24 
two, sometimes in three different directions ; and it appears to me more easy 
to conceive that these unequal forces should exist in a molecule which is of 
unequal dimensions in its different directions ; and it would be very easy to 
conceive that the form of the molecule is not a sphere. But, at the same time 
it is not a matter of great consequence, for, according to the Newtonian 
hypothesis, when the space between particles of matter is indefinitely large, 
compared with the magnitude of the particles themselves, it does not matter 
what we suppose the form to be. It appears to me rather more comprehen- 
sible that the forms are different in those atoms in which there are neces- 
sarily different attractive forces in different directions. I daresay the 
Professor would have given us some reason for superseding the view I have 
just enunciated. At the bottom of sec. 18 he speaks of the Newtonian 
expression, vis inertia. It appears to me that the consideration of inertia as a 
force, tends only to mystify and confuse our ideas in regard to what force 
means. Force, as copimonly defined, is that which tends to alter the con- 
dition of a body with respect to its state of rest or motion. Now, if this 
be the correct definition of the term force, clearly inertia does neither one 
nor the other. 
Mr. Challis. — He speaks of it as a quality. 
The Chairman. — As a force. 
Mr. Challis. — As reaction (in sec. 21). 
The Chairman. — He quotes Newton, and says : — 
“ He by no means affirms gravity to be essential to bodies ; that he takes 
vis inertia to be the only intrinsic (‘ insita ’) force, and that this force is 
invariable (‘ immutabilis ’), whilst, on the contrary, gravity diminishes with 
increase of distance from the earth. (These views accord with the rule I 
have adopted in sec. 11, of not admitting qualities susceptible of variation 
to be primary, which rule, of course, excludes gravitation from the class of 
primary qualities.) ” 
Mr. Challis.— I think that is discussed further on, where he speaks of 
it simply as a quality of resistance. 
The Chairman. — It does appear to me that it is an unfortunate expression 
of Newton’s ; to call inertia a force, confounds our ideas of force altogether. 
Mr. Challis. — I think the Professor in using that term does not adopt it 
in speaking of action, but only in speaking of re-action. 
The Chairman. — Then with regard to gravitation, he speaks of gravita- 
tion as a variable force. I think this again is a little looseness of language, 
because how do we estimate our measure of force ? We estimate it by its 
action on a unit of matter at a unit of distance. Now that is constant. 
The amount of gravitation will depend upon the distance. The force of 
gravitation I maintain to be constant and uniform, because we can only 
measure it by its action on a unit of matter at a unit of distance. 
Mr. Challis. — He speaks of it as not being a primary quality or fact, 
because it may be measured. 
The Chairman. — Because it is variable. 
Mr. Challis.— B ecause it is quantitative. 
