As has been shown, a "Law" is nothing but a statement that a certain kind of fact is found to occur in certain circumstances. Professor Haeckel has told us that the "Law of Substance" is a blend of two such statements, namely, "the Law of the persistency or indestructibility of matter," which signifies that in no instance within our knowledge is any particle of matter destroyed, and "the Law of the persistence of force, or conservation of energy," which signifies that the sum of force, at work in the world, and producing all phenomena, is similarly found to be unalterable.[34] It must here first be observed that the term "Conservation of Energy," is more correct and intelligible than "Conservation of Force"; by "Energy" being understood the power of doing "work," that is to say, of overcoming resistance.[35] It is in this form alone that Force becomes subject to observation and can be measured by Science, and the Law of Conservation which observation reveals is thus stated: The sum of all the various energies in the universe is a constant quantity, which can be neither increased nor diminished, though it may be changed from one form to another;[36] such forms being motion, heat, chemical action, electricity, magnetism. But another point is of far greater importance. The mode in which Professor Haeckel states this fundamental Law is altogether deceptive. He tells his readers only half the truth, and when the other half is told, not only is his whole doctrine found to receive no support from the Laws of Energy, but it is these very Laws which appear most incompatible with it. For, along with the Law of the Conservation, there is another, of the Dissipation of Energy. It is perfectly true, as Professor Haeckel often repeats, that the sum of Energy existing in the universe remains ever the same: but it is no less certain, as he unfortunately fails to remind his readers, that the stock of Energy _available for the work of the universe_ is growing less every day. Though none is ever destroyed, much is constantly _lost_, being dissipated, or radiated into space, in the form of heat which can never be recaptured or translated into any form which can be of any practical avail. "It is lost for ever as far as we are concerned."[37] From what we have heard concerning the Law of Substance it might naturally be supposed that it certified us of the continued existence of the power required to carry on the operations of Nature, and that, accordingly, reason bids us to suppose these operations to be everlasting. But this neglected element of the reckoning, or _Entropy_ as it is styled, leads scientific men to an entirely different estimate. Thus Professor Balfour Stewart writes:[38]