On the Measure of Work in the Theory of Energy. 291 



circuit is closed, the polarization, capillary constant, and depres- 

 sion are constant. 



By " increase of the surface of contact " of mercury and di- 

 lute acid two things might be understood — (1) moistening fresh 

 parts of the mercury which had hitherto been dry, or (2) 

 further recession from each other of parts already wetted. In 

 all the above phenomena the second is to be understood. This 

 can be shown by the following simple experiment. 



On a dry surface of mercury a broad drop of dilute sulphuric 

 acid is placed, which is then removed by a pipette, so that only 

 a wet spot remains. If this spot be perforated by an iron point 

 it polarizes and contracts immediately. But we only see a dis- 

 tortion of the entire surface, such as would take place upon the 

 surface of a stretched caoutchouc balloon upon which was a 

 damp spot and from which air emerged. The individual details 

 at what is ordinarily the jagged edge of the spot, as well as the 

 outlined part of the dry surface, remain separately recognizable 

 during the distortion, as if they had been drawn upon a caout- 

 chouc membrane ; and after the polarization has ceased they 

 return to their old position. 



On Young's view, according to which the constant of capil- 

 larity is regarded as a superficial tension, the result last 

 adduced (namely, that when the circuit is open the constant 

 increases on expansion) would be simply expressed thus — that 

 the surface of mercury behaves like an ordinary elastic 

 membrane, the tension of which increases when the membrane 

 is stretched. 



XXXV. On the Measure of Work in the Theory of Energy. By 

 Robert Moon, M.A., Honorary Fellow of Queen's Colelge, 

 Cambridge*. 



TO my former remarks on this subject f I desire to add the 

 following. 

 1. I cite the following sentences of Professor Maxwell as ex- 

 pressing generally accepted views : — 



" Work is done when resistance is overcome ; and the quantity 

 of work done is measured by the product of the resisting force 



* Communicated by the Author. 



t See ■ Philosophical Magazine ' for September 1873. 



U2 



