theory of the secular equation of the moon, we shall conclude, that since that time, the variation of the internal heat of the earth is insensible. It is true that the dilatation, the specific heat, the degree of permeability by heat, and the density of the various strata of the earth being unknown, may cause a sensible difference between the results relative to the earth, and those of the sphere we have supposed; according to which the diminution of the hundredth of a second, in the length of the day, would correspond to a diminution of two hundredths of a degree of temperature. But this difference could never extend from two hundredths of a degree, to the tenth; the loss of terrestrial heat corresponding to the diminution of a hundredth of a second in the length of the day. We may observe, even that the di minution of the hundredth of a degree, near the surface, supposes a much greater one in the internal strata; for it is known that ultimately the temperature of all the strata diminishes in the same geometric progression, so that the diminution of a degree near the surface, corresponds to a much greater diminution in the strata, nearer to the centre. The dimensions of the earth, therefore, and its inertial momentum would diminish more than in the case of the sphere we have supposed. Hence it follows, that if, in the course of time, changes are observed in the mean height of the thermometer placed at the bottom of the observatory caves, it must be attributed not to a variation in the mean temperature of the earth, but to change in the climate of Paris, of which the temperature may vary, with many accidental causes. It is remarkable that the discovery of the true cause of the secular equation of the moon, should at the same time make known to us the invariability of the length of the day, and of the mean temperature of the earth since the time of the most ancient observations.

This last phenomenon induces us to suppose that the earth has arrived at that permanent temperature, which accords with its position in space, and its relation to the sun. It is found by analysis, that whatever the specific heat, the permeability by heat, and the density of the strata of the terrestial spheroid, the increase of the heat, at a depth very small, compared to the

radius of that spheroid, is equal to the product of that depth, by the elevation of the temperature of the surface of the earth, above the state of which I have just spoken, and by a factor independent of the dimensions of the earth, and which depends only on the qualities of its first stratum relative to heat. From what we know of these qualities we find that if this elevation was many degrees, the increase of heat would be very sensible at depths to which we have penetrated and where nevertheless it has not been observed.

Note by the Editor of the Annales de Chimie, &c.

We have thought that our readers would not be displeased to meet here with some details of the method by which M. de Laplace, has established the invariability of the duration of the day.

A mean solar day, is equal to the time occupied by one revolution of the earth on its own axis, increased by the mean apparent motion of the sun, in the same interval. Theory has proved that the mean apparent motion of the sun, like that of all the planets, is constant; the duration of a solar day, therefore can only vary by a change in the velocity of the rotation of the earth.

The time in which the moon returns to the same position, relative to the sun, its conjunction for instance, is called a lunar month. This interval is evidently independent of the velocity of the earth's rotation. Our globe might even cease to turn on its centre, without the moon's advancement in its orbit suffering any alteration. From hence results a very simple method of discovering if the duration of the solar day has changed.

Suppose that at present, the duration of a lunar month be ascertained by direct observation; that is, how many days, and fractions of day, the moon occupies in returning to its conjunction with the sun. It is evident that on repeating this observation at another time, a different result will be found, if the length of the day has changed, if at the same time, the velocity of the moon has not changed. The month will appear longer, if the length of the day has diminished; and on the contrary, shorter, VOL. VIII.

I

if the day has increased. The constancy of the lunar month will indicate the invariability of the day.

All observations combine to prove that from the time of the Chaldeans, to our own days, the duration of the lunar month has been gradually diminishing. It follows, therefore, from what has been stated, either that the velocity of the moon has increased, or that the solar day has lengthened. But M. de Laplace has discovered by theory, that there is in the motion of the moon, an inequality known by the name of secular equation, which depends on the variation of the excentricity of the earth's orbit, and of which the value in each century may be deduced from the change of this excentricity. By the assistance of this equation, the increase of velocity above noticed is perfectly accounted for. There is, therefore, no reason to suppose that the duration of the day is not sensibly constant.

Let us admit for a moment, with M. de Laplace, that this duration, surpasses at present that of the time of Hipparchus, by the hundredth of a decimal second. The duration of a century now, or of 36,525 solar days, would be longer than the duration of a c9ntury 2,000 years ago, (Hipparchus lived about 120 years before our era), by 365."25. In this interval of time, the moon describes an arc of 534".6; this quantity, therefore, expresses the difference between two arcs traversed by the moon in a century now, and -in one of the time of Hipparchus; but as these arcs, determined 血 by observation and corrected by the secular equation, do not differ by a quantity so large, we may conclude that in this long interval the duration of the day has not varied by the hundredth of a second.

