If we grant, in accordance with modern views, that matter consists of minute particles, termed molecules, it must also be allowed that the distance between these ultimate particles must be very different, according to whether the matter is in the solid, or liquid, or in the gaseous state. Thus, a cubic centimeter of water at ioo° expands, when it is boiled into steam of the same temperature, to 1700 cubic centimeters; and a cubic centimeter of oxygen, measured at its boiling-point, -182% boils into 266 cubic centimeters of oxygen gas of the same temperature. In changing its state, therefore, from liquid or solid to gas, matter undergoes a great alteration of volume. It is accordingly to be expected that the molecules of a gas, being at so much greater a distance from each other than the molecules of a solid or liquid, should yield more readily to pressure, and should decrease in volume when the pressure is raised, much more than solids or liquids. It is also found, as appeared probable, that the expansion of a gas is much greater than that of a solid or a liquid, by a definite rise of temperature.