222 
OK THE DENSITY OF STEAM AT DD^FEREKT TE:VIPEEATrEES. 
Hence it appears that, as the steam becomes more and more superheated, the coefficient 
of expansion approaches that of a perfect gas, but that at and near to the maximum 
temperature of saturation the coefficient of expansion greatly exceeds that of a perfect 
gas. 
Explanation op the Plates. 
PLATE YII. 
Represents the apparatus for the experiments on the density of steam detailed in 
Section I. 
Fig. 1. Elevation showing the general arrangement of the apparatus, the copper 
boiler B, gauge G, thermometer t, oil-bath C, inner and outer tubes forming 
the saturation-gauge o, i, cathetometer yy. 
Fig. 2. A corresponding vertical section, and 
Fig. 3. A plan. 
PLATE VIII. 
Represents the apparatus for the experiments in Section II., shown in a vertical 
section : B copper boiler, f, t' thermometers, A the globe of glass containing 
the weighed portion of water, C the lower mercuiT-bath, ,r double-pointed 
pin for reading the lower mercury level, 1i lamp, and e, e gas-jets. 
Represents the cathetometer with its vernier and telescope. 
Represents the vernier and thumbscrew of the cathetometer seen on the other 
side. 
The glass globe, after the formation of the Torricellian vacuum, arranged with 
its cap k, supported by an india-rubber band for insertion in the boiler for 
one of the experiments at the higher pressures. 
A section of the cap and globe-stem of fig. 4, showing the platinum mre, &:c. 
Ditferent-sized globules for containing weighed portions of water. 
Cup and filled globule ready for transference to the glass globe. 
Bottom of the globe-stem arranged for a transference to the boiler for a low- 
pressure experiment ; the india-rubber cap c is removed during tlie experi- 
ment. 
PLATE IX. 
A diagram in which the relation of temperature and specific volume of steam is 
represented in a curve, the ordinates of which represent the increments of temperature, 
and the abscissse the increments of specific volume : the marks ( X ) indicate points deter- 
mined experimentally, and the three curves the relations of specific volume and tempe- 
rature as given by the common formula derived from the gaseous laws, by Mr. Rankine's 
formula, and by formula (7.), page 219, of this paper derived from our experiments. 
Fig. 1. 
Fig. 2. 
Fig. 3. 
Fig. 4. 
Fig. 5. 
Fig. 6. 
Fig. 7. 
Fig. 8. 
