on certain Theoretical Opinions. 63 



not that I go to the full extent of them all, as for instance that 

 of motion and rest; but I do not perceive their .bearing upon 

 the question, of whether conduction and insulation are different 

 properties, dependent upon two different modes of action of the 

 particles of the substances respectively possessing these actions, 

 or whether they are only differences in degree of one and the 

 same mode of action ? In this question, however, lies the whole 

 gist of the matter. To explain my views, I will put a case or 

 two. In former times a principle or force of levity was ad- 

 mitted, as well as of gravity, and certain variations in the 

 weights of bodies were supposed to be caused by different 

 combinations of substances possessing these two principles. 

 In later times, the levity principle has been discarded ; and 

 though we still have imponderable substances, yet the phae- 

 nomena causing weight have been accounted for by one force 

 or principle only, that of gravity; the difference in the gravi- 

 tation of different bodies being considered due to differences 

 in degree of this one force resident in them all. Now no one 

 can for a moment suppose that it is the same thing philoso- 

 phically to assume either the two forces or the one force for 

 the explanation of the phenomena in question. 



xxxiv. Again, at one time there was a distinction taken 

 between the principle of heat and that of cold : at present 

 that theory is done away with, and the phasnomena of heat 

 and cold are referred to the same class, (as I refer those of 

 insulation and conduction to one class,) and to the influence 

 of different degrees of the same power. But no one can say 

 that the two theories, namely, that including but one positive 

 principle, and that including two, are alike. 



xxxv. Again, there is the theory of one electric fluid and 

 also that of two. One explains by the difference in de- 

 gree or quantity of one fluid, what the other attributes to a 

 variation in the quantity and relation of two fluids. Both 

 cannot be true. That they have nearly equal hold of our 

 assent, is only a proof of our ignorance : and it is certain 

 whichever is the false theory, is at present holding the minds 

 of its supporters in bondage, and is greatly retarding the 

 progress of science. 



xxxvi. I think it therefore important, if we can, to ascer- 

 tain whether insulation and conduction are cases of the same 

 class, just as it is important to know that hot and cold are 

 phenomena of the same kind. As it is of consequence to show 

 that smoke ascends and a stone descends in obedience to one 

 property of matter, so I think it is of consequence to show 

 that one body insulates and another conducts only in conse- 

 quence of a difference in degree of one common property 



