Mr. Akin on Thermo-electric Currents of the Ritterian Species. 383 



which shows that in this case the less volatile constituent had boiled 

 the faster, the less volatile iodide of ethyle having a verv much 

 higher vapour-density than methyl-alcohol. 



It will be obvious that when the vapour-densities and tensions 

 are inversely proportional, the mixture must distil over unchanged. 

 This influence of vapour-density goes a great way to explain why 

 homologous bodies are so difficult of separation by means of frac- 

 tional distillation. The more complex the formula the higher the 

 boiling-point, but also the higher the vapour-density, and therefore 

 the greater the value of the vapour. Why oils, &c. distil so readily 

 in steam is also explained ; for aqueous vapour is one of the lightest, 

 while oily vapours are generally heavy. 



May 7- : — Major- General Sabine, President, in the Chair. 



The following communication was read : — 



" Notes, principally on Thermo-electric Currents of the Ritterian 

 Species." By C. K. Akin, Esq. 



The electromotive force of a thermo-electric couple is a function of 

 the nature of the metals of which it is composed, and of the tempe- 

 ratures of the junctions. It is expressed in this paper by 



[*i yJJ. 



where x and y are names of metals, and T and t are temperatures. 

 In this notation Becquerel's two laws become 



[«, »£=[* lf-\a, bf ti .... (I.) 

 and 



(«,«)?= [«, 4]7 + [ 4 , e j7 (II.) 



From (I.) we learn that the electromotive force of a couple may be 

 expressed as the difference of two quantities which are functions of 

 the temperature and of the nature of the circuit, or 



t**y$=l*>y\T—[*>y\t ( 1IL ) 



From (II.) we learn that any number of metals with their ends at 

 the same temperature may be introduced without effect, or 



0, b] t + [b, c],= [a, c] t (IV.) 



This equation will always be true if 



[■nfli-Mr-M* ( v -) 



whence we may write (III.) 



or, in other words, the electromotive force of a couple may be consi- 

 dered as the difference of the electromotive force of two metals, each 

 of which is found by subtracting its tension at the higher tempera- 

 ture from that of the lower one. 



Everything therefore depends on a knowledge of the value of what 

 may be called the electric tension of each metal at the various tem- 

 peratures. This for every metal is a function of temperature, and 

 may be called, in the language of the paper, a function of the nature 

 (or name) of the metal and the temperature. 



