¥ 
on the Sleam Engine. 327 
produced at the temperature of boiling water, has a foree equal to 
the pressure of the atmosphere. It has, in fact, a force a little 
greater than that, since it overcomes that pressure. If we introduce 
a few grains of water into a vessel, asa flask, and place the vessel] 
over the fire, the water will soon be converted into steam, which will 
expel the air of the vessel and fill its whole capacity. If we now 
close the orifice of the vessel and continue the heat, the steam will 
expand in the same manner as air would do under similar cir- 
cumstances, which is at a comparatively moderate rate, so that it 
might be heated red hot without exerting any very violent force. ‘If, 
however, the vessel is partly filled with water, and the heat is con- 
tinued -as before, then the elastic force is rapidly augmented, and 
becomes at length so great as to burst almost any vessel that can be 
provided; for every portion of new vapor that is raised from the 
surface of the water, adds to the density of that which was before i in 
= vessel, and proportionally increases its elasticity. 
Tn\ experiments made by Perkins, steam was heated toa tempera- 
ture at which, if of a corresponding density, it ought to have exerted 
a force of fifty six thousand pounds per square inch, but which did 
not exert a pressure of more.than one hundred and fifty pounds. ~ The 
tedson is obvious, for it was enclosed in a separa ate vessel, and its’ 
quantity remaining constant, it did not increase in’ density, Had, 
however, a smaall additional quantity of water heated under pressure 
toa high temperature, been injected, it might be inferred, that the 
Steam, wali have acquired the densi ty necessary to enable it to exert 
e force corresponding to its temperature. Perkins also established 
the truth of this inference by actual experiment. Water was heated 
‘in-one of his generators, the safety valve of w shich was loaded with 
‘Weight of sixty atmospheres, to a temperature of 900°; a receiver 
Was: prepared, void of both air and steam, and heated to upwards of 
;.asmall quantity of water was. then made to pass from the gene- 
ralor to the receiver ; this was instantly converted into steam, W a hanas 
heat Was suflicient to inflame the hemp that coated the tube, at, the 
eof ten feet from the generator. Its temperature was there- 
fore estimated at not less than 1400°. ‘In spite of this high tempera- 
ture at which the steam was formed, its pressure did not “exceed five 
atmospheres, But by injecting more water, although the temperature 
Was lessened, the clastic force was gradually Gaeoened to one hun- 
‘Atmospheres,—p. 95. 
> The space occupied by a 1 weight of vapor, depends on 
the degree of ie under which it is formed. Water converted 
