inference from these very experiments of Mr. Dalton, is the decreasing capacity of water, with the increase of its temperature. It deserves to be remarked, that my experiments on the relative times of cooling a globe of glass, successively filled with water, oil of vitriol, common oil, and oil of turpentine, give exactly the same results as Mr. Dalton had derived from mixtures of two ounces of ice with 60 of water, at different temperatures. This concurrence is the more satisfactory, since when my paper on the specific heats of the above bodies, published in the Annals of Philosophy for October 1817, was written, I had no recollection of Mr. Dalton's experiments.

In the Annals of Philosophy for March 1819, Dr. Thomson has made the following remarks in reviewing my paper on heat: "The second topic which Dr. Ure discusses in this paper, is Mr. Dalton's opinion that the common thermometer is an inaccurate measurer of heat, and that mercury and all liquids expand as the square of the tem perature, reckoning from the freezing point. It is not necessary to give a particular detail of the facts contained in this part, as Mr. Dalton's opinions on this subject had been already overturned by the experiments of Dulong and Petit. Dr. Ure's notion, that the capacity of bodies for heat diminishes as the temperature increases, is directly contrary to the results of the experiments of Dulong and Petit on the subject. It seems also contrary to analogy in other cases. We know that the capacity of elastic fluids increases as they become rarer, and that the rarest of all the elastic fluids has the greatest capacity. It is reasonable, I think, that this should be the case; for the further the particles of a body are removed from each other, the greater must the quantity of heat be, which shall be capable of producing a given effect on

a general inference from a particular case, a practice, it must be confessed, too common with some chemical writers, So far from asserting the proposition for all bodies, the idea is thrown out, of its being perhaps a property peculiar to water, like that of its expanding by dimiuntion of its heat, after being cooled down to 39°.

The total absence in the gases of cohesive attraction, that power which governs the phenomena of solids as to heat, and modifies those of liquids, renders the analogy of elastic fluids adduced by Dr. Thomson quite irrelevant. "The above circumstance in water, renders it peculiarly qualified for serving as the magazine and equalizer of the temperature of the globe. Since at our ordinary atmospherical heats, it possesses the greatest capacity for caloric, small variations in its temperature gave it a great modifying power over the circumambient air." See New Experimental Researches on some of the leading doctrines of Caloric, in the Philosophical Transactions for 1818, or in Tilloch's Magazine, vol. 53. I have looked with attention over MM. Dulong and Petit's paper, for the results of their experiments on the subject, which Dr. Thomson pronounces to be directly contrary to mine, but I could find nothing which affects my proposition with regard to water. Their experiments lead them to conclude, that the capacities of the following metallic bodies increase with the elevation of their temperature, in the following proportions.

TABLE V.-Of Capacities for Heat.

Mean capacity between 0° and 100°.

Mercury,

Zinc,

Antimony,

Silver,

Mean capacity be

tween 0 and 300°.

The capacity of iron was determined at the four following intervals:

Platinum,

Glass,

From 0° to 100°, the capacity is 0.1098

0° to 200

0° to 300

0° to 350

0.1150

0.1218

0.1255

If we estimate the temperatures, as some philosophers have proposed, by the ratios of the quantities of heat which the same body gives out in cooling to a determinate temperature, in order that this calculation be exact, it would be necessary that the body in cooling, for example, from 300° to 0°, should give out three times as much heat as in cooling from 100° to 0°. But it will give out more than three times as much, because the capacities are increas. ing. We should therefore find too high a

Copper, Platinum, Glass,

Experiments have been instituted, and theorems constructed, for determining the absolute quantity of heat in bodies, and the point of the total privation of that power, or of absolute cold, on the thermometric scale. The general principle on which most of the inquirers have proceeded is due to the ingenuity of Dr. Irvine. Supposing, for example, the capacity of ice to be to that of water as 8 to 10, at the temperature of 32°, we know that in order to liquefy a certain weight of ice, as much heat is required as would heat the same weight of water to 140° Fahr. Hence 140° represent two-tenths or one-fifth of the whole heat of fluid water; and therefore the whole heat would be 5 × 140° = 700° below 32°. It is needless to present any algebraic equations on a principle which is probably erroneous, and which has certain ly produced in experiment most discordant results. Mr. Dalton has given a general view of them in his section on the zero of temperature.

If we estimate the capacity of ice to that of water as 9 to 10, then the zero will come out 1400°

Gadolin, from the heat evolved 2936° in mixing sulphuric acid and water | 1710 in different proportions, and compa- 1510 ring the capacity of the compound (2637 with those of its components, dedu-3230 ced the opposite numbers, J1740

Mr. Dalton, from sulphuric acid and

6400° 4150 6000

Do. from nitric acid and lime, + 23837 Dr. Irvine placed it below 30°, = 900 Dr. Crawford do. do. = 1500 The above result of Lavoisier and Laplace on nitric acid and lime, shows the theorem in a very absurd point of view, for it places the zero of cold, above melting platina. MM. Clement and Desormes have been lately searching after the absolute zero, and are convinced that it is at 266.66° below the zero of the centigrade scale, or

448° F. This is a more conceivable result. But MM. Dulong and Petit have been led by their investigation to fix the absolute zero at infinity. "This opinion," say they, "rejected by a great many philosophers because it leads to the notion, that the quantity of heat in bodies is infinite, supposing their capacity constant, becomes probable, now that we know that the specific heats diminish as the temperatures sink. In fact the law of this diminution may be such, that the integral of heat, taken to a temperature infinitely low, may notwithstanding have a finite value." They farther infer, that the quantity of heat developed at the instant of the combination of bodies has no relation to the capacity of the elements; and that in the greatest number of cases this loss of heat is not followed by any diminution in the capacity of the compounds formed. This consequence of their researches, if correct, is fatal to the theorem of Irvine, and to all the inferences that have been drawn from it.

