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Here the spirit of wine acquired the temperature of 88° in 32', for which the water required 44'. Hence 44: 1:32:0.73 the relative specific heat. We think, therefore, that the fourth recommendation of mercury is proved, as acquiring any temperature in a shorter time than any other thermometric fluid, for of course the rapidity of cooling depends on the very same circumstances as that of heating.*

5. The fifth and last qualification of mercury is the perfect uniformity of its nature and properties, wherever it is procured, requiring only the operation of a careful distillation to render it fit for all thermometric purposes, and different specimens perfectly comparable. In this it differs remarkably from alcohol, which it is impossible to procure everywhere of precisely the same degree of purity, which greatly affects the extent and regularity of its expansions.

We have entered the more fully into these properties of mercury, as the consideration of them forms the whole basis of the first theory of the thermometer; we therefore see how great a benefit Reaumur and Fahrenheit conferred upon science by its introduction. The instruments made by the latter in Amsterdam were extremely neat, vying in portability and elegance with the productions of modern artists. Sometimes the scale was composed of paper, and being fixed in the form of a scroll round the tube, the whole was inclosed in a glass case, hermetically sealed. A very elegant instrument of this description we have seen, made by Prius, Fahrenheit's partner, at Amsterdam, which once belonged to the late Baron Maseres. Dr. Patrick Wilson of Glasgow used to make very beautiful thermometers on the same construction. Sometimes, particularly abroad, the scale was formed by a roll of paper inclosed within the glass tube BC, Plate DXXIV. Fig. 3, attached by a solid bend to the top of the thermometer at C. This plan is now out of use in this country; but we have often seen it employed by artists in Italy. Mercurial thermometers are more usually made in this country in a style resembling Fig. 4, the scale being of ivory, and sometimes the bulb and stem project below from the scale for the purpose of greater delicacy; the bulb too is then generally made elongated like the instrument of Fahrenheit.

The scale adopted by this philosopher was rather

an arbitrary one, and the motive of its adoption we have not been able quite satisfactorily to discover. Several of the older writers whom we have consulted, such as Boerhaave,† simply affirm that at the zero of the scale, the mercury was conceived to be divided into 11124 parts, an expansion of one of which indicated a degree, but how that arbitrary number was discovered is not indicated. Muschenbroek, on the other hand, leads us to believe from the details of the construction of the thermometer in his Essais de Physique, § 948, that the highest point of the scale was placed at the boiling point of mercury, and the space between that and the zero of the scale divided into 600 degrees. This, however, seems hardly probable, since Fahrenheit always employed boiling water in preference to boiling mercury in graduating his instruments. Were we to propose any theory, we should conceive the most probable one, that this philosopher, in conformity with the principle so much attended to, previous to his time, attempted to make his degrees equivalent to 0300 of the bulk of mercury at some fixed point, which, in fact, they are so extremely near, as to render the coincidence, if an accidental one, extremely remarkable. Some, we believe, suppose that he selected the number 64 as divisible by continued bisections for the interval in degrees between the freezing point and blood heat, but this appears a less probable hypothesis.

1

Be the origin of the scale what it may, its practical construction is well known. The space between freezing and boiling water is divided into 180°, and these degrees being carried down, the zero is placed 32° below freezing water, at a degree of cold, observed by Fahrenheit (though some insist by Römur) in Iceland, in 1709. The scale of the Mercurial Termometer has since been variously divided, but except two, these graduations have fallen into disuse. The first of these is usually called Reaumur's, though we shall presently show that it differs practically from his, and was really applied by Deluc: it divides the space between freezing and boiling water into 80°, whence the length of a degree is 180 or 24° of Fahrenheit. Its zero is at the freezing point. The second is the scale of Celsius, the Swedish philosopher, though revived during the frenzy of the French revolutionists for decimal measures, under the title of Centigrade: it has 100° between freezing and boiling water, and its scale begins at the former point; the principal defects of this and the preceding graduation are, that the degrees are too large, requiring the application of fractions, and that negative quantities are perpetually occurring, both of which are in a great measure avoided in the less natural scale of Fahrenheit. The relative values of the degrees on these scales are expressed by the following formulæ, in which the degree on each scale is indicated by its initial letter.

• We think it necessary to give only these decisive experiments of Martine's. For more modern researches, see Count Rumford's Essays. See also Deluc, Modifications de l'Atmosphere, i. 321. † Chem. i. 174.

