on the Sieam Ens'ine. , 327 



•i 



produced at the temperalure of boiling water, has a force 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, as a 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 in 

 the vessel, and proportionally increases its elasticity. 



In experiments made by Perkins, steam was heated to a 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 

 reason is obvious, for it was enclosed in a separate vessel, and its 

 quantity remaining constant, it did not increase in density. Had, 

 however, a small additional quantity of water heated under pressure 

 to a high temperature, been injected, it might be inferred, that the 

 steam would have acquired the density necessary to enable it to exert 

 the force corresponding to its temperature. Perkins also established 

 tlie truth of this inference by actual experiment. Water was heated 

 in one of his generators, the safety valve of which was loaded with 

 a weight of sixty atmospheres, to a temperature of 900°; a receiver 

 was prepared, void of both air and steam, and heated to upwards of 

 1800°; a small quantity of water was then made to pass from the gene- 

 rator to the receiver ; this was instantly converted into steam, whose 

 heat was sufficient to inflame the hemp that coated tlie tube, at the 

 distance of 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, altliough the temperature 

 was lessened, the elastic force was gradually increased to one hun- 

 dred atmospheres. — p. 95. 



3. The space occupied by a given weight of vapor, depends on 

 the degree of pressure under which it is formed. Water converted 



