For my own part, I think this imaginary critic would, at that date, have had something to say for himself—supposing always we are prepared to accept his presuppositions about scientific method. If sound reason and intellectual integrity require us to follow the lead of observation and experiment with no antecedent preference for one class of conclusions rather than another, then no doubt Joule and a long line of distinguished predecessors were the spoilt children of fortune. They made their discoveries in advance of their evidence, and in spite of their methods. If they turned out to be right, or, at least, on the right road, what can we do but criticise their credulity and wonder at their luck? unless, indeed, their luck be a form of inspiration. Before leaving beliefs of conservation, I must say one more word about the most famous of them all—the belief in the conservation of matter. This was an important article in the scientific creed of the early atomists, who had no better evidence for it than they had for the Atomic Theory itself. The material “substance” of the medieval Aristotelians was, I imagine, also conserved; though as all that could be known about it were its qualities, and as these were not necessarily conserved, the doctrine in practice did not, perhaps, amount to much. Then came the theory which, chiefly in the hands of Boyle[14] at the end of the seventeenth century, initiated modern chemistry. What was conserved, according to this view, was not a metaphysical substance with detachable qualities, but elementary kinds of matter with inseparable qualities; and out of these qualified entities was compounded the whole material universe. I may incidentally observe that a company promoter who should issue a prospectus based on no better evidence than Boyle could advance for this tremendous theory would certainly be in peril of the law. Yet Boyle was right: and, notwithstanding subsequent developments, his conjecture remains the corner-stone of modern chemical research. Now, what is it that we intend to assert when we say that matter is conserved, or is indestructible? We certainly do not mean that its qualities never suffer change: for most of those which are obvious and striking are always liable to change. If you sufficiently vary temperature or pressure; if you effect chemical composition or decomposition, the old characteristics will vanish and new characteristics will take their place. What, then, is conserved? In the first place, the lost qualities can (in theory) always be restored, though not always without the expenditure of energy. Water never ceases to be convertible into steam, nor steam into water. The characteristics may vanish, but in appropriate conditions they will always reappear.