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A very simple and useful rule may be added for the conversion of the Centigrade into Fahrenheit's scale. Double the number of degrees and subtract one tenth, and the result will be the number of degrees above the freezing point, to which add 32°. Thus, to convert 74° Centigrade into Fahrenheit, we say, 74 x 2 = 148 and 148 14.8+ 32° : 165.2. In De l'Isle's scale, the whole mercury at boiling heat is divided into 10,000 parts, each of which forms a degree counted downwards, so that the freezing point stands at 150°; a very unnatural graduation, but which is still used in Russia. The only other scale we shall notice, is one distinguished as being perhaps the most philosophical and convenient yet proposed, though it has never come into use; it is that of the late Dr. Murray of Edinburgh; he divides the whole range of mercury from freezing to boiling into 100 parts, whence if we assume - 40 and 655 Fahr. for those points, freezing water will be at 99°, and boiling water at 347°. Here the degrees would be of a very convenient length, (about 4th of those of Fahrenheit) and, negative quantities could hardly ever be required.*

Such is an account of the mercurial thermometer to the present day, for it has hitherto undergone no alterations of consequence in its simple form. We must now, in order, say a few words upon the real thermometer of Reaumur, which was constructed with spirit of wine, though in point of date it followed Fahrenheit's.

Reaumur conceived that he had ascertained that 1000 parts of strong spirit of wine expanded to 1087 at the heat of boiling water, he therefore diluted the fluid till be found the dilatation reduced to exactly 80 parts, which formed his degrees. Now this being a tentative process, ought by all means to be excluded from the practical construction of thermometers, and besides, all attempts which were then made to graduate upon the principles of absolute fractional expansion were fruitless in a philosophical point of view, for the expansion of the glass in which the fluid was contained, was always overlooked, as well as the inequalities of the dilatation of the substance itself. But the great error of Reaumur was in the determination of his fixed points, for since he never allowed the spirit to boil

in the tube at the upper extremity of the scale, instead of being 212 Fahrenheit, it was only about 175, as Martine has justly shown; the bulbs of his thermometers were likewise so large, that the freezing point was about 2° Fahrenheit too high. This thermometer was therefore a most imperfect instrument, and now wholly abandoned; we think it therefore unnecessary to enter upon it more fully, but refer for a full description of it to the work of Deluc,† in whose time the true value of Reaumur's degrees was of some importance, as the instrument had been much employed in France. That author afterwards applied the octogesimal division to the mercurial thermometer, which, as we have already observed, usually goes under the name of Reaumur's scale, for the conversion of which, into Fahrenheit's, we have already given directions.

Two more modern thermometers remain to be noticed. The expansion of metallic substances from its small amount, and the degree of heat to which it might be applied, early pointed them out as suitable indicators of intense temperatures, and under the name of pyrometers they were employed, with various modifications, during the last century (See PYROMETER.) The beautiful and refined application of the unequal expansibility of metals to the formation of a compensation pendulum, and especially the still more delicate contrivance for the compensation of the balance of a chronometer, probably suggested to Messrs. Breguet, the distinguished Parisian artists, the possibility of rendering the minute variation of metallic expansion sufficiently visible for ordinary thermometric purposes. Their instruments are of two descriptions. One in the form of a watch is shown at Plate DXXIV, Figs. 5 and 6, where, in the former, which represents the interior, the expanding metallic bar a a a a forms a double curve; it is composed of two lamine of steel and brass firmly united, the excess of the dilatability of the latter above the former causing the curve to expand and contract, thus acting upon the lever d, the opposite and longer arm of which consists of an arch-head, with fine teeth upon its edge, which act upon the small wheel g, upon the axis of which the needle which traverses the dial, shown in Fig. 6, is fixed. A fine gold wire spring acts upon the wheel, so as to keep the lever d always in contact with the extremity of the expanding bar. The most modern and beautiful form of the instrument is shown at Fig. 7, in which the metals employed are platina and silver, with a slip of gold interposed, as being of intermediate expansibility, the whole is flattened to an extreme tenuity and then formed into a spiral spring, to the lowest turn of which is attached an index which traverses a horizontal dial. This instrument is far superior to the last, and is altogether one of the most elegant philosophical instruments we ever saw. We had lately an opportunity of inspecting a number of them at M. Breguet's own manufactory.

