consequently the piston will be raised one foot above its first position in the tube, and the cubic foot beneath it will be completely filled with steam. Let us suppose, that to produce this effect required the lamp to be applied to the tube for the space of fifteen minutes. The water being again supposed at its original temperature, and the piston in its first position, let a weight be placed upon the piston equal to the pressure of the atmosphere, so that the water beneath the piston will be pressed down by double the atmospheric pressure. If the lamp be once more applied, the water will, as before, be converted into vapour; but the piston will now be raised to the height of only six inches[48] from the bottom, the steam expanding into only half its former bulk. The temperature at which the water would commence to be converted into vapour, instead of being 212°, would be 250°; but the time elapsed between the moment of the first application of the lamp, and the complete conversion of the water into steam, will still be fifteen minutes. [Footnote 48: Strictly speaking, the height to which the piston would be raised would not diminish in so great a proportion as the pressure is increased, because the increase of pressure being necessarily accompanied by an increase of temperature, a corresponding expansion would be produced. Therefore there will be a slight increase in the total mechanical effect of the steam. The difference, however, is not very important in practice, and it is usual to consider the density of steam as proportional to the pressure.] Again, if the piston be loaded with a weight equal to double the atmospheric pressure, the water will be pressed down by the force of three atmospheres. If the lamp be applied as before, the water would be converted into steam in the same time; but the piston will now be raised only four inches above its first position, and the steam will consequently be three times as dense as when the piston was pressed down only by the atmosphere. From these and similar experiments we infer:-- _First_, That the elastic pressure of steam is equal to the mechanical pressure under which the water producing the steam has been boiled. _Secondly_, That the bulk which steam fills is diminished in the same proportion as the pressure of the steam is increased; or, in other words, that the density of steam is always in the same proportion as its pressure. _Thirdly_, That the same quantity of heat is sufficient to convert the same weight of water into steam, whatever be the pressure under which the water is boiled, or whatever be the density and pressure of the steam produced.