550 



MR J. Y. BUCHANAN ON 



The temperature of saturated steam rises at a slower rate thau its teusiou. Hence 

 in our mechanical experiment with pure water, when the area is increased by a given 

 amount, and the pressure per unit of area is correspondingly reduced, the consequent 

 fall of temperature depends on the actual temperature, and is proportional to the 

 reciprocals of the figures representing the mean difference of tension per degree for the 

 interval. In the followino- table we have values of W. each of which is the double of 



w. 



a — A. 



t-T. 



Diff. 







a 



C. 



125 



08 



17-20° 





250 



0-4 



9-62° 



7-58° 



500 



0-2 



5-14° 



4-48° 



1,000 



01 



2-66° 



2-48° 



2,000 



005 



1-35° 



1-31° 



4,000 



0-025 



0-68° 



0-67° 



8,000 



0-0125 



0-34° 



0-34° 



16,000 



0-00625 



017° 



0-17° 



the one preceding it ; the concentration is therefore halved in each case and corresponds 

 exactly with the values of a — A, which give the excess of vapour tension above atmos- 

 pheric pressure. Under (-Twe have the corresponding excesses of temperature above 

 that of water boiling at atmospheric pressure (99 '09° C). In the last column we have 

 the differences of successive values of (t — T). Did the value of t — T fall in exact propor- 

 tion to the concentration, then each of these differences should be identical with the 

 values of t — T opposite it, because the temperature interval should be halved. When 

 the value of t — T falls below 0*68° C. the interval is apparently halved, and at temper- 

 atures within half a degree of that of water boiling at atmospheric pressure, the tension 

 of saturated steam seems to vary proportionately with its temperature ; but the differ- 

 ences which undoubtedly exist occur in places of decimals which are excluded from 

 the table. 



