Prof. J. H. Poynting on Osmotic Pressure. 291 



that which must occur in an electrolytic conductor. If D is 

 the " displacement " or " induction " in an electrolyte, and if 

 fi is the factor of decay per second, fiD is the quantity dis- 

 appearing per second and dissipating its energy as heat. This 

 may be equated in the steady state to the new " displace- 

 ment" or u induction" introduced per second per square 

 centimetre, or to the current- density C. Hence 



C=^=^, (4) 



where E is the slope of potential, and K is the specific induc- 

 tive capacity. But Ohm's law gives us 



0=5 (5) 



p 



where p is the specific resistance ; whence 



P= ^K (,,) 



Returning to equation (3) , we see that if n is constant, rj 

 varies inversely as X. For instance, when the temperature 

 rises the molecules have more energy, the breaking down of 

 structure is more frequent, and X is greater. Probably n is 

 not very much altered, though it doubtless tends to decrease. 

 Hence t] should decrease, and this is in accordance with 

 observation. On the other hand, when a salt is dissolved in 

 a liquid, if, as we are going to suppose, it makes the mole- 

 cules on the average less energetic by partially combining 

 the more energetic solvent molecules with the less energetic 

 salt molecules, they are on the average rather further from 

 instability, X is less and t) is greater. This again agrees with 

 observation. 



At the same time the specific electric resistance p is dimi- 

 nished. This would require that in (6) either //, or K, or 

 both, should be increased, probably both ; and this brings out 

 a point which must be noted, that the factor of decay X in (3) 

 is not likely to be the same as fi in (6) ; for while one relates 

 rather to the molecules and their relative positions, the other 

 most probably relates to the atoms and their positions in the 

 molecules. 



Maxwell (Proc. Roy. Soc. cxlviii. 1873) gave an account 

 of some experiments which he made to test this view of liquid 

 viscosity by shearing a liquid and looking out for double 

 refraction. He could only observe it in the case of Canada 

 balsam, in which it had already been found by Mach, and 

 here the " rate of relaxation" was so great that he could not 



Y2 



