12 The Rev. T. R. Robinson on the Relation beiiveen the Temperature of 



snapping, and its refraction may make the reading doubtful one or two tenths 

 of a division. All but the last are means of five ; that only of two, which, how- 

 ever, agree tolerably. 



In examining these tables it is evident, 



1. That the increased resistance is not occasioned by any condensation of 

 the current. In No. 40 it passes more easily than in No. 33, though of six-fold 

 power, and able to fuse a far thicker wire in air. It is also worth notice, that 

 in these two cases the productions of heat are as 29 : 1; and, therefore, it is not 

 by the mere employment of molecular forces in the production of that agent, 

 but by its accumulating and becoming sensible, that conduction is impeded. 



2. The magnitude of the change prevents me from attributing it to a mere 

 change of molecular distance. On this hypothesis we have seen that it will be 



A'-A _ 2 eP elP 



eP A' — A 



Now in No. 14, the highest of the set, -j = 0.1, which will give ^—^ — = 0.30, 



whereas it really = 4.68. 



3. Nor is it proportional to the expansion : up to a certain point it may be 

 expressed by the formula A=a + bP, but less accurately than by making it 

 depend on the temperature ; and 



4. It appears to be correctly determined by the equation 



A=a + bT; (1) 



wliere a is the resistance at 32° of Fahrenheit, and b its change for one degree. 



