186 
MR. W. FAIRBAIE^S’ AND MR. T. TATE ON THE DENSITY 
vapour of ether, which led him to conclude that at pressures somewhat above the atmo- 
spheric, the vapour of this substance does not follow the gaseous laws. Although these 
experiments were conducted on a scale too limited for deriving exact quantitative results, 
they enabled him to test the precision and delicacy of the saturation-gauge, which forms 
the leading feature of the method on which the following experiments were conducted. 
As yet, therefore, the results derived from the dynamical theory of heat require to be 
verified by direct experiments. 
The general features of our method of ascertaining the density of steam consist in 
vaporizing a known weight of water in a large glass globe ■v\T.th a stem, of known capa- 
city and devoid of air, and observing the exact temperature at which the whole of the 
water is just vaporized. Then, knowing the weight, volume, and temperatui*e of the 
steam, its specific gravity may be calculated. In order to pursue this method with 
safety and with the requisite amount of accuracy, the following peculiarities of construc- 
tion of the apparatus were adopted ; — 
First. In order to secure the thin globe from bursting, and at the same time to have 
it uniformly heated, it is placed in a strong closed copper steam-bath, ha'sing a thermo- 
meter and pressure-gauge attached, and a strong glass tube, closed at its exterior extre- 
mity, passing through a stuffing-box for receiring the stem of the globe. By this 
arrangement the glass globe is secured from bursting ; for whatever may be the elasticity 
of the steam, the internal pressure in the globe is always exactly balanced by the external 
pressure in the steam-bath. 
Second. When a given weight of water is vaporized in a closed vessel, devoid of air, 
the steam is said to be in a state of saturation so long as any portion of liquid remains 
in the vessel ; but after all the water is vaporized, heat being still applied, the steam 
becomes superheated, or heated beyond the temperature just requisite for vaporizing all 
the water ; by way of distinction, we shall call this point the maximum temperature of 
saturation. Now since we have to find hy observation the temperature of the steam 
exactly at the point when the whole of the water is vaporized, the determination of this, 
with sufficient accuracy and delicacy, has hitherto formed the great practical difficulty 
attending experimental researches on the density of vapours. We have overcome this 
difficulty by using what may be called a saturation gauge, the form of which varies 
according to circumstances, but the principle on which it is constructed may be illus- 
trated as follows : — 
Imagine two globes, A, B, fig. I, connected by a bent tube con- 
taining mercury and immersed in a large bath of liquid, to secure 
uniformity of temperature. Suppose these globes devoid of air, but 
containing weighed portions of water, say 20 grains in A and 30 in 
B. If heat be now applied to the liquid-bath so as to increase the 
temperature of the globes, this weighed portion of water will gra- 
dually pass into steam, and the elastic force in each globe will increase in a ratio corre- 
sponding with the temperature, but without in the least affecting the uniformity of level 