The only other thermometer we shall notice, is

* Dr. Blagden partly anticipated Dr. Murray in his paper on the Congelation of Mercury, (Phil. Trans. vol. Ixxiii.) by proposing that point as the zero of the thermometric scale.

† Modifications de l'Atmosphere, i. 352.

one proposed by M. Achard of Berlin for measuring high temperatures. The substance employed was the well known fusible metal composed of bismuth, lead, and tin, which liquefies below the temperature of boiling water; the bulb was proposed to be of porcelain. This idea is a very ingenious one, but from practical difficulties, never has, as far as we know, been brought into use, and we suspect it never will. The only instrument of the kind we have seen, failed, from the sudden expansion of the metal in the act of consolidation, which burst the bulb.

Having finished our account of the simple thermometer, let us pause a moment and apply this instantia crucis to the whole subject. How are we sure that any fluid corresponds by its expansions to equal increments of heat? The question admits of no mathematically demonstrable answer, for every attempt to measure heat, (a principle, as we have observed, admitting of no independent or absolutely comparable measure,) involves a certain degree of hypothesis. By far the simplest and most convincing plan which has been proposed, is the mixture of definite volumes of water at different temperatures, attempted first by Renaldini of Padua, but not accurately performed till undertaken by Dr. Brooke Taylor, who made his experiments in 1723 with the oil thermometer,* yet Deluc attributes the first adoption of the plan to Le Sage, who published an account of it in 1776.† Deluc himself is the first who has made these delicate researches with sufficient nicety, and has found that even mercury, which we have already stated to be the most accurate thermometric fluid, is liable to small deviations from the true scale of heat discovered by the method of mixtures just noticed; the accuracy of the principle depends on the assumption that the specific caloric of hot and cold water is the same, for if not, equal weights of both would not have a temperature exactly at the mean of the two, but somewhat different, as explained in the case of the mixture of mercury and water. Our limits necessarily confine us to a very brief sketch of this refined part of the subject.

The following results by Deluc of the absolute quantities of heat in every 5° of his thermometer may be taken as very correct, since one accurate observert has considered the deviation to be somewhat too small, and another too large.

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where is the true dilatation in parts of unity of volume at the temperature of melting ice, t being the temperature on the mercurial thermometer, and a b c coefficients depending upon the nature of the fluid. By this formula he has even satisfied the conditions of the singular phenomena of water, the only known fluid which does not in all parts of the scale expand with heat, its point of maximum density and minimum volume being between 39° and 40°, below which it expands until congelation.

We

The deviations of the mercurial thermometer from the truth we have seen to be small. But a far more practically important inquiry of the same nature regards the spirit thermometer, which, as it is necessarily employed for several purposes, it becomes desirable to ascertain whether these irregularities are so very great as has been stated. confess that it is with diffidence that we object to the conclusions which Deluc, and all who have succeeded him in the same investigation, have drawn as to the indications of the spirit thermometer; yet we think we can produce unanswerable arguments to show that they have done great injustice to the instrument, by subjecting it to trials for which it was never intended.

Deluc, in his work, does not describe, so far as we know, the precautions he took to ensure the accuracy of the upper term of adjustment in the spirit thermometer, for which he employs boiling water as in the case of mercury. He plunges the instru ment (the fluid of which boils at 175° Fahrenheit,) into freezing and then into boiling water, divides

† Deluc, Mod. de l' Atm. i. p. 223, 285.
Traité de Physique, i. 210.

