306 



EBULLITION. 



We have seen that the vapor into which water is converted by heat posses- 

 ses the leading qualities of common atmospheric air ; and if not submitted to 

 a minute examination, might be mistaken for highly heated air. It is perfectly 

 transparent and invisible ; for, in the first experiment* described in this dis- 

 course, when the water was boiled in the flask until the whole of the liquid 

 had been converted into steam, the flask had the same appearance as if it were 

 filled with air. It might be objected to this statement, that the steam which 

 issues from the spout of a boiling kettle, or which proceeds from the surface 

 of water boiling in an open vessel, is visible, since it presents the appearance 

 of a cloudy smoke. This appearance, however, is produced, not by steam, 

 but by very minute particles of water arising from the condensation of steam 

 in passing through the cold air. These minute particles, floating in the air, 

 become in some degree opaque, and are visible like the particles of smoke. 

 Such cloudy substances, therefore, are not true vapor or steam. 



But the most important property which steam enjoys in common with atmo- 

 spheric air and other gases, and on which, like them, all its mechanical prop- 

 erties depend, is its elasticity or pressure. If a quantity of pure steam be con- 

 fined in a close vessel, it will, like air, exert on every part of the interior sur- 

 face of that vessel a certain determinate pressure, directed outward, and having 

 a tendency to burst the vessel. A bladder might thus be inflated with steam 

 in the same manner as with atmospheric air ; and, provided the temperature 

 of the bladder be sustained at that point necessary to prevent the steam from 

 returning to the liquid form, its inflation would continue. 



By virtue of this property of elasticity, steam or air is expansible, and, 

 when freed from the limits which confine it, will dilate into any space to 

 which it may have access. Suppose a piston placed in a cylinder, in which 

 it moves steam-tight, and between the piston and the bottom of the cylinder 

 let any quantity of steam be contained ; if the piston be drawn upward, so as 

 to produce a larger space below it in the cylinder, the steam will expand, and 

 fill the increased space as effectually as it filled the more limited dimensions 

 in which it was first contained. As it expands, however, its elastic pressure 

 diminishes in exactly the same manner, and in the same proportion, as that of 

 atmospheric air. When the space it occupied is doubled, its temperature 

 being preserved, its elastic pressure is halved ; and, in like manner, in what- 

 ever proportion the space it fills be increased, its elastic pressure will be in 

 the same proportion diminished. 



It is found that the steam which is raised from water boiling under any given 

 pressure has an elasticity always equal to tlie pressure under which the water 

 boils. Thus, when water is boiled under the ordinary atmospheric pressure, 

 when the barometer stands at thirty inches, the steam which is dismissed at 

 the temperature of 212 has an elastic pressure equal to that of the atmo- 

 sphere. If water be boiled under a diminished pressure, and therefore at a 

 lower temperature, the steam which is produced from it will have a pressure 

 which is diminished in an equal degree. Thus, water boiled under pressure 

 corresponding to fifteen inches of mercury, and at a temperature of 180, will 

 produce steam, the elasticity of which will be equivalent to a column of fifteen 

 inches of mercury. 



Numerous experiments have been made, and investigations instituted, with 

 a view to determine some fixed relation between the temperature at whioh 

 water boils, and the elasticity of the steam which it produces ; but hitherto 

 without success. That some fixed relation does exist, there can be no doubt ; 

 because at the same temperature steam of the same elasticity is invariably pro- 

 duced. Tables are constructed expressing the elasticity or pressure corre- 

 sponding to different temperatures, and empirical formulae or rules have been 



