PYRONOMICS. 93 
does not reach its surface, so that the former has the same temperature as 
the water, the latter as the steam. A thermometer is sunk in each barrel, 
with its upper end bent horizontally ; this horizontal portion, as represented. 
more clearly in fig. 20, is maintained at a constant temperature by a stream 
of water. From the boiler rises a vertical tube, 0, in which the steam 
ascends, and at w presses against the top of a column of water which fills 
the tube wdb, and the upper part of the cast iron vessel, vv’. This pressure 
of the vapor is transmitted to the surface of the mercury in vv’, and produces 
a compression of the air in the manometer tube, mm’, by means of which 
the tension of the vapor may be ascertained. To determine the varying 
height of the mercury in the vessel, vv’, a glass tube, nn’, is employed, com- 
municating with both the upper and under part of the vessel; in this tube 
the height of the mercury may be ascertained by means of.a movable 
slide in the graduated rod, z. 
Observations with this apparatus are conducted in the following manner :— 
Water is poured into the boiler, until the gun barrel containing the smaller 
thermometer stands just above the surface. This is kept boiling for fifteen 
or twenty minutes, with the safety-valve and the vertical tube, 6, remaining 
open, in order to expel all the atmospheric air. When this is effected, fuel 
is placed in the grate of the furnace, and all the openings in the boiler 
closed. Both thermometers, and the mercury in the manometers, then 
quickly rise to a maximum, which being attained, the height of the mercury 
in the above-named instrument is ascertained by two observers, and 
carefully noted down. 
To determine from experiments already made, degrees of tension which 
have not been observed, or ?n other words, to interpolate the series, it 
becomes necessary to develope certain empirical formule for the purpose, 
whose results shall agree in the closest possible manner with the observations 
already made. In these formule the force of tension, E, and the corres- 
ponding temperature, T, must occur, of which one or the other must be 
known. Such a formula, with which the observations made by Arago 
and Dulong agree closely, is that of Tredgold, available to a pressure of 
23.94571T ae 
800 L3T 2:2960383. For higher ten- 
sions, even up to fifty atmospheres, we have the formula E = (1 + 0.7153T)’ 
where T indicates the temperature above 212° F. 
Hitherto investigations have been instituted principally with reference 
to the degree of elasticity of the vapor of water; quite recently, however, 
experiments have been made with the vapors of alcohol, sulphuret of 
carbon, and sulphuric ether, by Ure, Schmidt, and Muncke. Bunsen has 
investigated the tension of some condensed gases, particularly of sulphurous 
acid, cyanogen, and ammonia. 
The density of watery vapor is best ascertained by means of the apparatus 
invented by Gay Lussac (pl. 19, fig. 21). Upon the furnace, f, stands the 
cast iron vessel, c, containing mercury; in this a graduated tube, g, is 
267 
four atmospheres, where log. EK = 
