OF STEAM AT DIFFERED TEMPERATURES. 
187 
of the mercury columns a and because the pressure on each side will be the same. 
But after the whole of the water in the globe A has been evaporated, this equality of 
pressure no longer exists, and the column a will rise. The pressure in B increases in 
the ratio for saturated steam, whilst that in A increases in the much smaller ratio of 
superheated steam, and hence the difference of level of the columns. The instant at 
which the columns begin to rise on one side and to fall on the other, is the point at which 
the whole of the water in A is converted into steam, and the temperature then noted is 
the maximum temperature of saturation. The following theoretical Table gives, approxi- 
mately, the rise of the mercury column at several temperatures : — 
Increments of pressure for 1° Fahe. 
Iba. 
o 
for expansion. 
for vaporization. 
Difference. 
At 
4 
and 
152 
0-012 
0-222 
0-21 
At 
7 
and 
176 
0-022 
0-32 
0-30 
At 
15 
and 
213 
0-044 
0-60 
0-56 
At 
20 
and 
228 
0-060 
0-80 
0-74 
At 
61 
and 
295 
0-16 
2-00 
1-84 
At 
74 
and 
308 
0-20 
2-22 
2-02 
In Plate VII. fig. 1 is an elevation, fig. 2 a section, and fig. 3 a plan of the apparatus 
employed in these researches for pressures varying from 15 to 70 lbs. on the square inch, 
or from one to five atmospheres. A is the glass globe for the reception of the weighed 
portion of water, drawn out into a stem about 32 inches long. The average size of the 
globes was 5;^ inches in diameter, or 75 cubic inches capacity; the stems were about 
f to inches bore. B B is the copper boiler or steam-bath in which the globe was 
heated uniformly throughout. The copper bath is prolonged by a strong glass tube, o o, 
Ij inch in diameter, and hermetically sealed at the bottom : this tube is fixed to the 
boiler by a stuffing-box, s s, its upper part being trumpet-mouthed to prevent its being 
forced down by pressure ; the joint in the stuffing-box was made by a thick ring of 
^’ulcanized india-rubber, which, with the temperatures employed in this apparatus, 
answered its purpose perfectly. To heat this outer tube uniformly, which, from being 
exposed to pressure as well as temperature, was exceedingly liable to explode, an outer 
oil-bath, C C, was used, made of blown glass, 20 inches long, and resting in a sand-bath, 
1 1. This bath was supported on the tripod m m m. The copper bath was heated by 
a spiral coil of jets of gas, E E, and the oil-bath by a large and powerful wire-gauze 
lamp, h h, protected from draughts by a simple ring of sheet iron, k k. The tempera- 
tm’e thus obtained and distributed uniformly throughout the glass tube and steam-bath 
by convection, was measured by the thermometer u in the oil, and the thermometer t in 
the steam, exposed naked, and fixed by the stuffing-box v. Opposite to the -thermometer, 
t, is the stopcock, y), by which the steam is, if necessary, blown off ; and on the top of the 
boiler is a gauge, G, of the Schsefier construction, for roughly indicating the pressure in 
the boiler. The two mercury columns, the outer in the tube o o, and the inner in the 
globe stem i i, separating the vapour and water in the steam-bath from that in the 
2 c 2 
