INSULATING PROPERTY OF HEAT. 231 



whereas, in the intimate union of two non-conductors, the insulating power remains 

 perfect. Since, then, heat does not impair the insulating power of a given volume of 

 air, heat, if a substance, should necessarily have non-conducting properties. 



51. The converse of this reasoning furnishes additional evidence in favour of the 

 above conclusion ; it is a well-known fact, that the excitation of heat in good con- 

 ductors, such as the metals, is inimical to their conducting power. This result always 

 ensues in mixing a conducting with a non-conducting substance, and is also evident 

 in amalgamating a good conducting metal with an inferior one *. 



This curious effect of heat in impairing the conducting power of metals, has been 

 clearly and beautifully illustrated by Sir Humphry Davy -f. I have also arrived at 

 similar results X, and find, as stated by him, that heat in any way excited in metallic 

 conductors, whilst transmitting an electrical current, tends to impair their conducting 

 power. Mr. Christie, likewise, has observed the same fact, as appears in his last in- 

 teresting paper on the Laws of Magneto-electric Induction §. 



52. Although the experiments in evidence of this influence of heat or metallic con- 

 ductors are numerous and very conclusive, yet opposite views have been advanced by 

 Dr. Ritchie in his paper on Electric Conduction ||. Dr. Ritchie's principal experi- 

 ment consists in transmitting common electricity over a forked iron rod, one of the 

 legs of which he heated to redness : he finds, under these circumstances, that the 

 electricity will rather pass from the heated side, than from the cool side ; but this 

 result cannot be taken in evidence of the superior conducting power of the heated 

 iron, so long as the experiment is made in air, since, as has been just shown (48.), air 

 rarefied by heat, loses to a greater or less extent, its restraining power. Now the air 

 immediately in contact with an iron rod heated to redness, is necessarily in an ex- 

 tremely rare state : hence the impaired conducting power of the metal becomes more 

 than compensated by the diminished resistance on its surface ; so that the conducting 

 power of the metal, together with the greatly diminished density of the air on the one 

 side, may still afford an easier passage to the electricity than the conducting power of 

 the metal alone on the other (44.). It is hence essential, in such an experiment as 

 that proposed by Dr. Ritchie, to place the bent iron rod in a well-exhausted receiver 

 before any fair conclusion can be drawn as to the influence of heat on its conducting 

 power. Of this the talented author of the paper alluded to seems to be in a great 

 measure aware, as appears in his account of his seventh experiment. Dr. Ritchie 

 has, however, taken an objection to one of the many phenomena so decisive of this 

 important question : he appears to think that the effect of a heated wire would be a 

 species of electrical evaporation from its surface ; but it will be immediately perceived 

 that this notion is purely hypothetical. Electricity is never found to escape from a 



* Philosophical Transactions, 1827, p. 18. t Ibid., 1821. 



X Transactions of the Royal Society of Edinburgh, 1832. - § Philosophical Transactions, 1833. 



II Philosophical Trans^actions, 1828, p. 373. 



2 h2 



