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THERMOMETER; from the Greek words gues, heat, and μrgo, a measure; an instrument adapted for the comparative estimation of the temperature

of bodies.

That the idea of any instrumental measurement of heat is wholly of modern invention seems beyond a doubt, since in the writings of the ancients we find no trace of any such comparative scale except the rude indications of our natural feelings. Since we are absolutely ignorant of the state of substances deprived of, or saturated with heat; since our senses, though adapted to the perception of it, are incapable of forming any accurate measure of its effect, or of computing its real quantity upon any independent standard as we can do the pressure of the atmosphere by knowing the weight of an equiponderant column of mercury, we are obliged to have recourse to a relative, not an absolute scale, and establish an arbitrary measure of heat by the extent of one of its acknowledged effects, that of expansion. (See the Article HEAT.)

Hence, it is obvious, that in order to conceive the thermometer to be a true measure of heat, we assume that by equal increments of that agency, the expanding substance employed is uniformly enlarged in bulk, a postulate not of easy proof, but which it has been ascertained in most cases of importance is nearly true. But this we shall explain more fully afterwards; in the meantime we propose to give some account of the invention and history of the simple thermometer, with the various improvements which it has gradually undergone, and after some directions for its practical construction, proceed to the various ingenious modifications which it has received for different scientific purposes, and we propose to conclude with some considerations on the application of the thermometer to use, and the defects to which it is liable, without the most scrupulous attention.

Many

the thermometer has had several claimants, the res-
pective merits of whom it seems nearly impossible
at the present day definitely to determine. Though
by some Galileo has been considered as the true
inventor, and by others, Father Paul Sarpi, the Ve-
netian, this honour chiefly rests between Sanctorius,
an Italian physician, and Drebbel, a Dutchman,
both men of ingenuity and original minds.
moderns support each of these claims, but Sancto-
rius (or Santorio) has, on the whole, most in his
favour, since, as Martine justly observes,* no one
but he demanded the honour of an inventor during
his lifetime. His Commentaries on Avicenna, pub-
lished in 1626 are very interesting, since they con-
tain descriptions and curious wood cuts of the va-
rious forms of the thermometer which he proposed
for medical purposes; and the extent to which he
carried it in other matters, is shown by his attempt
described and figured at folio 22 of that work, to
compare the heat of the sun's and moon's rays, an
attempt which afterwards became one of the most
delicate in natural philosophy.

Perhaps Sanctorius and Drebbel invented much about the same time this instrument, a circumstance by no means improbable, considering the then advancing state of physical science. Be this as it may, the instrument contrived by each was precisely similar, in which air was employed as the expanding substance, which in several respects is remarkably well fitted for this application.

The air thermometer, as shown at Plate DXXIV. Fig. 1, consisted of a ball of glass A with a stem B nearly filled with air, but having the lower part of the tube occupied by a coloured liquor, which, when the air in A was expanded, was forced into the recipient C, and hence a scale being applied to the stem, the dilatation of the enclosed air by heat was marked by the descent of the coloured fluid. In this form the instrument was obviously unfit for

Like most other important discoveries, that of trying with any ease the temperature of fluids.

Essays on Thermometers, p. 2, where the authorities in this controversy are quoted.

VOL. XVIII. PART I.

A

Boyle therefore subsequently proposed to include both the air and the coloured fluid in one bulb as shown at Fig. 2, where the tube AB which is open at top, reaches below the surface of the fluid nearly to the bottom of the receptacle C, into the neck of which the tube is hermetically sealed. The same philosopher, however, demonstrated the grand defect of the air thermometer, that by a change of pressure in the atmosphere, as shown by the barometer, the elasticity of the inclosed air is altered independently of temperature, so as to render the indications of the same instrument not comparable at different times. The air thermometer was subsequently modified by the ingenious Hooke, in order to act as a barometer, which it obviously does, if the effects of temperature be corrected and those of pressure alone shown, just as on the other hand, if the result of variable pressure were neutralized, that of temperature would be truly expressed. Hooke attached a mercurial thermometer to the original instrument, the temperature of which thus indicated gave the data for separating the influence of dilatation caused by heat, which was performed by means of a sliding scale. By this elegant modification Hooke converted the air thermometer into a marine barometer, which, however, was soon abandoned from the absorption that was found to take place of the excluded air by the coloured fluid. This defect has more lately been in a great measure remedied by the substitution of hydrogen gas instead of the common included air, by Mr. Adie, who has revived this instrument under a very elegant and portable form, and under the name of the sympiesometer. See METEOROLOGY.

