74 Geological Society : — 



be imparted to the junction to keep its temperature constant 

 will be different for different metals. 



From the important equation (7) it follows that, supposing 

 the constant k not to be zero, when contact is made between 

 two pieces of the same metal at different temperatures, there 

 will be a temporary current across the junction which is in 

 the same direction for all metals. From this rule those metals 

 are excepted which, on being heated, undergo some permanent 

 molecular change, so that their state is not completely defined 

 by the pressure and temperature. 



When a thermoelectric circuit is formed of two metals, 

 equation (9) for the electromotive force takes the simple form 



E = S-8o, ...... (17) 



which gives at once the well-known equation 



When the temperatures of the two junctions differ by an 

 infinitesimal quantity r, we have, instead of (10) , the following 

 form for the electromotive force : — 



i^^y.T, (19) 



a result due to Sir W. Thomson. 



In general, if a thermoelectric circuit be formed of any 

 number of different metals, and if h be the abrupt rise of po- 

 tential at any junction as we travel in the direction of the 

 current, then evidently E = !ES (20) 



If the junctions be all at the same temperature, we have S8 = 0, 

 and therefore E = 0. 



I wish to add, in conclusion, that I have lately learned that 

 the subject of thermoelectricity has been already considered 

 by Duhem {Annal. de C/iim. Dec. 1887), and that the results 

 at which he has arrived are in complete accord with those 

 which I have obtained in my two papers on the subject. 



X. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from vol. xxvi. p. 314.] 



November 7, 1888.— W. T. Blanford, LL.D., F.R.S., President, 

 in the Chair. 



T^HE following communications were read : — 



-'- 1. " The Permian Rocks of the Leicestershire Coal-field." By 



Horace T. Brown, Esq., E.Gr.S. 



The author considers that whilst rocks belonging to the Car- 



