262 § —=«-—Prrof» Whewell’s- Demonstration 
losophical truth be “ futile” in the one case, because inapplicable 
in the other? Because the inertias of A and B have been discov- 
ered, by means of their gravitation, does it follow that the inertias 
of X and Y cannot be discovered by their self-repellent power ? 
Why should the inapplicability of gravitation in the one case 
render its employment futile in the other? 
. It is self-evident, that matter without weight cannot be 
estimated by weighing, but I deny that on that account such 
weightless matter may not be otherwise estimated. The inertias 
of A and B cannot be better measured by gravitation than those 
of X and Y by repulsion, as already shown. 
12. You seem to infer, in paragraph second, page sixth, (p. 268,) 
that we should be equally destitute of the means of measuring 
matter accurately, “ were any kind of matter heavy indeed, but not 
, in proportion to its quantity of matter, as other kinds.” 
13. If in the case of all matter weight be admitted to be the only 
measure of quantity, it were inconsistent to suppose any given 
quantity of matter, of any one kind, to have less weight than an 
equal quantity of another kind; but upon what other than a 
conventional basis is it to be assumed, that there is more matter 
in a cubic inch of platinum than in a enbie inch of tin; ina 
cubic inch of mercury than in a cubic inch of iron? Judging 
by the chemical efficacy of the masses, although the weight of 
mercury is to that of iron as 13.6 is to 8, there are more equiva- 
lents of the latter than the former in any given bulk, since by 
weight twenty-eight parts of iron are equivalent to two hundred 
and two parts of mercury. 
14, Weight is one of the properties of certain kinds of matter, 
and has been advantageously resorted to, in preference to any 
other property, in estimating the quantity of the matter to which 
it appertains. Nevertheless, measurement by bulk is found ex- 
pedient or necessary in many cases. But may we not appeal to 
any general property which admits of being measured or esti- 
mated? Faraday has inferred that the quantity of electricity, is 
as the quantity of gas which it evolves. Light has been con- 
sidered as proportional in quantity to the surface which it illu- 
minates with a given intensity at a certain distance. ‘The quan- 
tity of caloric has been held to be directly as the weight of 
water which it will render aériform ; and has also been estima- 
