To render these general principles more intelligible, let us suppose that the water in B is raised to the temperature of 213°, the stopcock C being open; the vessel A will then be filled with steam of the same temperature, and having a pressure of 15 lbs. per square inch. This will be common steam. If the stopcock be now closed, and the whole apparatus be exposed to the temperature of 243°; the steam in A will preserve the same density, but its pressure will be [Pg171] increased from 15 lbs. to a little more than 16 lbs. per square inch. Let the stopcock C be then opened and while the temperature of the steam in A shall continue to be 243°, the pressure will suddenly rise from 16 lbs. to about 26 lbs. per square inch. The weight of the steam in A will be at the same time increased in the same proportion of 16 to 26 as its pressure. The steam thus produced in A will then be common steam, and any abstraction of heat from it would be attended with partial condensation. (96.) The law, according to which the pressure of elastic fluids in general, whether gases or vapours, increases with their temperature, was simultaneously discovered by Dalton and Gay Lussac. If the pressure which the gas or vapour would have at the temperature of melting ice, were expressed by 10,000, then the increase of pressure which it would receive for every degree of temperature by which it would be raised, its volume being supposed to be preserved, would be expressed by 208-1/3. Thus, if the pressure of gas, or vapour, on a surface of a certain magnitude at the temperature of 32° were 10,000 ounces, then the same gas or vapour would acquire an additional pressure of 208-1/3 ounces for every degree of temperature which would be imparted to it above 32°. This law is common to all gases and vapours. It may be objected that water cannot exist in the state of vapour under the usual pressures at so low a temperature as melting ice. This, however, does not hinder the application of the above law, for that law will equally hold good by computing the pressure which the vapour would have if it were a permanent gas, and if it could therefore exist in the elastic form at that low temperature.