The force of vapour from water between 80° and 212', may also be determined by means of an air pump; and results exactly agree with those determin ed above. Take a florence flask, half filled with hot water. into which insert the bulb of a thermometer, then cover the whole with a receiver on one of the pump-plates, and place a barometer gage on the other; the air being slowly exhausted, mark both the thermometer and barometer at the moment ebullition commences, and the height of the barometer gage will denote the force of vapour from water of the ob served temperature. This method may also be used for other liquids. It may be proper to observe, the various thermometers used in these experiments were duly adjusted to a good standard one.
After repeated experiments by all these methods, and a careful comparison of the results, I was enabled to digest the following table of the force of steam from water, in all the temperatures from to 212°.
Dissatisfied with his own experiments, in the re sults of which he observed irregularities which he could not explain, Mr. Watt, in the year 1796, requested Mr. Southern to try them over again, and, in fulfilling his request, he was assisted by Mr. William Creighton. The results of these experi ments, which were first published in Dr. Brewster's edition of Professor Robison's works, vol. ii. p. 16. are as follows:— The next experiments on the elasticity of steam were those of Dr. Ure, which were made at tempera tures from 24° to 312°. They are as follows:— As Dr. lire's experiments have been regarded as furnishing us with the most accurate and uniform series of results, we shall lay before our readers an account, of the apparatus and methods by which they were made.
Fig. 1. Plate DIV. " represents the apparatus used for temperatures below and a little above the boiling point, and Fig. 2. and 3. for higher temperatures, Fig. S. being the most convenient. " One simple principle," says Dr. Ure, " pervades the whole train of experiments, which is, that the progressive in crease of elastic force developed by heat from the liquid incumbent on the mercury at / /' /'" (Fig. 1, 2, 3.) is measured by the length or columns which must be added over L, the primitive level below, in order to restore the quicksilver to its primitive level above at 1. These two stations or points of departure are nicely defined by a ring of platina wire twisted firmly round the tube.
At the commencement of the experiment after the liquid now freed from the air has been let up, the quicksilver is made a tangent to the edge of the up per ring, by cautiously pouring mercury in a slender stream into the open leg of the syphon D. The level ring below is then carefully adjusted.
From the mode or conducting these experiments, there remained always a quantity of liquid in contact with the vapour, a circumstance essential to accuracy in this research.
Suppose the temperature of the water or the oil in A to be 32°, F, as denoted by a delicate thermometer, or by the liquefaction of ice, communicate heat to the cylinder A by means of two argand flames playing gently on its shoulder at each sick. When the ther mometer indicates 42', modify the flames, or remove them, so as to maintain an uniform temperature for a few minutes. A film, or line of light, will now be perceived between the mercury and the ring at 1, as is seen under the vernier of a mountain barometer, when it is raised a few feet off the ground. Were the tube at 1 and L, of equal area, or were the rela tions of the areas experimentally determined, then the rise of the quicksilver above L would be one half, or a known submultiple of the total depression, equi valent to the additional elasticity of the vapour at 42°, above that at 32°. Since the depressions, how ever, for 30 or 40 degrees in this part of the scale are exceedingly small, one half of the quantity can scarcely be ascertained with suitable precision, even after taking the above precautions; and besides, the other sources of error, or, at least, embarrassment, from the inequalities of the tube, and from the lengthening space occupied by the vapour as the temperature ascends, renders this method of reduc tion very ineligible." By the other plan, we avoid all these evils. For whatever additional elasticity we communicate to the vapour above 1, it will be faithfully represented and measured by the mercurial column, which we must add over L, in order to overcome it, and restore the quicksilver under 1 to its zero, or initial level, when the platina ring becomes once more a tangent to the mercury.
At E, a piece of cork is fixed between the parallel legs of the syphon to sustain it, and to serve as a point, by which the whole is steadily suspended.