180 On Steam and Brines 



tangent, yet the difference is none the less real on that 

 account. 



In our mechanical experiment the excess of pressure of 

 the water at any moment above that of the atmosphere is 



-JY^, and when this is multiplied by the quantity of water 



present, lV n> the product is constantly b. Now b is the 

 equivalent of the salt dissolved, therefore our mechanical 

 experiment represents the case wJiere the increase of steam 

 tension, neutralised by the salt, is proportional to the quantity 

 of the salt. 



For convenience in reference, the values of the tension of 

 saturated steam, at temperatures from 90 C. to 120 C., 

 expressed in kilogrs. per sq. cm., are collected in Table VIII, 

 page 224. 



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

 elastic tank. The atmospheric pressure is 733'5 mm., or 

 i kilogr. per sq. cm. The depth of the tank, which can be 

 enlarged laterally, is uniformly i cm., therefore its volume at 

 the beginning is 100 cub. cms., and its surface is 100 sq. cms. 

 The boiling temperature of pure water at a pressure of 

 i k./cm.'- is 99'O9 C. The tank is securely covered, and its 

 temperature raised to ii9*57C., 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.-'. 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 ii9'57 C. is 2 k./cm. 2 . 

 Therefore, our extra load, , 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. 



