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XXVI. On the Law of Expansion of Superheated Steam. " 
By William Faiebaien, Esq., LL.B., F.B.S., and Thomas Tate, Esq. 
Eeceived Marcli 20, — Bead April 3, 1862. 
The following experiments have been undertaken to verify the law of expansion for 
superheated steam indicated in a previous paper*. 
The earliest experiments on the subject were made by Mr. Feost in America, but 
without sufficient accuracy to be of scientific value. Mr. Siemens has also experimented 
on the expansion of steam isolated from water ; his results give a much higher rate 
of expansion for steam than for ordinary gases ; but, owing to the obvious defects of 
Mr. Siemens’s method of conducting the experiments, we consider that his results are not 
reliable. 
For gases, the rate of expansion is expressed by the formula, for constant volume. 
P E + ^ 
Px~E + #i’ 
( 1 .) 
where E is a constant derived from experiment and determined by Regnault to be 459 
in the case of air In the paper alluded to, it was shown that, with a certain proviso, the 
rate of expansion of superheated steam nearly coincided with that of air. Within a 
short distance of the maximum temperature of saturation, the rate of expansion of steam 
was found to be exceedingly variable ; near the saturation-point it is higher than that of 
air, and it decreases as the temperature is increased until it becomes sensibly identical 
with that of air. The results on 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 afibrding experimental data of the expansion of steam at higher tem- 
peratures 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 researches, however, confirm the general law deduced from the previous ones. 
The annexed diagram represents the experimental apparatus employed where the pres- 
sures did not exceed that of the atmosphere. It consists of a glass globe a, about three 
inches in diameter, and with a stem about thirty-five inches long. The capacity of this 
globe was known to a point b on the stem, where a piece of fine platinum wire was twisted 
round it to mark accurately the level to which the mercury column in the stem was to be 
brought to maintain a constant volume in the globe. The stem dipped below into the 
* “ Experimental Eesearches on the Density of Steam at different Temperatures, and to determine the 
Law of Expansion of Superheated Steam,” Phil. Trans. 1860, p. 185. 
MDCCCLXII. 4 M 