3. Of the general habitudes of heat, with the different forms of matter.

The effects of heat are either transient and physical; or permanent and chemical, inducing a durable change in the constitution of bodies. The second mode of operation we shall treat of under COMBUSTION. The first falls to be discussed here; and divides itself naturally into the two heads, of changes in the volume of bodies while they retain their form, and changes in the state of bodies.

1st, The successive increments of vola ume which bodies receive with successive increments of temperature, have been the subjects of innumerable researches. The expansion of fluids is so much greater than that of solids by the same elevation of their temperature, that it becomes an easy task to ascertain within certain limits the augmentation of volume which liquids and gases suffer through a moderate thermometric range. We have only to enclose them in a glass vessel of a proper form, and expose it to heat. But to determine their expansions with final accuracy, and free the results from the errors arising from the unequable expansion of the recipient, is a problem of no small difficulty. It seems, however, after many vain attempts by preceding experimenters, to have been finally solved by MM. Dulong and Petit. The expansion of solids had been previously measured with considerable accuracy by several philosophers, particularly by Smeaton, Roy, Ramsden, and Trough. ton, in this country, and Lavoisier and Laplace in France. The method devised by Genl. Roy, and executed by him in conjunction with Ramsden, deserves the preference. The metallic or other rod, the subject of experiment, was placed horizontally in a rectangular trough of water, which could be conveniently heated. At any aliquot distance on the rod, two micrometer microscopes were attached at right angles, so that each being adjusted at first to two immoveable points, exterior to the heating apparatus, when the rod was elongated by heat, the displacement of the microscopes could be determined to a very minute quantity, to the twenty or thirty thousandth of an inch, by the micrometrical mechanism. The apparatus of Lavoisier and Laplace was on Smeaton's plan, a series of levers; but differed in this respect, that the last lever gave a vertical motion to a telescope of six feet focal length, whose quantity of displacement was determined by a scale in its field of view from 100 to 200 yards distant. This addition of a micrometrical telescope was ingenious; but the whole mechanism is liable to many objections, from which that of Ramsden was free. Still, when managed by such hands and heads as those of Lavoisier and Laplace, we must regard its results with veneration. MM. Dulong and Petit have measured the dilatations of some solids, as well as mercury, on plans which merit equal praise for their originality and philosophical precision. They commenced with mercury. Their method with it is founded on this incontestable law of hydrostatics, that when two columns of a liquid communicate by means of a lateral tube, the vertical heights of these two columns are precisely the inverse of their densities. In the axis of two upright copper cylinders, vertical tubes of glass were fixed, joined together at bottom by an hori. zontal glass tube resting on a leveiled iron bar. One of the cylinders was charged with ice, the other with oil to be warmed at pleasure by a subjacent stove. The rectangular inverted glass syphon was filled nearly to the top with mercury, and the height at which the liquid stood in each leg was determined with nicety by a telescopic micrometer, revolving in a horizontal plane on a vertical rod. The telescope had a spirit level attached to it, and could be moved up or down a very minute quantity by a fine screw. The temperature of the oil, the medium of heat, was measured by both an air and a mercurial thermometer, whose bulbs occupied nearly the whole

vertical extent of the cylinder. The elongation of the heated column of mercury could be rigorously known by directing the eye through the micrometer, first to its surface, and next to that in the ice-cold leg. Having by a series of careful trials ascer. tained the expansions of mercury through different thermometric ranges, they then determined the expansion of glass from the apparent expansions of mercury within it. They filled a thermometer with well boiled mercury, and plunging it into ice, waited till the liquid became stationary, and then cut across the stem at the point where the mercury stood. After weighing it exactly, they immersed it for some time in boiling water. On withdrawing, wiping, and weighing it, they learned the quantity of mercury expelled, which being compared with the whole weight of the mercury in it at the temperature of melting ice, gave the dilatation of volume. This is precisely the plan employed long ago by Mr. Crighton, as well as myself, and which gave the quantity 1-63d, employed in my paper for the apparent dilatation of mercury in glass.

Their next project was to measure the dilatation of other solids; and this they accomplished with much ingenuity by enclosing a cylinder of the solid, iron for example, in a glass tube, which was filled up with mercury, after its point had been drawn out to a capillary point. The mercury having been previously boiled in it, to expel all air and moisture, the tube was exposed to different temperatures. By determining the weight of the mercury which was driven out, it was easy to deduce the dilatation of the iron; for the volume driven out obviously represents the sum of the dilatations of the mercury and the metal, diminished by the dilatation of the glass. To make the calculation, it is necessary to know the volumes of these three bodies at the temperature of freezing water; but that of the iron is obtained by dividing its weight by its density at 32°. We deduce in the same manner the volume of the glass from the quantity of mercury which fills it at that temperature. That of the mercury is obviously the difference of the first two. The process just pointed out may be applied likewise to other metals, taking the precaution merely to oxidize their surface to hinder amalgamation.

In the years 1812 and 1813 I made many experiments with a micrometrical apparatus of a peculiar construction, for measuring the dilatation of solids. I was particularly perplexed with the rods of zinc, which after innumerable trials I finally found to elongate permanently by being alternately heated and cooled. It would seem that the plates composing this metal, in sliding over each other by the expansive force of heat, present such an adhesive friction as to pre