University of Edinburgh, by the most elegant experiment which has

the interval into 80 degrees, which he compares with the mercurial thermometer at every 5o, and gives the defect as the error of expansion. But we are disposed to ask Deluc, who blames Reaumur for not suffering the spirit in his thermometer to boil, what does he gain by this procedure; how does he pretend to mark the dilatation of a fluid above a temperature when a considerable portion of it must confessedly pass into vapour? Deluc gives no explanation of the point, and we find his table of the inequality of the expansion of alcohol quoted in almost every work on the subject, without the slightest comment on its derivation; the discrepancies are so enormous, (amounting to above 12° Fahr. at a maximum,) as to convince us, if correct, that such a fluid is absolutely unfit for thermometric measurement. M. Biot, however, in his excellent work already quoted, says expressly, that the ordinary method of graduating alcohol from mercurial thermometers, is extremely incorrect, for nothing is easier than to raise the spirit thermometer to the temperature of the boiling point by carefully freeing the tube of air and permitting an elastic atmosphere of vapour to be formed above it, which shall prevent, by means of pressure, the vaporization of the fluid, and enable it to attain an unnatural temperature.* This we readily admit; but in the first place, it presupposes the conversion of a portion of the fluid into an invisible and elastic form, which obviously lowers the height of the remainder in the tube, since it is abstracted from it; and what deserves notice, the quantity of vapour required to repress by its elasticity the tendency of the alcohol to vapourize, must vary with the length which the tube may happen to be, so as to occupy it with vapour, having the great elasticity of 67 inches of mercury† to enable it to reach the temperature of boiling water. Not to speak of the natural contraction of the fluid itself under this pressure, or the expansion of the glass bulb, we would ask by what rule we can expect a substance in so unnatural a condition to be a true measure of equal increments of heat by expansion, when that very substance would be naturally expanded at the same temperature, not by the previous laws of dilatation, but after rendering latent an immense quantity of heat, into a subtile vapour many hundred times rarer than in its fluid state. We are aware that in a real vacuum it may be said that alcohol boils far below 175°, and that it is repressed below as above that temperature, by the force of its own vapour, but this very force is so small, differing from that at the natural boiling point by a fraction of an atmosphere, and therefore the conversion of so small a quantity of fluid into vapour being necessary, that it may well be overlooked; besides, that in theory we do not think there ought to be, and in practice there never is an exclusion of air from above the thermometric column, for we shall soon show how much the value of such a vacuum has been overrated in mercurial thermometers.

* See Traité de Physique, i. 40.

But the most satisfactory proof of the inaccuracy of Deluc's method is obtained from the following consideration, which is the only attempt made, though an indirect one, which we have been able to discover, to correct the important misconceptions which have prevailed on the subject. Dalton, whose accuracy as an experimentalist is undoubted, has confirmed the assertion of Deluc, that while spirit of wine expands 35 parts between the freezing point and 122° Fahr., it expands 45 between 122° and 212°, but the same chemist has stated, that taking 170° as the upper point, which may be determined with great accuracy by a careful comparison with the mercurial thermometer, he found the spirit which occupied 1000 parts at 50° to fill 1079 at 170° and at 110°, the mean distance where the error should have been at a maximum, he found it to occupy 1039 parts, or half a division below the entire error exists, as was to be expected, in the 40° true mean; from which it appears that almost the F. of Deluc's thermometer nearest the boiling point of water, which, therefore, it is manifestly unfair whole instrument with an enormous error of exto carry down through the scale, and charge the pansion, amounting at a maximum, to one sixteenth of the whole interval between boiling and freezing

water.

expansions of alcohol, from experiments performed We shall now quote the latest table of the on the method of Deluc by Dr. De Wildt, for every 5° of the octogesimal scale.

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Here we have another question to ask regarding this table, which shows a violence done to the natural properties of bodies not less than that of forcing alcohol to bear the heat of boiling water. How, we should be glad to know, does Dr. De Wildt pretend to measure the dilatation of mercury at R = 76.025 Fahrenheit? He must have forgot that the congelation of mercury has not hitherto been measured by this fanciful scale of the alcohol thermometer, but by the mercurial one itself, and found to coincide with 39° of Fahrenheit's scale,§ a point determined beyond a doubt, and which is somewhat greater, though not nearly so much as De Wildt would have us to believe, than the temperature expressed by most spirit thermometers constructed in the ordinary way, as we hope it ever

† According to Dr. Ure's Table.

Kastner Archiv fur die Gesammte Naturlehre, Dec. 1825. § See the most ample testimony by Hutchins, Philosophical Transactions, Ixxiii. 308. Cavendish, ibid.

Phil. Trans. 1818.