The only other air thermometer we intend to notice is that of Amontons, who made the indicial fluid a column of mercury twenty-eight French inches long, so that the included air was subjected to the pressure of two atmospheres; by this method he was enabled to measure high temperatures, such as that of boiling water, without a scale of such great length as the dilatation of air under the common pressure would have required. It was, however, subject to the same great defect as the Sanctorian thermometer, and was besides very unwieldy and liable to accident. In the seventeeth century a modification of the air thermometer was proposed by Van Helmont and by Sturmius, which has recently been revived under the names of Thermoscope and Differential thermometer; as this, however, does not belong to the simple form of the instrument, we shall give some account of it in another part of this article.

*

The defects of the air thermometer having been duly appreciated by the Florentine Academia del Cimento, that enterprising body published, in the first volume of their transactions, a description of a new thermometer, in which spirit of wine was used as the expanding substance, which, as it might be hermetically sealed up in a glass tube or bulb, was free from any defect arising from pressure, as well as the possibility of any loss of fluid

* Saggi di Naturali Esperienze.

by evaporation. This instrument was constructed much in the same way as at present, the spirit being dilated till it filled the whole tube, when it was quickly sealed, and on cooling, the fluid retired, leaving nearly a vacuum above it. The great defect of the Florentine weather glass, as it was commonly called, was the want of any fixed scale of graduation, on which account no instrument except those graduated by the original one of the academy, could be comparable with any other, the only direc tion being that the cold of ice and snow should make it stand at 20 deg. and the greatest summer heats at Florence, at 80 degrees.

The spirit thermometer was faulty in several other respects, yet it cannot but be thought fortunate that this fluid, which is esteemed the second best for filling thermometers, should have been so early thought of. The Florentine academicians sometimes bent the tube of their instrument into a toruous form, but this does not appear to be the first attempt of the kind, for we observe at fol. 220 of Sanctorius's Commentaries on Avicenna, a curious figure of a similar contrivance.

From the invention of the spirit thermometer may be dated the epoch of the application of truly scientific effects to the art of thermometry. Boyle having received one of the Florentine instruments in England, set himself to effect some radical improvements upon a contrivance which it was obvious was capable of shedding new light and precision over every branch of natural inquiry. He therefore proposed, as a fixed point for graduation, the thawing of oil of aniseeds, which he preferred to the freezing of water, (another point he also mentioned,) as being more easily obtained in all seasons of the year, and because he had great doubts as to the constancy of the point of freezing of different kinds of water. Indeed this doubt served for long after to delay the adoption of the fixed points at present employed. Derham, Halley, Musschenbroek, and other philosophers, till far on in the last century, believed that water froze at different temperatures in different latitudes, which Martine, † only ninety years since, cites some experiments of his own to disprove. Besides this, Boyle falls into another mistake in not taking two fixed points, but endeavouring to compute the absolute expansion of the spirits, and graduating the scale to ten-thousandths, or some other fixed number of parts of expansion, a proposal which, though extremely philosophical in the abstract, is too difficult of execution to be successfully adopted in practice. A similar plan was proposed by Hooke.‡

The acute Halley afterwards turned his attention to this subject of growing importance. He first proposed the temperature of deep pits for a standard, naturally enough suggested by the constancy of the thermometer observed by De La Rive and others, in the caverns of the Observatory at Paris. But the difficulty of finding such situations, independently of the variation of their temperature in different latitudes, was enough to set aside this pro

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posal. Halley afterwards devised the excellent standard derived from boiling liquids, though he did not appreciate its real merit. He tried the point of vaporization of water, mercury, and spirit of wine, and unfortunately preferred the latter; a point very uncertain, as depending greatly upon the strength of the fluid. The boiling point of water, though Halley very much overlooked it, he found to be very steady in its indication. Nor is it easily affected by minute accidental admixtures. The principal source of error is that the vaporization of fluids is affected by the state of atmosphere at the time, as to pressure, the temperature being highest when the pressure is greatest, and the reverse. By subsequent examination, this source of error has become merely the object of an equation table which will be given in the course of this article.

