M Royal Society : — 



"Experimental Researches to determine the Density of Steam at 

 different Temperatures, and to ascertain the Law of Expansion of 

 Superheated Steam " (Phil. Trans. 18G0, p. 185), it was shown that 

 although Dumas, Gay-Liissac, and other distinguished physicists 

 had determined the density of steam at 212°, it was, however, "left for 

 these researches to ascertain the law of density, volume, &c. at all 

 temperatures, and also the law of expansion of superheated steam. 

 These experiments have therefore been continued, and have elicited 

 remarkable results as regards the rate of expansion at various tempe- 

 ratures. 



The earliest experiments on this subject were made by Mr. Frost 

 in America, but without sufficient accuracy to be of scientific value. 

 Mr. Siemens has also experimented on steam isolated from water ; 

 his results give a much higher rate of expansion for steam than for 

 ordinary gases ; but, owing to some obvious defects of Mr. Siemens' s 

 method of conducting the experiments, we consider his results are 

 not reliable. 



For gases, the rate of expansion is expressed by the formula for 

 constant volume, 



P.lH' W 



where E is a constant determined by experiment, and decided by 

 Regnault as 459 in the case of air. In the paper alluded to, it was 

 shown that, with a certain proviso, the rate of expansion of super- 

 heated steam nearly coincided with that of air. Within a short 

 distance of the maximum temperature of saturation the rate of ex- 

 pansion of steam was found to be exceedingly variable ; near the 

 saturation-point it is higher than that of air, and decreases as the 

 temperature is increased, until it becomes sensibly identical with 

 that of air. The results upon which this law was based were too 

 limited in their range for much numerical accuracy in the constants 

 deduced. 



Hence it has been our object in the present paper to supply the 

 deficiency in the previous one, by affording experimental data of the 

 expansion of steam at higher temperatures and with a greater range 

 of superheating than was possible with the apparatus employed in 

 ascertaining the density of steam. The results obtained in these 

 later experiments, however, confirm the general law deduced from the 

 previous ones. 



The figure represents the apparatus used when the pressure did 

 not exceed that of the atmosphere, consisting of a glass globe (a) 

 6 inches in diameter, and stem 35 inches long; the capacity was 

 known to a point (b), where a piece of platinum wire was twisted to 

 mark accurately the point at which the mercury column in the stem 

 was to be brought to maintain a constant volume in the globe. 



A l|-inch tube (d), filled with mercury, rested upon the frame 

 (e e). The weight of the tube and frame was counterbalanced by 

 weights (w w). By such an adjustment the tube (d) could be regu- 

 lated with facility, preserving the upper level of the mercury column 



