In a first approximation we agree to consider ponderable matter only. Chemistry acquaints us with its different forms. They are the different simple bodies, metalloids, metals, and the compound bodies, mineral or organic. Hence we may say that chemistry is _the history of the transformations of matter_. From the time of Lavoisier this science has followed the transformations of matter, balance in hand, and ascertains that they are accomplished without change of weight. _Law of the Conservation of Matter._—Imagine a system of bodies enclosed in a closed vessel, and the vessel placed in the scale of a balance. All the chemical reactions capable of completely modifying the state of this system have no effect upon the scale of the balance. The total weight is the same before, during, and after. It is precisely this equality of weight which is expressed in all the equations with which treatises on chemistry are filled. From a higher point of view we recognize here, in this _law of Lavoisier_ or of the _conservation of weight_, the verification of one of the great laws of nature which we extend to every kind of matter, ponderable or not. It is the _law of the conservation of matter_, or again, of the indestructibility of matter—“Nothing is lost, nothing is created, all is transformation.” This is exactly what Tait held, this impossibility of creating or destroying matter which at the same time is a proof of its objective existence. This indestructibility of ponderable matter is at the same time the fundamental basis of chemistry. Chemical analysis could not exist if the chemist were not sure that the contents of his vessel at the end of his operations ought to be quantitatively, that is to say by weight, the same as at the beginning, and during the whole course of the experiment.[6] [6] It must be added that the absolute rigour of this law has been called in question in recent researches. It would only have an approximate value. § 2. ENERGY.