ART. IX. On the Preparation of Oxygenated Water. By M. Thenard.

THE preparation of oxygenated water requires certain precautions, without which success will only be partial. That none may be omitted, I shall describe the process in the most minute

manner.

1. Nitrate of barytes should first be obtained perfectly pure, and, above all, free from iron and manganese. The most certain mean of procuring it is to dissolve the nitrate in water, to add to the solution a small excess of barytes water, to filter and crystallize.

2. The pure nitrate is to be decomposed by heat. This ought not to be done in a common earthen-ware retort, because it contains too much of the oxides of iron and manganese, but in a perfectly white porcelain retort. Four or five pounds of nitraté of barytes may be decomposed at once, and the process will require about three hours. The barytes thus obtained, will contain a considerable quantity of silex and alumine, but it will have only very minute traces of manganese and iron, a circumstance of essential importance.

3. The barytes, divided by a knife into pieces as large as the end of the thumb, should then be placed in a luted tube of glass. This tube should be long, and large enough to contain from one kilogramme to 14 kilogrammes, (from 2lbs. 4oz. to 3lbs. 6oz. nearly). It is to be surrounded with fire, and heated to dull redness, and then a current of dry oxygen gas is to be passed through it. However rapid the current, the gas is completely absorbed; so that when it passes by the small tube which ought to terminate the larger one, it may be concluded that the deutoxide of barium is finished. It is, however, well to continue the current for seven or eight minutes more. Then the tube being nearly cold, the deut-oxide, which is of a light gray colour, is taken out, and preserved in stoppered bottles.

4. A certain quantity of water, for example, two decilitres, (41 pints) is then taken; to which is added as much pure and fuming hydrochloric acid as will dissolve 15 grammes, (232 grains) of barytes. The acid solution is put into a glass with a foot, and ice placed round it, which must be renewed as it melts. Then 12 grammes, (185 grains,) of the deut-oxide are to be very slightly moistened, and rubbed, by portions, in a mortar of agate or glass. As these portions are reduced into a fine paste, they are to be removed by a box-wood knife, and placed in the fluid; they will soon dissolve without effervescence, especially if

slightly agitated. When the solution is made, pure and concentrated sulphuric acid is added, drop by drop, the fluid being stirred at the time with a glass rod, until there is a slight excess of it, which is easily known by the property possessed by the sulphate of barytes formed at the moment, of readily depositing in flocculi; then, as at first, a fresh quantity of deut-oxide is dissolved in the fluid, which is again precipitated by sulphuric acid. The deut-oxide is always easy to distinguish from the sulphate. It is important to add enough sulphuric acid to precipitate all the barytes, but not too much. If enough is not added, the fluid filters with difficulty, and slowly; if too much is added, the filtration also goes on badly. On arriving at the exact point mentioned, the filtration takes place with the utmost facility. When the filtration is completed, a small quantity of common water is to be passed through the filter, and added to the first fluid; in this way the latter does not lose in volume: then, that nothing may be lost, it is necessary to spread the filter on a glass plate, to separate the substance from it, diffuse it through a small quantity of fresh water, and filter it. The water, thus obtained, is but slightly charged, but it is useful to wash the future filters.

This operation being finished, another is made exactly similar to it, i. e., deut-oxide of barium is to be dissolved in the fluid; the barytes is to be precipitated by sulphuric acid, and so on; and the fluid is not to be filtered until after two solutions, and two precipitations. It is on this new filter that the water obtained by washing the precipitate in the preceding operation is to be poured; after which fresh weak water is procured by washing the matter on the last filter.

The second operation is followed by a third, that by a fourth, and thus, until the fluid is sufficiently charged with oxygen. By using the quantity of hydro-chloric acid mentioned, from 90 to 100 grammes (29 to 32ozs.) of deut-oxide of barium may be operated on, and a fluid will be obtained charged with 25 or 30 times its volume of oxygen. If it is required to be further oxygenized, more hydro-chloric acid must be added.

I have many times succeeded by this means in charging the