or STEAM AT DIEEEEENT TEMPEEATUEES. 
189 
globe, in which a Torricellian vacuum had been formed. Here the weight of water 
introduced 
To form the Torricellian vacuum, the globe, filled with warm dry mercury, was heated 
upon a sand-bath until the mercury boiled ; the stem was then filled with dry mercury 
and the globe inverted, with its stem inserted in a basin of mercury. The globule was 
then introduced into the stem, and allowed to ascend through the mercury column into 
the globe. In order to transfer the globe from the basin to its place in the steam-bath, 
a cap, k (Plate VIII. figs. 4 & 5}, filled with mercury, was suspended from the stem by 
an india-rubber strap, a platinum wire being placed between the cap and globe-stem to 
ensure free passage for the mercury. The cover of the boiler, B B (Plate VII.), being 
then taken off, and the outer tube, o o, partially filled with dry mercury, the globe was 
raised and inserted into its place, resting on a tripod in the boiler, as shown in the dia- 
gram. The cover was then fixed with a fiax and red-lead joint, and the cock con- 
nected with a single barrel air-pump ; exhaustion was effected so that the columns in 
the globe-stem and outer tube came nearly to a level ; tile air-pump was removed, and a 
portion of water allowed to enter through the cock. The gas-lights were then kindled, 
and until the water reached the boiling-point, the columns were maintained in position 
by the air-pump. The object of this, it will be seen, was to prevent any water passing 
round from the boiler to the globe. After boiling for a time the cock jp was closed, 
and the process of vaporization went on simultaneously in the bath and globe, the tem- 
perature being kept sufficiently high in the oil-bath C C to maintain the water in the 
outer tube in a state of ebullition. 
Vffien all the parts of the globe have attained the same temperature as the steam in 
the bath, the mercury in the stem of the globe will stand a little above the mercury in 
the outer tube C C, the difference, a h, balancing the column of water, h c, and pos- 
sibly the pressure of any air left in the steam-bath. The temperature of the baths is 
slowly and uniformly increased until the temperature of the vapour in the globe is con- 
siderably above the maximum temperature of saturation : after having been maintained 
for a considerable period at this temperature, the levels of the columns were observed ; 
then, the temperature being allowed to sink some degrees, the operation was repeated, 
and the temperature again reduced until the columns became stationary, indicating 
saturated steam in the globe as well as the bath. The readings of a series of file marks 
on the globe-stem were taken at the same time, by which the capacity of the globe in 
each position of the mercmy column could be calculated. All the elements were thus 
ascertained for calculating the density of the steam. 
Let w be put for the weight of distilled water, at a temperature of 1 Fahe., filling 
the globe to the point at which the mercury columns stood at the maximum tempera- 
ture of saturation ; let w be the weight of water vaporized ; v = the specific volume of 
the steam, or the number of times the volume of steam exceeds the volume of the water 
from which it is raised ; then 
w 
^ V)' 
( 3 .) 
