of inordinate length, which take the direc- 

 tion of the electrified wire, and follow all its motions. The re- 

 sultant interior current is in this case directed along the axis of 

 the proboscis from its root to its extremity, which thus becomes 

 an indication of a very powerful radiation along its surface in an 

 opposite direction. In others, the mercury flattens throughout 

 its whole extent, and, when this is the case, it is always covered 

 with a thick coat of oxide. In these circumstances the super- 

 ficial currents tend from the circumference towards the centre 

 of the flattened mass, and the interior stream tends from the 

 centre outwards in all directions, in a horizontal plane, thus 

 continually urging the circumference farther and farther out, by 

 diminishing the radius of curvature of the vertical section of its 

 edge, 



