A few years ago such a question would have seemed singularly audacious.  And yet the law of Lavoisier is so far from self-evident that for centuries it escaped the notice of physicists and chemists.  But its great apparent simplicity and its high character of generality, when enunciated at the end of the eighteenth century, rapidly gave it such an authority that no one was able to any longer dispute it unless he desired the reputation of an oddity inclined to paradoxical ideas.

It is important, however, to remark that, under fallacious metaphysical appearances, we are in reality using empty words when we repeat the aphorism, “Nothing can be lost, nothing can be created,” and deduce from it the indestructibility of matter.  This indestructibility, in truth, is an experimental fact, and the principle depends on experiment.  It may even seem, at first sight, more singular than not that the weight of a bodily system in a given place, or the quotient of this weight by that of the standard mass—­that is to say, the mass of these bodies—­remains invariable, both when the temperature changes and when chemical reagents cause the original materials to disappear and to be replaced by new ones.  We may certainly consider that in a chemical phenomenon annihilations and creations of matter are really produced; but the experimental law teaches us that there is compensation in certain respects.

The discovery of the radioactive bodies has, in some sort, rendered popular the speculations of physicists on the phenomena of the disaggregation of matter.  We shall have to seek the exact meaning which ought to be given to the experiments on the emanation of these bodies, and to discover whether these experiments really imperil the law of Lavoisier.

For some years different experimenters have also effected many very precise measurements of the weight of divers bodies both before and after chemical reactions between these bodies.  Two highly experienced and cautious physicists, Professors Landolt and Heydweiller, have not hesitated to announce the sensational result that in certain circumstances the weight is no longer the same after as before the reaction.  In particular, the weight of a solution of salts of copper in water is not the exact sum of the joint weights of the salt and the water.  Such experiments are evidently very delicate; they have been disputed, and they cannot be considered as sufficient for conviction.  It follows nevertheless that it is no longer forbidden to regard the law of Lavoisier as only an approximate law; according to Sandford and Ray, this approximation would be about 1/2,400,000.  This is also the result reached by Professor Poynting in experiments regarding the possible action of temperature on the weight of a body; and if this be really so, we may reassure ourselves, and from the point of view of practical application may continue to look upon matter as indestructible.