tion, the lute gets hot, and would at last be detached, unless care were taken to cool it perpetually by a current of water.

The box thus constructed was solidly fixed on a furnace, supported on all sides by iron bars. This furnace, in the figure, is supposed to be cut in two, that we may see the pieces in its inside.

We shall terminate this preliminary description by saying, that the copper cylinder is filled with a fixed oil, which is gradually heated till it reach the requisite temperature. Then all the mouths of the furnace are shut; the heat then spreads itself uniformly through the whole mass, and the temperature remains stationary during a time sufficient to take all the requisite measures. But that nothing may alter the exactness of these determinations, it is necessary that the copper be always completely filled with oil, and that the hot column of mercury terminate at a very small height above the cover. We easily fulfil this second determination by adding, or withdrawing by means of a sucker (pipette), the requisite quantity of mercury, some instants before the observation. As to the first, it is obtained by filling the vessel with oil, when cold, and by putting at the top of the vessel, a tube, L Q, whose orifice, Q, is on a level with the under side of the cover. Through this tube the soil flows out as it dilates.

to Let us now proceed to the measurement of the temperatures, and of the heights of the columns.

The oil bath contains two thermometers, the one mercurial and analogous to that which we have had occasion to describe already, and in which the temperature is calculated by comparing the weight of the mercury which has made its escape from the instrument, with that which it contains at zero. Such is the sensibility of that which we employed, that an increase of temperature of one degree made about one decigramme of mercury Sissue out. Its reservoir, D E, is every where of the same diameter, and is plunged into the oil to the same depth as the column, A' B'. Of course, it gives the exact mean temperature of the column.

22 The second is an air thermometer, whose cylindrical reservoir, D' E', placed like that of the preceding, is terminated by a very fine tube, E'G' H', curved horizontally beyond the furnace. This tube is united at H', with a vertical tube, a little darger, and well calibred, which is plunged into the mercurial bath, K. To regulate this thermometer, the bath was in the first place heated nearly to the boiling point of the oil, while the extremity, K', of the tube remained open. When the whole excess of air had been driven out by the heat, the orifice, K', was plunged into the mercury, and by the cooling of the oil, the mercury rose gradually in the tube. It is by measuring the height of this cohimn, at the maximum of temperature, and that of the barometer, that the augmentation of the elasticity of the

air is ascertained, whence, by a very simple calculation, the temperature of the air thermometer is deduced. It is scarcely necessary to add, that the tube had been carefully dried, and that for each measurement the correction arising from the capillary depression was made.

The indications of this thermometer add nothing to the precision of those furnished by the mercurial thermometer. But we took that opportunity of again comparing the two thermometers. The results deduced from this comparison entered into the determination of the means inserted in Table I.

It remains now to describe the kind of micrometer which we employed to measure the height of the columns. This instrument (fig. 4) is composed of a thick copper rule, A B, along which moves stiffly, but smoothly, a piece of copper, MNPRS, carrying at its two ends, M and S, two rings, in which a micrometer telescope, O O', turns, furnished at its focus with a horizontal wire. From the telescope is suspended a very sensible level, the graduated tube of which serves to regulate the optical axis. This piece of copper, M N P R S, is susceptible of two movements, one very rapid, by unscrewing the lateral screw, C; the other very gentle, by turning the adjusting screw, D. The whole instrument turns round a vertical axis, which rests upon a thick triangular plane of copper, furnished with a screw at each of its summits.

The construction of this instrument enables us, as is evident, to measure the difference between the height of two columns, which are not situated in the same vertical. It is necessary for this, after having directed the glass to one of the points, to cause the axis to turn, in order to bring it in the azimuth of the other point. It is then raised or depressed the requisite quantity, which is measured on a scale engraven on the opposite face of the rule, A B, by means of a vernier moved by the piece, MNPRS. A micrometer screw would probably have been preferable had it not been for the rapidity which our experiments required. The vernier enabled us to appreciate the 50th of a millimetre, a degree of precision which we thought sufficient.

To give to this instrument all the requisite exactness, it was necessary that the smallest differences between the two heights should be appreciable; and that in the passage from one observation to another, the glass should preserve its horizontality, or at least that we should be able to appreciate the derangement. The first of these conditions was satisfied by giving the telescope a sufficiently high magnifying power; and as for the second, the particular care with which the micrometer was made, the solidity of the support on which it rested, and which was independent of the rest of the apparatus, might have led us to consider it as satisfied. However, we measured beforehand for the distance at which the telescope pointed, to what difference of height would correspond a change of inclination equal to one degree

of the level. This datum was sufficient to enable us to correct the observations in which the level was deranged.

The processes employed for regulating such instruments are too well known to require any details here. It is known that by the requisite turnings of the telescope, both upon itself and on its rings, and by observations in the different azimuths in which it may be placed, by turning the axis of the instrument, we have it in our power to render that axis vertical, and the optical axis of the telescope horizontal.

