If we ask for the evidence on which this generalisation is founded, we have to appeal to various delicate weighings, conducted chiefly by chemists for practical purposes, and very few of them really directed to ascertain whether the law is true or not. A few such direct experiments are now, indeed, being conducted with the hope of finding that the law is not completely true; in other words, with the hope of finding that the weight of a body does depend slightly on its state of aggregation or on some other physical property. The question has even been raised whether the weight of a crystal is altogether independent of its _aspect_: the direction of its plane of cleavage with reference to the earth's radius; also, whether the _temperature_ of bodies has any influence on their weight; but on these points it may be truly said that if any difference were discovered it would not be expressed by saying that the amount of matter was different, but simply that "weight" was not so fundamental and inalienable a property of matter as has been sometimes assumed; in which case it is clear that there must be a more fundamental property to which appeal can be made in favour of constancy or persistency or conservation. Now the most fundamental property of matter known is undoubtedly 'inertia'; and the law of conservation would therefore come to mean that the _inertia_ of matter was constant, no matter what changes it underwent. But, then, inertia is not an easy property to measure,—very difficult to measure with great accuracy: it is in practice nearly always _inferred_ from weight; and in terms of inertia the law of conservation of matter cannot be considered really an experimental fact; it is, strictly speaking, a reasonable hypothesis, an empirical law, which we have never seen any reason to doubt, and in support of which all scientific experience may be adduced in favour. It is possible, however, to grant to Professor Haeckel—not positively, but for the sake of argument, and giving him the benefit of our present ignorance—that it is unlikely that matter in its lowest denomination can by us be created or destroyed. For, although it is now pretty well known that atoms of matter are not the indestructible and immutable things they were once thought (seeing that, although we do not know how to break them up, they are liable every now and then themselves to break up or explode, and so resolve themselves into simpler forms), yet it can be granted that these simpler forms are likewise themselves atoms, in the same sense, and that if they break up they will break up likewise into atoms: or ultimately, it may be, into those corpuscles or electrons or electric charges, of which one plausible theory conjectures that the atoms of matter are really composed.