Messrs. Negretti and Zambra’s arrangement of the instrument is shown in figures 72 and 73.
The thermometer is made with an e
C, is a copper boiler, supported by a tripod stand so as to allow a spirit-lamp, A, made of metal to be placed underneath. The flame from the lamp may be surrounded by a fine wire gauze, B, which will prevent it being extinguished when experimenting in the external air. E E E, is a three-drawn telescope tube, proceeding from the boiler, and open also at top. Another tube, similarly constructed, envelops this, as shown by D D D. This tube is screwed to the top of the boiler, and has two openings, one at the top to admit the thermometer, the other low down, G, to give vent to the steam. As the steam is generated, it rises in the inner tube, passes down between the tubes, and flows away at G. The thermometer is passed down, supported by an india-rubber washer, fitting steam tight, so as to leave the top of the mercury, when the boiling-point is attained, sufficiently visible to make the observation. The telescopic movement, and the mode of supporting the thermometer, enable the observer always to keep the bulb near the water, and the double tube gives all the protection required to obtain a steady boiling-point. Some boiling-point thermometers are constructed with their scales altogether exposed to the air, which may be very cold, and consequently may contract to some extent the thread of mercury outside the boiler. The steam, having the same temperature as the boiling water, keeps the tube, throughout nearly its whole length, at the same degree of heat, in the apparatus described. The whole can be packed in a tin case very compactly and securely for travelling, as in fig. 72.
Directions for Using.â€”When the apparatus is required for practical use, sufficient water must be poured into the boiler to fill it about one third, through an opening, F, which must be afterwards closed by the screw plug. Then apply the lighted lamp. In a short time steam will issue from G; and the mercury in the thermometer, kept carefully immersed, will rise rapidly until it attains a stationary point, which is the boiling temperature. The observation should now be taken and recorded with as much accuracy as possible, and the temperature of the external air must be noted at the same time by an ordinary thermometer.
The water employed should be pure. Distilled water would therefore be the best. If a substance is held mechanically suspended in water, it will not affect the boiling-point. Thus, muddy water would serve equally as well as distilled water. However, as it cannot be readily ascertained that nothing is dissolved chemically when water is dirty, we are only correct when we employ pure water.