84 
THE NATURE AND 
[sect. It. 
= 2-55 ^ 459 + t' y 
30 x 2-55 (459 + t') _ 76-5 (459 + t') 
And / : 30 : : 
2-55 ( 459 + t' ) : 
/ 
/ 
the volume at the force / and temperature t'. 
121. Hence, we have this convenient rule for finding the volume or space 
the steam of a cubic foot of water occupies, when the steam is of any given elastic 
force and temperature. 
Rule. To 459 add the temperature in degrees, and multiply the sum by 76*5. 
Divide the product so obtained by the force of the steam in inches of mercury, 
and the result will be the space in cubic feet the steam of a cubic foot of water 
will occupy. 
Example. If the force of the steam be 4 atmospheres, or 120 inches of 
mercury, the temperature to that force being, according to Mr. Southern’s experi 
ments 295° (art. 77.); then 459 + 295 = 754 and 
57681 
120 
754 x 76-5 
120 
= 480-7. 
Its volume found by experiment was 404; and considering the difficulty of 
ascertaining the volume, on account of the allowances to be made for escape of 
steam of such a high temperature, it agrees very well with the calculated result. 
According to Dr. Ure’s experiments, the force of steam at 295° is 129 inches, 
which gives 446 for the number of times the volume is increased by converting 
into steam of that force and pressure. 
Of the Mixture of Air and Steam. 
122. It is a well-known fact that common water contains a considerable por 
tion of air or other uncondensible gaseous matter; and when water is converted 
into steam, this air mixes with it, and when the steam is condensed, remains in the 
gaseous state. If means were not taken to remove this gaseous matter from the 
condenser of an engine, it would collect so as to obstruct the motion of the piston : 
but even when means for removing it are employed, a certain quantity constantly 
remains in the condenser of an engine; and, in order to determine its state, we 
must consider the effects produced by mixing air with steam, or vapour, at different 
temperatures and pressures. 
Let us suppose that we have air and vapour of the same temperature t, and 
elastic force p ; and that the volumes are v and v'. If they were now put one on 
the other in a closed vessel of the capacity v+v', it is plain they could preserve an 
equilibrium, because the temperature is the same, and the mutual pressures are 
equal; but this equilibrium would not be stable.