VAPOR. As all solifis. with the exception of carbon (an exception most probably due to our not being able to pi educe a sufficiently high temperature), are melted, or rendered liquid by the application of heat (q.v.), so a further application of heat converts them into taper. A vapor is really a gas, but it requires a little consideration to convince our selves of the tact. Perhaps the best proof that can be given is that supplied by the beau tiful experiments of Faraday (q.v.) and others on the liquefaction of gases. Hydrogen, oxygeu, and nitrogen were long exceptions; but now all gases have been liquefied by a proper application of pressure or cold, or of cold and pressure combined. The differ ence, in common language only, between a vapor and a gas is this: A gas is a substance which at ordinary temperatures and pressures exists in a state of vapor; while a vapor is produced by the application of heat to a substance which is ordinarily found in the solid or liquid form. In other words, gases are the vapors of substances which, in the liquid form, boil at very low temperatures.
The most familiar instance of vapor is aqueous vapor, or steam (q.v.). At all tem peratures, even as low as the freezing-point, ice and water give off vapor; and the quan tity produced is determined by the temperature alone; that is, evaporation (q.v.) at any temperature continues (more or less slowly according to the quantity of air or other gas which is present) until the pressure exerted by the vapor upon the containing vessel attains a certain definite value, depending on the temperature alone. If the temperature be such that the corresponding vapor-pressure is equal to the pressure of the air, vapor comes off freely, and we have the phenomenon called boiling.
Vapor in a vessel which contains some unevaporated water is thus always saturated, as it is called, i.e., the full amount of vapor capable of existing at the temperature of the vessel is present. If it be compressed, some is liquefied; if allowed to expand, more vapor is formed.
If, however, there be no water present in the liquid form, and the temperature be gradually raised, the pressure of the vapor will rise, but much more slowly than when water is present, because no more vapor can be formed. In this state—that of super
heated steam—vapor behaves almost exactly as an ordinary gas.
Chlorine, carbonic acid, sulphurous acid, etc., thus exist at ordinary temperatures as superheated vapors; and can therefore be reduced by cold and pressure to the condition of saturated vapor, when they are easily liquefied by carrying the process further.
Aqueous vapor may be liquefied by cold alone, or by pressure alone, as we have seen; and at ordinary temperatures it is easy to liquefy sulphurous acid, ammonia, and even carbonic acid and laughing gas, by mere compression. Gases absorbed by charcoal, or by spongy platinum, i.e., condensed by intense molecular forces on the large surface presented by the interstices in these bodies, must in all probability exist in the state of liquids. Carbonicacid is liquefied when exposed to a pressure of 35 atmospheres at ordinary temperatures; and some varieties of charcoal absorb from 80 to 100 times their bulk of this gas. Remembering that, on account of the impenetrability of matter, the gas can only be in the pores of the charcoal, and that their whole bulk forms but a small fraction of that of the charcoal itself, we sec that in all probability the absorbed gas must be condensed so enormously as to have become liquid. It is probable that in Graham's recent process, for separating by dialysis'(see OsmosE) the oxygen and nitrogen of the atmosphere, the film of vulcanized india-rubber which is employed as septum compels these gases to pass through its pores in a liquid form.
Some extraordinary experiments, due to Cagniard de la Tour (the inventor of the Sirene, q.v.), have given us valuable information on the subject of vapors. He showed that when water, ether, and other liquids are hermetically sealed in glass tubes, so as to fill from a quarter to a half of the tube, the application of the requisite amount of heat is sufficient to convert the whole into vapor. This vapor, therefore, has a density equal to half or quarter of that of the liquid! Ordinary steam from boiling water has only of the density of water (in common language, a cubic inch of water gives a cubic foot of steam), These experiments are very dangerous.