It is known that the metals have a strong attraction for oxygen gas, and this attraction is promoted by elevating their temperature. If a glass tube be filled with iron wire heated to redness, and to one end of this tube a small vessel of boiling water be attached, the steam evolved from the water will force its way through the spaces between the red-hot wires in the tube, and would be expected to issue from the remote end; but if the substance issuing from the remote end of the tube be examined, it will be found to be not steam, but hydrogen gas. If the quantity of this gas be ascertained by weight, and also the quantity of weight lost by the vessel of water at the other end of the tube, it will be found that the loss of weight of the water by evaporation will be nine times the weight of the hydrogen which has issued from the remote end of the tube. If the weight of the tube with the wire contained in it be next ascertained, it will be found to be increased by eight times the weight of the hydrogen which has issued from its remote end. From this it follows that the weight of the hydrogen which has escaped from the tube, added to the increase of weight which has been given to the wire in the tube, makes up the whole weight of the water evaporated. If the wire in the tube be next examined, it will be found that it has suffered oxydation, or, in other words, that a new substance has been formed in it called the oxyde of iron,—such substance being a chemical compound formed of oxygen gas and iron. It follows, therefore, that in this process the vapour of the water, in passing through the tube, has been decomposed, and that, having given up to the iron its oxygen, the hydrogen [Pg305] alone escaped from the other end; and for every nine grains weight of steam which passed through the tube, eight grains of oxygen have been combined with the iron, and one grain of hydrogen has escaped from the end of the tube. Such are the class of effects on which the modern discovery of the composition of water has been based. The merit of that discovery has been shared between the celebrated English chemist, CAVENDISH, and the not less celebrated French chemist, LAVOISIER, the chief merit, however, being ascribed to the former. We shall now briefly state the facts which led to this discovery, with their dates, which will necessarily show the share which Watt had in it.