ponderable Bodies, and Caloric and Electricity. 1 1 L 



with, bodies, as measured by their power of imparting or abs- 

 tracting heat, is exactly proportional to the quantity of any 

 ponderable (simple) substance separable from, or combining 

 with, the same bodies, as ascertained by weighing. Thus it 

 requires, according to the law of Dulong and Petit, twice as 

 much caloric to elevate the temperature of a pound of sulphur 

 10 degrees, as it does to raise that of a pound of zinc to 

 the same extent^ and a pound of heated sulphur will warm 

 twice as much water, or any third substance, as a pound of 

 equally hot zinc. Now this is exactly what takes place among 

 ponderable substances. A certain weight of sulphur combines 

 with twice as much oxygen, or chlorine, as the same weight 

 of zinc does, in forming the correspondent compound and 

 gives up twice as much on entering into a fresh combination. 

 If we were to adapt to the ponderable body oxygen, the 

 phraseology which we are accustomed to use in regard to ca- 

 loric, we might say then, and the fact would be correct, how- 

 ever unusual the expression, that the " capacity " of sulphur 

 for oxygen is twice as great as that of zinc. Again, " the 

 quantities of oxygen combined with by sulphur and zinc are 

 in inverse ratio to the weights of the atoms of those bodies." 



Again, a given quantity of oxygen will raise, in the scale of 

 oxides, portions of zinc and of sulphur represented by their 

 atomic weights. Once more, " the weights of oxygen in the 

 corresponding oxides, multiplied by the atomic weights, give 

 a constant quantity." For 16 parts of sulphur combine with 

 8 oxygen, 16 zinc with 4 oxygen, but 8x16 = 4 x 32. This 

 ground of analogy between caloric and ponderable bodies is 

 so obvious, that it would seem as if it must have occurred to 

 many, yet I have never seen it adverted to. If it has really 

 escaped some who might have been expected to notice it, the 

 difference in our modes of expressing facts thus closely corre- 

 sponding, when the subject is caloric and when it is a ponder- 

 able body, furnishes one reason for the oversight. Another 

 is found in the circumstance that in our experiments and re- 

 ports of experiments on the relation of bodies to heat7 we ac- 

 custom ourselves to equal quantities of the bodies compared, 

 and variable quantities of caloric. 



We are accustomed, on the other hand, to take a fixed 

 quantity of any ponderable substance, and a variable quantity 

 of those whose combinations with it we compare together. 

 But how this analogy has remained without mention is of 

 minor consequence. 1 wish to draw attention to its existence, 

 and to a fact, exactly corresponding to it in the history of elec- 

 tricity. Mr. Faraday, in his 7th series of researches in that 

 science, says that the quantity of electricity set free during 



