202 PROFESSOR WHEWELL's DEMONSTRATION 



I may remark, that the same conclusion is easily extended to the case in 

 which phlogiston is supposed to have absolute levity ; for in that case, a 

 certain mixture of phlogiston and of heavy matter would have no weight, 

 and might be substituted for phlogiston in the preceding reasoning. 



I may remark, also, that the same conclusion would follow by the same 

 reasoning, if any kind of matter, instead of being void of weight, were 

 heavy, indeed, but not so heavy, in proportion to its quantity of matter, as 

 other kinds. 



On all these hypotheses there would be no possibility of measuring quan- 

 tity of matter by weight at all, in any case, or to any extent. 



But it may be urged, that we have not yet reduced the hypothesis of 

 matter without weight to a contradiction ; for that mathematicians measure 

 quantity of matter, not by weight, but by the other property, of which we 

 have spoken, inertia. 



To this I reply, that, practically speaking, quantity of matter is always 

 measured by weight, both by mechanicians and chemists : and as we have 

 proved that this procedure is utterly insecure in all cases, on the hypothesis 

 of weightless matter, the practice rests upon a conviction that the hypo- 

 thesis is false. And yet the practice is universal. Every experimenter 

 measures quantity of matter by the balance. No one has ever thought of 

 measuring quantity of matter by its inertia practically : no one has con- 

 structed a measure of quantity of matter in which the matter produces its 

 indications of quantity by its motion. When we have to take into account 

 the inertia of a body, we inquire what its weight is, and assume this as the 

 measure of the inertia; but we never take the contrary course, and ascertain 

 the inertia first in order to determine by that means the weight. 



But it may be asked, Is it not then true, and an important scientific 

 truth, that the quantity of matter is measured by the inertia ? Is it not true, 

 and proved by experiment, that the weight is proportional to the inertia f 

 If this be not the result of Newton's experiments mentioned above, what, it 

 may be demanded, do they prove ? 



To these questions I reply : It is true that quantity of matter is mea- 

 sured by the inertia, for it is true that inertia is as the quantity of matter. 



