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 there are any explanations other than mechanical. The illustrious English physicist, Lord Kelvin, does not seem willing to admit this. "I am never satisfied," he said, in his Molecular Mechanics, "until I have made a mechanical model of the object. If I can make this model, I understand; if I cannot, I do not understand."

This tendency of so vigorous a mind to be content only with mechanical explanations, has been that of the majority of scientific men up to the present day, and from it has arisen the scientific idea of matter.

What is matter, in fact, to the student of mechanics? It is mass. All mechanics is constructed of masses and forces. Laplace said: "The mass of a body is the sum of its material points." To Poisson, mass is the quantity of matter of which a body is composed. Matter is therefore confused with mass. Now, mass is the characteristic of the motion of a body under the action of a given force; it defines obedience or resistance to the causes of motion; it is the ''mechanical parameter''; it is the co-efficient proper to every mobile body; it is the first invariant of which a conception has been established by science.

In fact, the word matter appears to be used in other senses by physicists, but this is only apparently so. They have but broadened the idea of the mechanicians. They have characterized matter by the whole series of phenomenal manifestations which are proportional to mass, such as weight, volume, chemical properties—so that we may say that the notion of matter does not intervene scientifically with a different signification from that of mass.

''Two kinds of Matter. Ponderable and Imponder''-