We have already mentioned that Amontons constructed an air thermometer which could measure temperatures as high as boiling water, which he held as a fixed point in graduation, but hitherto the importance of the points of freezing and boiling water was imperfectly estimated, and had never been united in graduating the same instrument. It was reserved for the powerful mind of Newton to put this branch of science upon the same secure footing with every other which he had studied. Had not his mind been led to more magnificent fields of investigation, he would probably have effected the admirable refinements in thermometric science to which he only led the way, rejecting the partial views of his predecessors, and laying the foundation, as that of every philosophical superstructure should be laid, in principles as rigidly accurate in theory as they are simple and consistent in their application.

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Newton rejected spirit of wine as a thermometric fluid, as having too low a range, and selected linseed oil as being capable of bearing unaltered intense heat and cold. Like Boyle he computed the absolute expansion of the fluid in ten thousandths of its bulk, but he showed his judgment in not making this the basis of practical graduation, which was always to be performed by means of two fixed points, and dividing the interval into an arbitrary number of degrees. He found that supposing the mass of oil at the temperature of melting ice to be 10000; at the heat of the human body it was 10256, boiling water = 10725, and of melting tin 11516. As an empirical measure he divided the space between melting snow and the common temperature of the body into 12 parts, and proceeding from the data above given, he num725×12 bered the boiling point 34. The principle of his thermometer, and a scale of various temperatures examined by it are to be found in a paper by Newton, printed anonymously in the 22d Vol. of the Philosophical Transactions in 1701, and are also partly given in the Principia.

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256

The oil thermometer of Newton, though a happy approach to a universal standard, had striking defects. The fluid, indeed, does not boil below 600°

• Philosophical Transactions.

of Fahrenheit, nor does it freeze with a great degree of cold; its expansibility too is great, being not much less than that of alcohol; but with all these recommendations, linseed oil has been found so viscid as to render any accuracy of observation impossible, and from the quantity which always adheres to the tube, to make the actual height very uncertain. We now come to mention the greatest improvement made upon the thermometer since the period of its invention,-the practical introduction of mercury as the expanding fluid. Halley had proposed this substance, but was prevented from trying it practically, by the consideration of its low expansible power, which indeed he underrated, making it between freezing and boiling water, instead of 1 as has been most lately determined. This defect was certainly of consequence in the 17th century, when the art of making thermometers with bores of great tenuity was neither understood not appreciated; and for the advantageous introduction of mercury as a fluid, a proportionate improvement in manipulation was requisite.

35.5

This practical discovery appears, as far as the testimony of contemporary writers can guide us, to be due to Olaus Römer, the ingenious discoverer of the progressive motion of light, who, according to Boerhaave,† likewise proposed the scale now known under the name of Fahrenheit's, and so early as the year 1709 observed the mercury to sink by a natural cold to the zero of that scale; but it is more commonly conceived that Fahrenheit himself made that observation, and founded his scale upon it, which was contrived in the year 1720, and described to the Royal Society of London in 1724.‡ From this period the thermometer became of scientific utility, and its indications being founded upon fixed principles, gave the assurance that all instruments made with equal care would be strictly comparable. There is even now, after the lapse of a century, no prospect of materially improving the plan of the simple thermometer, or of finding any substance fitter than mercury for forming a scale of heat. In fact it unites the most important qualifications.

Ist, It is found to measure with more accuracy than any fluid hitherto employed by equal spaces, equal increments and decrements of heat, which being the fundamental postulate in all thermometric fluids, is of the most primary consequence. 2d, The facility with which it is divested of air. Though this might at first sight appear a trifling recommendation, it is found in practice one of the highest value; for the difficulty with which spirit of wine is freed from particles of air is one great obstacle in forming good instruments with that fluid, the air deranging much its operations, and giving an independent degree of expansibility. Oil is still more faulty in this respect, but mercury, from its immense cohesion of parts, the high temperature to which it may be brought, and the power of distilling it readily into a state of almost chemical purity, has a decided superiority over every other substance yet applied to the purpose. 3d,