Blagden, p. 329, &c.

will be constructed, by a careful comparison with a good mercurial one. Hence it appears that De Wildt actually compared alcohol with mercury 37 degrees below its freezing point, if we are by any means to judge of temperature measured by a solid just reduced from fluidity. This, however, is impossible, since mercury contracts enormously after solidification, we may therefore approximate to the real temperature of the first point in the table in this way; supposing the contractions of absolute alcohol equable with the temperature, as we have endeavoured to prove they are nearly, when at a moderate distance from the boiling point of the fluid, we may say, comparing it with the mercurial thermometer at the first term given above congelation (-30 R) as 20.32: 30:: 28.50: 42.07. Whence we may estimate the thermometer to have sunk about 3 R7 F. too low, a circumstance quite accidental, since below the point of congelation, mercury sometimes contracts suddenly, so as to indicate a cold of several hundred negative degrees, sometimes contracts but little, indicating a temperature of 60 without moving. On the whole, therefore, we conclude that these experiments have done gross injustice to the spirit thermometer, and though the irregularities of that fluid are in all probability much greater than those of mercury, they have been exaggerated by the injudicious choice of the highest fixed point. Below zero of Fahrenheit, alcohol appears to be defective in its expansion, though by no means so much so as stated in De Wildt's table, (which makes the error about twenty-two degrees of Fahrenheit at the freezing point of mercury,) for in the numerous experiments of Hutchins, the spirit thermometers, at extremely low temperatures, sometimes did not differ so much from the mercurial ones, as these from one another, and they occasionally indicated temperatures not only as low, but even lower than the mercurial ones at a point higher on the scale than some of those at which Dr. De Wildt has attempted to compare the instruments.

50 or

We are happy to be able to subjoin some extremely satisfactory evperiments from one of Captain Parry's late voyages,t on the comparison of alcohol thermometers by the best London makers, with mercurial ones, which fully confirm our ideas; the differences of the alcohol from the mercury, and with a single exception Spirit of (+57 which perhaps is a Mercury. Alcohol. misprint for 54) the spirit of wine, are remarkably+ 58 + 53 constant, and the indica-+50.2 + 44 tions of the spirit are al-+ 3 0.5 ways lower than those of

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We own that our limits have enabled us to give but a hurried exposition of our opinion on this subject, but we hope that enough has been said to show that the subject requires a thorough re-investigation.‡

Before quitting the theory of the thermometer, we must mention an instrument quite original in its idea, and unconnected with the principles that have hitherto been detailed, and therefore requiring a separate place. Dr. Brewster, with that fertility of application in scientific subjects for which he is remarkable, has proposed the measure of the refractive power of a fluid (which varies with the density) as an index of temperature. In this plan, which we believe has not hitherto been published, "a hollow prism is filled with oil or any other fluid which changes its refractive power much with heat. The prism is fixed in a circular scale, and the angular distance between the two stationary points, when the refraction is at a maximum, is a measure of the temperature." The same philosopher ingeniously proposes the time which a bubble of air takes to pass through a viscid fluid from one end of a cylinder hermetically sealed to the other, as a measure of temperature. Indeed the viscidity of all fluids is sensibly altered by heat; we have seen a species of water clock which is used by the Indians, in which time is measured by the sinking of a copper cup floating in a vessel of water, and having a minute hole in the bottom through which so much water passes in an hour as exactly to make it sink. We have seen experiments made, in which the water employed was at different temperatures, and the time of descent was always diminished as the temperature was raised; this might be in a minute degree owing to the increased size of the hole in the copper cup, but principally from the diminished viscidity of the fluid.

We must now shortly detail the process of constructing thermometers, and the forms best fitted for their future application in practice.

*See the above authorities, and Deluc's Account of Braun's experiments; also Walker's Papers, Philosophical Trans. vol. Ixxviii. † Parry's second voyage. Appendix.

We cannot help adding that M. Biot, while he upholds the principle of graduation from the boiling point, gives the strongest testimony to the regularity of the motions of alcohol within moderate limits; for in deducing the apparent expansion of this fluid in glass, his formulæ are founded on the admirable experiments of Blagden and Gilpin, which extend only from 32° F. to 100° F. In proving their accuracy by a correspondence with the total expansion given by Dalton for alcohol in glass, from 8° Fahr. to + 162°, M. Biot

is obliged to relinquish the formulæ derived from the expansion of alcohol for the whole 80 degrees of Deluc's scale, and calculate separately the positive or negative temperatures by that scale, and yet the remarkable fact is, that the principle deduced from Blagden and Gilpin's small range of 68 deg. Fahr. coincide to every decimal when applied to Dalton's range of 180 degrees. B

VOL. XVIII. PART I.

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