On Steam and Brines 169 



and the vapour tension of water at the temperature of the 

 boiling mixture, we see that it is practically constant. The 

 mean of the sixteen values is 07435. The average deviation 

 from the mean is O'OOO4, and the maximum deviation O'ooog. 

 The mean of the observations made at Edinburgh is O'oooi 

 below this. The five observations made at Pontresina and St 

 Moritz give a mean of 0^0004 above, and the three observations 

 on the Schafberg a mean of O'OOO5 below 07435. Taking 

 07439 for the value at Pontresina, the value of t- T would 

 be o'Oi5 C. less than when the general mean 07435 is used, 

 and on the Schafberg, using the factor 07430, the value of 

 / T comes out o'O25 C. higher than with the general mean. 

 Table II (page 187) has been constructed on the basis that 



P -h 



= 07435, an d therefore |j-= 1*345. The barometric pressure, 



P, is given for intervals of 10 mm. from 790 mm. to 550 mm. 

 The temperature of saturated steam at pressure, P, is given 

 under T. Under / the vapour tension of water at the tem- 

 perature of a boiling mixture of steam and NaCl at baro- 

 metric pressure, P, is given, where /= r^^P. The temperature 

 of this boiling mixture is found from Regnault's tables con- 

 necting the temperature and pressure of saturated steam, and 

 it is given under /. The difference (t- T} gives the elevation 

 of the boiling-point of saturated NaCl brine above that of 

 pure water at barometric pressure P. 



The figures in this table show that a boiling mixture of 

 steam and NaCl at a known barometric pressure gives the 

 means of obtaining an independent fixed point on a thermo- 

 meter about 8 to 8'5 C. above that furnished by the boiling- 

 point of pure water at the same pressure. In most cases the 

 normal pressure of 760 mm. would be used, but as the mean 

 pressure in inhabited countries is less than 760 mm., it is con- 

 venient to be able to use directly the values observed at the 

 existing pressure, and our table affords the means of doing 

 so, assuming that the readings of our thermometers as cor- 

 rected are exact. 



In Table III are given the saturation values for the simple 

 salts. They are arranged in order of the temperature of the 