Let us return now to the apparatus of the dilatation. The micrometer was placed upon a marble plane, T, supported by mason work. The axis of the instrument was at an equal distance from the centres of the tubes, A B and A' B′, and the point, R. Hence we could measure immediately the excess of the height of this point above the summits of the columns of mercury; that is to say, the heights rh and rh', calling r the absolute height of R. To be certain that the refraction across the tubes produced no deviation in the vertical direction, we placed artificial horizons in the centre of each, on which we directed our telescope, and we ascertained that the coincidence of the wire was not altered whether we raised or turned the tube. Nothing further remained than to ascertain r. But this height remained constant in all the experiments, since the bar supporting the arch, R, was always surrounded with ice. To measure it, a vertical graduated rule was employed, the zero of which was placed upon the iron bar, M N. This rule, constructed for another purpose with very great care, gave the height within the tenth of a millimetre. But the heights measured above the bar, MN, are too great; for h, h', and r, ought to be reckoned from the axis of the horizontal tube. Hence from the height given by the rule, we must subtract half the total thickness of the tube.

To enable the reader to judge of the accuracy to which these different operations lead, let us state one of the measures taken at 100°. The height of the arch, R, above the axis of the horizontal tube was 0.58520 metre, the heights r h, r h', were respectively 0-03855 and 0.02875; therefore h = 0.54395, and l' h = 0·00980. And consequently the mean coefficient of the absolute dilatation of mercury between 0° and 100° = 5550 We see by this, that an error of two or three tenths of a millimetre on the measure of would produce only an uncertainty of two or three unities in the denominator of the preceding fraction. Thus by a particular effect of the disposition of our apparatus, those measures susceptible of the least precision can only occasion errors that may be entirely overlooked. Supposing that even the iron bar were a little deranged by the effect of the fire (though we always took care to make it horizontal by means of the levels), it would produce but very little effect upon the final result.

In this respect our apparatus is greatly superior to those employed to determine the dilatation of solids. In them the smallest derangement of the fixed point during the long duration of the experiment, does not merely affect the total length of the rule; the dilatation itself is augmented or diminished, which occasions the most serious errors. We see, on the contrary, that when, in our experiments, the heights, h and h', are affected by the cause of which we spoke, the difference h-h' which measures the dilatation is not so. For it is absurd to suppose that the instrument becomes deranged during the very short interval which elapses between the successive observation of the hot and cold column.

We have collected in the following table the mean results of a great number of observations made in the way just described. The first column contains the temperatures such as they are deduced from the dilatation of air; the second contains the mean absolute dilatations of mercury between freezing water and each of the temperatures indicated in the first column; the third column exhibits the temperatures which we should obtain, on the supposition that the dilatation of mercury is uniform, or, in other words, those which should be indicated by a thermometer formed of that fluid inclosed in a vessel, whose expansion followed the same law as its own.

molts ji neder enzoll Temperatures indicated by the dilatations of mercury

supposed uniform.

100-00

204.61

314-15 2015

Each of the results contained in this column is the mean of a great number of measures, which it would have been too tedious to have given in detail; we shall satisfy ourselves with giving the extreme values for each of the three temperatures.

SIR,

Defence of Dr. Murray's New Theory of Acids.
By John Murray, M.D. F.R.S. Edin.

(To Dr. Thomson.)

Edinburgh, Dec. 8, 1818.

In the account which you give in your number for Dec. of the mutual action of sulphurous acid and sulphuretted hydrogen, and in which an important experimental result is established, you remark of the compound which you find to be formed of these two gases, that though containing both oxygen and hydrogen, united to a combustible base, it possesses the properties of acidity in a very weak degree, and you consider this as affording a proof that my notion of the greatest degree of acidity being given to bodies by the joint union of oxygen and hydrogen is not countenanced by chemical facts, nor consistent with the phenomena of the science.

Unwilling to engage in controversial discussion, I should not probably have alluded to the subject, with the view merely of obviating an objection. But the fact becomes more interesting when it affords, as I am led to believe it does, an important illustration and confirmation of the truth of my opinion.

In a memoir read at the close of last session before the Royal Society of Edinburgh, on the Relation of the Law of Definite Proportions to the Constitution of Acids, and which I have published lately as an appendix to the new edition of my System of Chemistry, I had given the example of sulphuric acid (oil of vitriol) as affording an argument in support of my views. It is composed of 100 of sulphur with 150 of oxygen, and 56.7 of combined water; that is, of 100 of sulphur with 200 of oxygen, and 6-7 of hydrogen. Sulphurous acid is a compound of 100 of sulphur with 100 of oxygen. The proportion of 200 of oxygen, therefore, in sulphuric acid is the regular multiple conformable to the usual law. The proportion of hydrogen is that which constitutes sulphuretted hydrogen. It appears, therefore, I remark, that the proportions of both these elements are determined by their relation to the sulphur as the radical of the acid, and are those which the quantity of sulphur would separately require. This, so far as theory can discover, is not a necessary result. The oxygen and hydrogen might each have required the quantity of sulphur with which they combine; that is, the existing relations might have been those of sulphur to oxygen, and sulphur to hydrogen, in their several proportions. It is otherwise; there is the relation of sulphur to oxygen, and in addition to this of hydrogen to the same sulphur. And thus, since the same quantity of sulphur receives the acidifying influence of both elements, we discover the source of the higher degree of acid