52 



when this convection is in the nominal " direction of the current," 

 and I shall apply the same expressions to " resinous electricity " 

 when the convection is against the nominal direction of the current. 

 It is established then that one or other of the following three hypo- 

 theses must be true : 



Vitreous electricity carries heat with it in an unequally heated 

 conductor whether of copper or iron ; but more in copper than in 

 iron. 



Or Resinous electricity carries heat with it in an unequally heated 

 conductor whether of copper or iron; but more in iron than in 

 copper. 



Or Vitreous electricity carries heat with it in an unequally heated 

 conductor of copper, and Resinous electricity carries heat with it in 

 an unequally heated conductor of iron. 



Immediately after communicating this theory to the Royal Society 

 of Edinburgh, I commenced trying to ascertain by experiment which 

 of the three hypotheses is the truth, as Theory with only thermo- 

 electric data could not decide between them. I had a slight bias in 

 favour of the first rather than the second, in consequence of the 

 positiveness which, after Franklin, we habitually attribute to the 

 vitreous electricity, and a very strong feeling of the improbability of 

 the third. With the able and persevering exertions of my assistant, 

 Mr. M c Farlane, applied to the construction of various forms of ap- 

 paratus and to assist me in conducting experiments, the research 

 has been carried on, with little intermission, for more than two 

 years. Mr. Robert Davidson, Mr. Charles A. Smith, and other 

 friends have also given much valuable assistance during the greater 

 part of this time, in the different experimental investigations of 

 which results are now laid before the Royal Society. Only nu- 

 gatory results were obtained until recently from multiplied and 

 varied experiments both on copper and iron conductors ; but the 

 theoretical anticipation was of such a nature that no want of expe- 

 rimental evidence could influence my conviction of its truth. About 

 four months ago, by means of a new form of apparatus, I ascertained 

 that resinous electricity carries heat with it in an unequally heated iron 

 conductor. A similar equally sensitive arrangement showed no re- 

 sult for copper. The second hypothesis might then have been ex- 

 pected to hold ; but to ascertain the truth with certainty I have 



