Propagation of Electric Waves along Conductors. 199 



41. Similar considerations hold for maximum critical tem- 

 perature and minimum vapour-pressure. A minimum vapour- 

 pressure at low temperature means, according to the equation, 

 a maximum for a/b : and a maximum of a/b is the same as a 

 maximum of the hypothetical ci'itical temperature, and there- 

 fore a fortiori of the real critical temperature. The minimum 

 pressure will move towards the substance of lower vapour- 

 pressure, with rise of temperature, and may either disappear 

 or else exist up to the critical region. It is easy to under- 

 stand what the p-t diagram would be like in this case. But 

 the case hae not been observed yet. A minimum vapour- 

 pressure was found by Roscoe and Dittmar * for hydrochloric 

 acid and water and for nitric acid and water, by Konowalow | 

 for formic acid and and water, by Friedel % for hydrochloric 

 acid and methyl ether. I hope to have an early opportunity 

 of trying the latter combination, which will not require tem- 

 peratures as high as the combinations with water. 



42. One remark may be added. In the condensation of 

 some substances it is assumed, that an association takes place 

 of molecules to more complicated systems. Substances with 

 which that happens do not obey the laws of corresponding 

 states and van der Waals's formula would be even more in- 

 adequate in those cases than otherwise. We may expect 

 similar phenomena to occur in the liquefaction of some 

 mixtures. An association of that kind one would feel inclined 

 to identify with what is called the formation of molecular 

 compounds. This would naturally have a great influence on 

 the vapour-pressure, and it is not unlikely, that rules like 

 those given above and deduced from van der Waals's formula 

 would fail entirely in those cases. But these considerations 

 can only be tested by experiment. 



University College, Dundee. 



XXVIII. On the Propagation of Electric Waves along Cylin- 

 drical Conductors of any Section. By Lord Rayleigh, 

 F. R.S.I 



rpHE problem of the propagation of waves along conduc- 

 J- tors has been considered by Mr. Heavisideand Prof. J. J. 

 Thomson, for the most part with limitation to the case of a 

 wire of circular section with a coaxal sheath serving as a 



* Itoscos & Dittmar, Lieb. Ann. cxii. p. 327, cxvi. p. 327. 

 f Kouowalow, Wied. Ann. xiv. p. 34. 



\ Friedel, Comptes Hendiis, lxxxi. p. 152. 



§ Communicated by the Author. 



