Mr. W. H. Preece on the Electric Light. 29 



which is within the limits of the quantities obtained in the 

 case of the spheroidal drops on liquids. 



That by this formula K varies nearly as Q, and not as Q 2 , 

 is not to be wondered at, because in the first place the for- 

 mula only professes to represent an approximation to the true 

 state of affairs, and in the second place it is only at distances 

 and pressures at which the ordinary laws of conduction of heat 

 cease to apply that it professes even approximately to repre- 

 sent it. 



The whole of these investigations are unsatisfactory to this 

 extent — that I have been unable, from a consideration of the 

 molecular encounters themselves, to discover what is the actual 

 distribution of velocities even in the simple case of two parallel 

 surfaces. This is hardly to be wondered at ; for the problem 

 is extremely complicated, and evidently depends upon the 

 undecided point in molecular physics, namely the proportion 

 of the molecules encountering in a given direction that are 

 thrown off in the various other directions. We might very 

 well assume, with Maxwell, that they are uniformly distributed 

 in every direction after the encounter ; but even this does not 

 simplify the question sufficiently to bring it within my present 

 powers of solution. 



III. The Electric Light. By W. H. Pkeece, Memb. Inst. 

 C.E., V.P. Soc. T.E., Electrician General Post- Office, 8fc* 



1. rilHB theory of the electric light cannot be brought abso- 

 J- lutely within the domain of quantitative mathematics, 

 for the reason that we do not yet know the exact relation that 

 exists between the production of heat and the emission of light 

 with a given current ; but we know sufficient to predicate that 

 what is true for the production of heat is equally true for the 

 production of light beyond certain limits. 



The work done in a battery, or any source of current-elec- 

 tricity, is expended outside the battery in a closed circuit in 

 the form of heat. When this heat acquires a certain tempe- 

 rature per unit mass, we have light. If the heat be confined 

 to a mass of metal wire like platinum, we have light by in- 

 candescence ; if it be expended in the transference of minute 

 particles of incandescent matter like carbon across an air-space, 

 we have the electric arc. The exact relations between current, 

 heat, temperature, mass, and light have yet to be determined 

 by experiment. 



2. The arc is thus a form of energy developed in one point 

 * Communicated by the Author. 



