On the Conservation of Electricity. 13 



(2) The formula for the exposure correction in terms of 

 degrees Centigrade is given incorrectly. 



(3) In the investigation of the movement of the zero with 

 temperature, we think that, by a mistaken interpretation of 

 the formula for y, Dr. Mills has inferred the existence of 

 turning-points, of which in reality it affords no indication. 

 If, owing to the ambiguous definition of y, we are wrong in 

 the above supposition, we think that the supposed higher 

 turning-points are too far outside the range of the experi- 

 ments to admit of the application of the formulae to these 

 determinations. 



(4) For the reasons given we consider Dr. Mills's ex- 

 periments with the open thermometer inconclusive; and the 

 inference drawn from them is rendered still more doubtful by 

 the results of our own observations. 



II. On the Conservation of Electricity, and the Absolute Scale 

 of Electric Potential. By Professor Silvanus P. Thompson, 

 D.Sc.,B.A* 



1. r T^HE Law of Conservation of Electricity . — In art. 35 of 

 Clerk Maxwell's ' Electricity and Magnetism ' the 

 following paragraph occurs : — 



" While admitting electricity; as we have now done, to the 

 rank of a physical quantity, we must not too hastily assume 

 that it is, or is not, a substance, or that it is, or is not, 

 a form of energy, or that it belongs to any known category 

 of physical quantities. All that we have hitherto proved is 

 that it cannot be created or annihilated', so that, if the total 

 quantity of electricity within a closed surface is increased or 

 diminished, the increase or diminution must have passed in or 

 out through the closed surface." 



In the paragraphs immediately preceding that quoted, it 

 had been pointed out what experimental evidence there is for 

 the conclusion that, when the electrification is in any way pro- 

 duced, the algebraic sum of the + and — electrifications is 

 zero. It results at once from this that, whatever the ultimate 

 nature of electricity may be, it so far behaves like an incom- 

 pressible fluid as to be subject to a law, resembling the Hydro- 

 kinetic Law of Continuity, of the general form 



du dv die n 



dx dy dz 



where u, v } to are the three components of the flow of electri- 

 city resolved along rectangular coordinates. 



* Communicated by the Author. 



