STEAM AND BRINES. 541 



steam was stopped previous to making the first weighing. These particular epochs 

 were always noted, but, as a matter of fact, a complete time-log was kept of every 

 experiment. In experiments with mixtures the temperature was noted every minute, 

 and often every half-minute, so long as salt remained undissolved. A complete time 

 record of this kind often furnishes valuable incidental information, and is often useful 

 in detecting and rectifying errors of observation. 



Chloride of Sodium. — The series with this salt is very complete, including sixteen 

 independent experiments; the atmospheric pressure varied from 550*4 to 772 mm., and 

 the temperature of saturated steam from 91*2° C. to 100-44° C. The corresponding 

 temperatures of the boiling mixture ranged from 99*3° C. to 108*98°, so that the elevation 

 of boiling point caused by saturation with NaCl ranges from 8-1° C. to 8 '54° C, or 

 nearly half a degree Centigrade of increase for a rise of boiling point of the salt solution 

 of 9*68° C. Roughly, it diminishes 0*05° C. for every degree that the boiling tempera- 

 ture of the boiling solution falls. The results of the observations in different localities 

 are collected in Table I., page 551. 



If we consider the relation between atmospheric pressure 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 

 0*0004, and the maximum deviation 0*0009. The mean of the observations made at 

 Edinburgh is 0*0001 below T 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 0*0005 below 0*7435. Taking 0*7439 for the value at Pontresina, the value of 

 t-T would be 0*015° C. less than when the general mean 0*7435 is used, and on the 

 Schafberg, using the factor 0*7430, the value of t — T comes out 0*025° C. higher than 

 with the general mean. 



p 



Table II. (page 552) has been constructed on the basis that — - = 0*7435, and 



Po 



therefore ^ = 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 p the vapour tension of water at the temperature of a boiling mixture of 

 steam and NaCl at barometric pressure, P, is given, where p= 1*345 P. The tempera- 

 ture of this boiling mixture is found from Regnault's tables connecting the temperature 

 and pressure of saturated steam, and it is given under t. 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 

 thermometer 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 convenient to 



