valve S. The piston is now at the bottom of the barrel, and the globe is full of mercury,-if the piston be now drawn up, a vacuum would be formed in the barrel, but the mercury in the globe must descend as it is above the level of the piston the whole height T, and the vacuum in the globe K would be Torricellian were there not a communication between it and the receiver R. When the mercury again ascends into the globe, it expels every particle of air provided the mercury rises into the aperture at S; and to ensure this the cap O is formed into a rim so as always to supply the contraction or waste, and it is admitted towards the end of the exhaustion by raising the valve S with the finger. The air is admitted through a hole a in the cock P, a section is shown, fig. 11. The cap O should be strong, and, if brass, should be coated with the cement used in attaching it to the glass (that used for nautical machines is best), the gauge may be attached to the cap, or inclosed in the receiver.

The stiff wire, with the valve T and the ball U, may be entirely removed; and for it may be substituted a glass tube open at both ends cemented into the cock P, and reaching almost to the bottom of the globe. The mercury, when it rises to the lower end of this tube, cuts off the communication with the receiver. This will perhaps be the simplest and best plan. It may be made a double pump by connecting the cap O with the barrel G, as on the dotted line b-one valve opening in and one out. The weight of the mercury will be no objection as the machine is small-the diameter of the globe about four inches, the height of the barrel about eight, and the whole height to the plate R, 15 or 20 inches.

ARTICLE VII.

Reply to the Remarks of X. on certain Subjects in Mr. Daniell's Meteorological Essays. By J. F. Daniell, Esq.

(To the Editors of the Annals of Philosophy.)

GENTLEMEN,

Gower-street, Sept. 4, 1824.

YOUR Correspondent X. has committed a great mistake in his remarks upon my work, which, as having found its way into the Annals of Philosophy, it may not be unnecessary to correct.

He observes, " It has always been understood that, other circumstances being alike, mercury in the barometer will have its altitude affected by the existing temperature in no other way than as that temperature alters its specific gravity." It is scarcely worth while, perhaps, to remark the inaccuracy of this expression, but the fact is, the altitude of the mercury in the barometer, New Series, VOL. VIII.

S

in measuring heights, is much more affected by the alteration in the specific gravity of the air by the existing temperature, than by that of the metal. He proceeds, "So that whether the tube expand or contract, or were it possible, do neither, whatever the material of which it is made, whatever its sectional form, equality or inequality of calibre, still the absolute dilatation and not the apparent must regulate the correction for difference of temperature."

It is evident that X. here speaks of the change in specific gravity as if it were to be ascertained by weight, and not by measure. He forgets that in the barometric experiment the only way in which the result is affected is by alteration of volume; which alteration of volume is ascertained by measure. Now as this measure cannot be taken upon any scale which is not itself liable to expansion and contraction from changes of temperature, it is clear that the alterations of the latter must be taken into account; so that if the expansion of the mercury be measured upon brass, the absolute dilatation per degree of the former must be taken minus that of the latter, or if it be compared with glass, minus that of the glass.

I shall leave MM. Dulong and Petit, whom your correspondent asserts are so" egregiously wrong," to defend themselves, should they think it worth while, being perfectly assured, in my own mind, that they are fully competent to the task; and I have not much doubt that even M. Biot would be able to rescue himself from the imputation of having made "a false conclusion from his own premises.'

With regard to the filtration and distillation of mercury, I must beg to assure X. that notwithstanding his knowledge of that metal "derived from a peculiar application of it," he may acquire much further information by inquiry of any competent workman.

If I shall have succeeded in making myself intelligible to X. I may, perhaps, be permitted to hope, that he will see the propriety of hereafter excluding from scientific controversy such expressions as that of "mere drivelling."

I remain, Gentlemen, with great esteem,
Yours faithfully,
J. F. DANIELL.

ARTICLE VIII.

New Researches on the Sulphuric Acid of Saxony. By M. Bussy.*

We have given an account in the present number of the Annals (p. 307), of M. Bussy's experiments on anhydrous sulphurous acid. He has lately examined the properties of the fuming sulphuric acid from Nordhausen, which is prepared by the distillation of green vitriol previously deprived of its water of crystallization. The results which M. Bussy obtained confirm the statement given by Dr. Thomson (System, vol. ii. p. 113), that the fuming property of the Nordhausen acid is owing to its containing a portion of anhydrous, or absolutely pure sulphuric acid, which may be separated by distillation. The properties of the anhydrous sulphuric acid (which, as our readers know, is a crystalline solid), as detailed by M. Bussy, agree very nearly with the account given of it by Dr. Thomson. We proceed to give a short extract of the most important and novel parts of M. Bussy's researches.

The Nordhausen acid boils at first at a temperature between 104° and 122° Fahr. A portion flies off in thick vapours, and when these cease to come over, a very considerable increase of heat is requisite to maintain the ebullition of the liquid, which is now reduced to the state of common sulphuric acid. When the anhydrous acid is exposed to the air, a portion evaporates at ordinary temperatures, and the remainder gradually attracts moisture, and is converted into common liquid sulphuric acid; it chars vegetable substances, such as paper and wood, the instant it comes in contact with them. No gas is disengaged by the action of the concrete acid on water; and M. Bussy ascertained that it is perfectly anhydrous by passing its vapour over caustic barytes, slightly heated in a glass tube, connected with a mercurial apparatus; a lively incandescence of the whole mass ensued, but neither sulphurous acid nor any other gas was given out. Nothing but sulphate of barytes was formed, perfectly free both from sulphite and sulphuret. By the mean of three experiments, 100 parts of concrete acid gave 288-6 parts of sulphate of barytes, which is composed of 78 parts of base united to 40 parts of dry sulphuric acid; therefore the concrete acid must have contained 97.8 parts per cent. of real acid, and 2-2 water. But the smallest proportion in which water can combine with dry sulphuric acid is that of 9: 40, and 100 parts of such acid contain 1836 parts of water; consequently the concrete acid contains none that properly belongs to its composition; and the minute quantity of 2.2 per cent.

* Journal de Pharmacie.