heated, at a uniform temperature, and to prevent it from dividing into strata unequally heated. To obviate this, the temperature may be observed at various depths and the arithmetical mean taken, or the length of the barometer may be decreased as the temperature rises. Or a barometer with two limbs may be employed, as in the researches of Dr. Ure; or, lastly, the varying temperature of the atmosphere may be substituted for that of the liquid bath, as in the experiments of Kaemtz,* intended to supply data for meteorological purposes, which extended over a period of two years, and ranged from - 15° to +80° Fahr.

Fig. 148.

Dr. Ure's† modification enables the experiments to be carried to pressures higher than that of the atmosphere. The space in the barometer tube occupied by the vapour need never be large, and the increase of elastic force is measured by the quantity of mercury which must be added to a second limb of the barometer in order to maintain the quicksilver in the first at a constant level. Thus, in fig. 148, a b c is the bent barometer tube for experiments above the atmospheric pressure, the shorter limb being enclosed in a glass vessel, which can be filled with oil and heated progressively to any required temperature. Fine rings of platinum wire are firmly fixed round the tubes at the level d d, and as the temperature rises the mercury in the limb in the bath is maintained at this level by adding mercury in the other limb, when the column de, supported by the steam, measures its elastic force.

Dalton, whose experiments were, on the whole, accurate, inferred from the results which he obtained with water and alcohol, that the tension of all vapours was equal at temperatures equally distant from their boiling points under atmospheric pressure. This law, which has since borne his name, has not

* Traité de Météorologie, vol. i. p. 290.

† Phil. Trans., 1818, p. 338.

been confirmed by experiments on a larger number of liquids. For many liquids, however, it is nearly true, at small distances above the boiling point.

Thus :

In the above table the boiling point of water is 212°, of alcohol 173°, and of ether 104°.

The following table gives a few of Dalton's results for comparison with those of other experimenters which will be given presently.

TABLE II.-ELASTIC FORCE OF THE VAPOUR OF WATER, ACCORDING TO Dalton.

In 1823 the French Government, then legislating on the subject of steam, and requiring some further knowledge of its properties, intrusted to the French Academy the conduct of some important experiments on this subject. The Academy appointed a Commission, consisting of MM. Prony, Arago, Girard, and Dulong, to investigate the subject, and their report was published in the Memoirs of the Academy for 1831.* The experiments detailed in this Report were made chiefly by MM. Dulong and Arago, by a new method, and with all the care and accuracy which was possible in the state of science at the time. They were also on a scale which is only possible where

* Mémoires de l'Institut, tom. x. p. 194, and Annales de Chimie et de Physique, tom. xliii. p. 74.

private effort is seconded by the munificence of the Govern

ment.

Their apparatus consisted of, 1st, a boiler to generate the steam, 2nd, a manometer to measure the pressure.

The pressure, which extended to twenty-four atmospheres, was in fact measured by the column of mercury it would support in an open glass tube, but as the length of tube necessary for this purpose rendered it very inconvenient, they employed an intermediate measurer, consisting of a closed air manometer, graduated by experiment with the open mercury column. At the centre of the tower of the ancient church of St. Geneviève they erected a firmly supported wooden column, to which they attached the glass tubes containing the mercury column. These tubes, thirteen in number, were each 6 feet in length, so that the mercury column for the graduation of the manometer could be as much as 86 feet in height, corresponding to a pressure of thirty atmospheres, or 450 lbs. per square inch. This column was adjusted precisely vertical, and communicated with a cistern containing 100 lbs. of mercury. The manometer, which consisted of a carefully dried glass tube, closed at the upper extremity, and 67 inches long, communicated with the same cistern, and was maintained at a uniform temperature by a stream of water circulating round it. The height of its mercury was read by means of a vernier, similar to that of a standard barometer. It is easy to see how, by means of a force pump, the pressure in the cistern of mercury could be increased at pleasure, and how the pressure could be registered by reading off simultaneously the height of the mercury in the open tube and its corresponding level in the manometer. When the value of the divisions of the manometer had been thus determined up to twenty-seven atmospheres, it became an instrument for measuring pressure of as great accuracy and delicacy as could be desired.

The boiler for generating the steam was of a capacity of 17.6 gallons, to ensure a uniform temperature, and communicated with the manometer by a tube filled with water, cooled by a refrigeratory apparatus. The temperature was measured by means of mercurial thermometers, placed in thin metal tubes, containing mercury, to protect them from pressure.

The boiler being charged, and a convenient quantity of fuel introduced into the furnace, the temperature was allowed to rise until it nearly attained a maximum. A series of readings were then taken simultaneously from the manometer and four thermometers, until the temperature passed its maximum, and began sensibly to decrease. The readings at the maximum were alone retained for calculation. Fresh fuel was then added, and a second experiment obtained.

The method, carried out with the skill for which MM. Arago and Dulong have earned so high a reputation, possesses most of the essentials of complete accuracy. Its chief defect, as M. Regnault has pointed out, lies in this, that when the pressure and temperature are changing, however slowly, it is impossible to be absolutely certain that the thermometers have followed that change with the necessary rapidity, and that they do really register the temperature at the time the observation is made. There is in these experiments one other source of possible error, namely, the use of the mercurial thermometer, which, in the higher parts of its scale does not possess the accuracy necessary in experiments of this nature. Be this as it may, these experiments are of high value and permanent importance. The results obtained in thirty experiments are given in Table IV. on next page.

Next to the experiments of the French Academy, the most important experiments on the relation of temperature and pressure of steam were those of the Franklin Institute in America. They differed considerably from those of the French physicists, and are probably less reliable. The following table gives an abstract of the results :

TABLE III.

ELASTIC FORCE OF STEAM FROM THE EXPERIMENTS OF THE FRANKLIN INSTITUTE.

TABLE IV.-RESULTS OF MM. ARAGO AND DULONG'S EXPERIMENTS ON THE RELATION OF PRESSURE AND TEMPERATURE OF SATURATED STEAM.

The experiments of Arago and Dulong give a temperature of 358°.88 Fahr. for a pressure of ten atmospheres, or 6°.38 higher than that of the American Institute. This notable difference, too great to be merely accidental, Regnault, whose experience in the matter entitles him to speak with certainty, attributes to the use of mercurial thermometers, which, although agreeing perfectly between 32° and 212°, often present at elevated temperatures a difference of many degrees. Regnault's own experiments give 356.54 on the air-thermometer as the temperature at the pressure of ten atmospheres, which is 2°.34 lower than the French Academy, and 4° higher than the Franklin Institute. As at this temperature the mercurial thermometer gives higher