VAPORIXATION. 51 



This is a small lid in the upper part of the boiler, properly 

 loaded, according to the force of the steam to be generated. 

 The steam of boiling water occasions a severe scald, if allowed 

 to condense upon the body. But when steam from water under 

 pressure, or " high pressure" steam, which may be of a much 

 higher temperature than boiling water, issues into the air, the 

 hand may be directly exposed to it with impunity ; and a ther- 

 mometer placed in it, shows that its temperature is greatly 

 below that of boiling water. This singular property of high 

 pressure steam is connected with the great expansion which it 

 undergoes on escaping into the air from the vessel in which it 

 was confined ; elastic bodies having a tendency when com- 

 pressed to fly asunder, not only to their original dimensions, 

 but beyond them. The steam is greatly expanded and at the 

 same time mixed with air, which prevents it from afterwards 

 collapsing. Now after being incorporated with several times 

 its bulk of air, steam is not easily condensed, but becomes 

 low-pressure steam, and may have its condensing point re- 

 duced from above 212 to 120 or 130. Hence the heat which 

 it is capable of communicating, while condensing upon the 

 hand held in it, is of much less intensity than that of ordinary 

 steam, and inadequate to occasion scalding. 



Steam, when heated by itself, apart from the liquid which 

 produced it, does not possess a greater elasticity than an equal 

 bulk of air confined and heated to the same degree, and may 

 be heated to redness, without acquiring great elastic force. But 

 if water be present, then more and more steam continues to 

 rise, adding its elastic force to that of the vapour previously 

 existing, so that the pressure becomes excessive. 



The elastic force of steam at temperatures above 212 is 

 determined by heating water in a stout globular vessel con- 

 taining mercury m, (see Figure) and water w, and having a 

 long glass tube ft, screwed into it, open at both ends, and 

 dipping into the mercury, having a scale a, divided into inches 

 applied to it. The globular vessel has two other openings, 

 into one of which a stopcock b is screwed, and into the other 

 a thermometer I, having its bulb within the globe. The water 

 is boiled in this vessel for some time, with the stopcock open, 

 so as to expel all the air. On shutting the stopcock, and con- 

 tinuing the heat, the temperature of the interior, as indicated 

 by the thermometer, now rises above 212, at which it was 



E 2 



