STEAM AND BRINES. 549 



peratures from 90° C. to 120° C, expressed in kilogs. per sq. cm., are collected in 

 Table VIII, page 572. 



In Table IX. we have the case of 100 grms. water in the elastic tank. The 

 atmospheric pressure is 735-5 mm., or 1 kilogr. per sq. cm. The depth of the tank, 

 which can be enlarged laterally, is uniformly 1 cm., therefore its volume at the begin- 

 ning is 100 cub. cms., and its surface is 100 sq. cms. The boiling temperature of pure 

 water at a pressure of 1 k/cm 2 . is 99-09° C. The tank is securely covered, and 

 its temperature raised to 119-57° C, at which temperature the tension of saturated steam 

 is exactly 2 kilogrs. per sq. cm. Neglecting, or allowing for, any thermal expansion of 

 the tank and its contents, the area of it has remained the same, namely, 100 cm 2 . Let 

 the cover be loaded until its fastenings just become slack, then the surface of the water is 

 pressed by the loaded cover and by the atmosphere ; the latter on a surface of 100 cm 2 , 

 amounts to 100 kilogrs., and the former makes up the difference between this weight 

 and 200 kilogrs., because the pressure of the steam at 119-57° C. is 2 k/cm 2 . There- 

 fore, our extra load, b , is 100 kilogrs., and it may be looked on as the weight of the cover, 

 which, like the tank, is supposed to be capable of lateral extension of its area without 

 alteration of weight, keeping pace with the lateral increase of volume and area of the 

 tank, while its contents are being increased by the condensation of steam. 



Let the temperature of the water in the tank be now reduced to 118-03° C, at which 

 temperature its vapour tension is 1*9 k/cm 2 ., and let steam of this temperature be con- 

 densed in it. The volume of water in the tank increases while its area expands in the 

 same ratio, until the weight pressing on its surface is at the rate of 1 -9 k/cm 2 ., when 

 the steam will lift the cover and escape. The weight of the cover has remained con- 

 stant (= 100 kilogs.), but the area exposed to the atmospheric pressure has increased. 

 The resulting area and volume of water is given by the equation 1'9W= 100 + W, whence 



0-9 



Again, let the temperature of the water in the tank be reduced to 116-29° C, at 



which temperature its vapour tension is 1*8 k/cm 2 ., and let steam of this temperature be 



passed into it. It will be condensed until the volume and surface of the water have 



increased to such an extent that the total pressure on its surface is at the rate of 1*8 



k/cm 2 . As before, we find the value of W, when this point has been reached, to be 



100 



-^Tq = 125, and so on. 



In the principal table the pressure is reduced by 0'1 k/cm 2 . at a time, from 2*0 to 

 1-1 k/cm 2 ., then by 0-01 k/cm 2 . down to 1-01 k/cm 2 ., and by 0-001 down to 1*001 

 k/cm 2 . The value of W is inversely proportional to the difference of pressure, a — A, 

 and becomes infinite when a — A = 0. On the other hand, it diminishes rapidly at high 

 temperatures and pressures, and would become when a — A— 00. 



The table is carried upwards to a=10 k/cm 2 ., and downwards to a= 1-001 k/cm 2 . 

 In these limits the value of "W varies from 11-1 to 100,000. 



