118 Answer to Dr. Hare's Letter. 



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 admitted 

 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 phenomena concerning weight 

 have been accounted for by one force or principle only, that of 

 gravity ; the difference in the gravitation of different bodies be- 

 ing considered due to differences in degree of this one force resi- 

 dent in them all. Now no one can for a moment suppose that it 

 is the same thing, philosophically, to assume either the two for- 

 ces or the one force, for the explanation of the phenomena in 

 question. 



xxxiv. Again ; — at one time there was a distinction taken be- 

 tween the principle of heat and that of cold : at present that the- 

 ory is done away with and the phenomena of heat and cold are 

 referred to the same class (as I refer those of insulation and con- 

 duction 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 inclu- 

 ding two, are alike. 



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

 that of two. One explains by the difference in degree or quantity 

 of one fluid what the other attributes to the variation in the quan- 

 tity and relation of two fluids. Both cannot be true ; that they 

 have nearly equal hold of our assent is only a proof of our igno- 

 rance ; and it is certain, whichever is the false theory is at present 

 holding the minds of its supporters in bondage and is greatly re- 

 tarding the progress of science. 



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

 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 shew that smoke 

 ascends and a stone descends in obedience to one property of 

 matter, so I think it is of consequence to shew that one body 

 insulates and another conducts only in consequence of a differ- 

 ence in degree of one common property which they both possess, 



