98 



Dr. G. Gore. 



The mechanical law of action and reaction of equal and opposite forces 

 also requires the primary movements of the mercury and solution to 

 be in opposite directions. If this hypothesis is correct, the unequal 

 adhesion of the mercury and solution to the glass is a necessary con- 

 dition of the simultaneous advance of the two liquids in the same 

 direction. If the tube was as free to move as the liquids, it would of 

 course move in an opposite direction to them. The number of physical 

 actions, however, involved in the case is probably much greater than 

 is represented by the above explanation. 



Herschel observed that " the peculiar action is only exerted at the 

 common surface of the fluids;" and nearly all the evidence supports 

 the conclusion that it is primarily an action of films and not of the 

 mass of mercury or of solution within the capillary tube. 



Even when during the passage of a current, the column of liquid is 

 prevented by adhesion from rising or falling in the capillary tube, 

 there is probably a motion occurring in the contiguous portions of the 

 mercury and solution, though not usually visible even by the aid of a 

 microscope. 



Armstrong's experiments ("Phil. Mag." vol. 23, 1843, pp. 194- 

 202) also indicate that the movement of the mercury is affected by an 

 action of adhesion between the surface of the liquid and glass within 

 the tube, because in his experiments, when the silk thread, acting 

 as a fulcrum, was removed, the amount of liquid flowing in one 

 direction was equal to that flowing in the other, and there was no 

 manifest transfer of the mass. 



In all cases with a globule of mercury in a pool of liquid, the outer 

 film of mercury and the film of liquid in contact with it appears to 

 move in the same direction ; the mass of the liquid, and also that of 

 the interior of the mercury, must, therefore, of necessity, move in a 

 direction opposite to that of the films (compare Herschel, " Phil. 

 Trans." 1824, p. 165). 



The opposite movements obtained with dilute sulphuric acid and 

 with solution of potassic cyanide might also be explained on another 

 supposition, viz., that one of these liquids adheres more strongly, and 

 the other less strongly, than mercury to glass ; but this does not appear 

 to be a fact. 



Although the phenomenon of to-and-fro motion appears to be 

 essentially due to a combination of surface actions, it is not confined 

 entirely to capillary spaces, the motion (as is well known) takes place 

 as readily with a large globule of mercury in a pool of liquid as in a 

 capillary tube. (See also " Movements of Liquid Metals and Electro- 

 lytes in the Voltaic Circuit," " Proc. Roy. Soc," vol. 10, par. 35.) 

 The to-and-fro movement, in the case of a globule of mercury in a 

 pool of liquid, is dependent upon the circumstance that the solid 

 surface of the containing vessel, upon which the mercury and film of 



