me : 
propositions of Euclid, and not till then, it becomes evident to him, 
that parallelograms on the same base and between the same parallels 
are equal; and he cannot even conceive the ri When he has 
a little further cultivated his geometrical powers, the equality of the 
square on the hypothenuse of a dace: ica to the squares on 
the sides, becomes also evident; the steps by which it is demonstrated 
being so familiar to the mind as to be apprehended without a conscious 
act. And thus, the contrary of a necessary truth cannot be distinctly 
conceived ; but the incapacity of forming such a conception is a condi- 
tion which depends upon cultivation, being intimately connected with 
the power of rapidly and clearly perceiving the connection of the 
necessary truth under consideration with the elementary principles on 
which it depends. And thus, again, it may be that there is an absolute 
impossibility of conceiving matter without weight; but then, this impos- 
sibility may not be apparent, till we have traced our fundamental con- 
ceptions of matter into some of their consequences. 
“The question then occurs, whether we can, by any steps of rea- 
soning, point out‘an inconsistency in the conception of matter without 
weight. This I conceive we may do, and this I shall attempt to shew. 
“ The general mode of stating the argument is this :—the quantity of 
matter is measured by those sensible properties of matter which undergo 
quantitative addition, subtraction and division, as the matter is added, 
subtracted and divided. The quantity of matter cannot be known in 
any other way. But this mode of measuring the quantity of matter, in 
order to be true at all, must be universally true. If it were only partially 
true, the limits within which it is to be applied would be arbitrary ; 
and therefore the whole procedure would be-arbitrary, mre asa method 
of obtaining philosophical truth, altogether futile. 
. “ We may unfold this argument further. Let the contrary be sup- 
posed, of that which we assert to be true: namely, let it be supposed 
that while all other kinds of matter are heavy, (and of course heavy 
in proportion to the quantity of matter,) there is one kind of matter 
which is absolutely destitute of weight; as, for instance, phlogiston, or 
any other element. Then where this weightless element (as we may 
term it) is mixed with weighty elements, we shall have a compound, in 
which the weight is no. longer proportional to the quantity of matter. 
If, for example, 2 measures of heavy matter unite with 1 measure of 
Phlogiston, the weight is as 2, and the quantity of matter as 3. In all 
Such cases, therefore, the weight ceases to be the measure of the quan- 
tity of matter. And as the proportion of the weighty and the weight 
less matter may vary in innumerable degrees in such compounds, the 
Weight affords no criterion at all of the quantity of matter in them. 
