(100.) Common steam being raised from water at any pressure and temperature, and being afterwards separated from the water, if the same steam be compressed into a small volume, or allowed to expand into a greater volume, it will still maintain its quality of common steam, and will have the same pressure and temperature, whatever volume it may assume, as it would have if immediately raised from water at that pressure. Thus if steam be raised from water under a pressure of 30 lbs. per square inch, and, being separated from the water, be allowed to dilate, until its pressure is reduced to 15 lbs. per square inch, its temperature will then be reduced to 213°, which is that temperature which it would have if immediately raised from water under a pressure of 15 lbs. per square inch; and if any heat be abstracted from such steam, whether under its original pressure, or under the diminished pressure of 15 lbs. per square inch, a condensation will be produced, the amount of which will be the same, if the same quantity of heat be abstracted from the steam. These are consequences which immediately flow from the fact, that the sum of the latent and sensible heats of steam is always the same.[20] It appears, therefore, that supposing the steam used in an engine to receive no additional heat after it leaves the boiler, however it may be changed in its density by subsequent expansion, it will still retain its character of common steam, and cannot lose any portion of heat, however small, without suffering partial condensation. The mechanical force also exerted by such steam, after expansion, must be computed in the same manner as if it were raised immediately. (101.) If the law of Mariotte were strictly applicable to steam, its mechanical effect would be the same as has been already explained in all states of density; but since its temperature will rise and fall as its density is increased or diminished, a corresponding change will be produced in its [Pg174] mechanical efficacy. It is therefore necessary in the calculation of the mechanical effect of steam, whether it be used at a uniform pressure without the principle of expansion, or with the application of that principle to any given extent, to take into account the combined operation of the laws of Mariotte and Dalton. Formulæ exhibiting the relation between the temperatures, pressures, volumes, and densities of steam, and the mechanical effect produced by the evaporation of water, whether acting with or without expansion, together with the tables necessary for the practical application of these, will be found in the Appendix.