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, etic, 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 phenometia 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 ieee of the agi 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 som but one positive pane and that inclu- 
ding two, are alike. 
‘xxxv. Again, there i 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, sol 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, 