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The range of the mercurial scale is so great, and between such limits as to be extremely convenient for all ordinary experiments; it does not boil under a temperature of 660° of Fahrenheit according to Dalton, or 656° according to Petit and Dulong, and it does not freeze until the extreme cold of 39 of the same scale. Deluc, from whom the principal facts of the thermometric properties of mercury have been taken,* is much mistaken in asserting that mercury bears a more intense cold than alcohol. The experiments made about the middle of the last century were very erroneous in regard to extreme degrees of cold, since as mercury contracts enormously at the moment of solidification, it was supposed that a temperature of - 261 of Deluc's scale, or - 489 Fahrenheit, had been measured without the freezing of mercury. On the other hand it was asserted, † that the French Academicians had found the spirits in Reaumur's thermometer to freeze at 37 at Tornea in Lapland. This proves nothing but the weakness of the spirits employed, which indeed were always adulterated with water, as will be shown when we describe that thermometer. Pure alcohol has never yet been frozen by any cold, natural or artificial, though it has been reduced as low as -91 Fah. by Mr. Walker, of Oxford, and it has even been alleged that a temperature of 150° has been produced.‡ For intense cold, therefore, such as was frequently experienced in Captain Parry's late voyages, spirit thermometers are requisite, but within all ordinary bounds, as we have already stated, mercury enjoys an ample and convenient range.

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4th, Mercury accommodates itself more speedily to the temperature of the medium in which it is placed, than any other fluid, gases excepted. The theory of the heating and cooling of bodies is a very curious one, and till lately was very imperfectly understood. From its great importance, a short outline of it may here find a place. Equal quantities of heat thrown into different bodies do not necessarily produce the same temperatures in each. An equal bulk of mercury having a temperature of 100° and of water at 40° will not produce a mixture having a temperature at a mean of these, or 70°, as equal volumes of water would have done; the resulting temperature will only be 90°, showing that the 40° of heat lost by the mercury when transferred to the water had only the power of raising it 20° of temperature. Speaking generally, therefore, mercury is capable of receiving only half as much heat as water without being raised to a higher temperature, or to use the language of modern chemistry, the specific caloric of water being 1, that of mercury by equal volumes is 0.5.§ Now it is easy

to see, that as the quantity of heat required to raise mercury to any temperature is less than is requisite for any other fluid, the acquisition of temperature will, cæteris paribus, be more quickly accomplished. Add to this that mercury combines the rapid conducting power of a metal with conduction by locomotion, which is peculiar to the fluid state, and which, communicating the temperature through all its parts, facilitates the change of condition. Dr. Trail found, (Nicholson's Journal, xii. p. 137.) by a series of experiments, in which the real conducting power of substances was alone indicated, that mercury conducted a definite portion of heat in 15" for which alcohol required 10′ 45′′ making the conducting power as 43 to 1. From these circumstances a mercurial thermometer acquires much sooner the temperature of any medium than one of alcohol; a fact which, though long since ascertained, was a subject of great surprise and perplexity to philosophers. The determination of specific heats being one of great nicety, experimenters vary very much in their results, but probably we are not far wrong if we consider alcohol to possess double that of mercury. As a satisfactory proof of the superiority of mercury in this respect, we shall quote a specimen of Martine's experiments on the subject, which were among the first to throw light on the subject, taken from his little volume of Essays on Heat,|| a work which at any period would have done honour to its author, and for the time at which it was written, certainly one of uncommon merit. The following experiment was made by placing two thermometers, one containing 3 ounces of water, the other nearly 48 of mercury (or equal volumes) before a large fire.

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† Maupertuis, figure de la Terre, p. 58.

* Modifications de l'Atmosphere, i. 285. We know not upon what authority this is stated in a late publication. $ Henry's Chemistry, 10th edition, vol. i. p. 146. The difference between specific caloric and capacity for caloric, is sometimes ill explained, or not explained at all, in chemical works, which use both terms. The former expression is properly applied to substances taken by equal weights and the latter by equal volumes; yet they are sometimes interchanged. See Henry, p. 151; and we think that the latter term might very advantageously be done away with. See also Murray's Chemistry, i. 392, &c. To prevent confusion, it is proper to add, that the specific heat of French writers is identical with the absolute caloric of Crawfurd and others. Such abuses of nomenclature ought to be discarded as fast as possible from science.

Page 77. The specific heat of mercury by volume is usually stated at .445, water being 1. That of alcohol varies exceedingly under the hands of different observers, some making it almost equal to water, which seems too great, and on the other hand, Mr. Leslie (Essay on Heat, p. 548) has certainly rated it too low in making it only .57.

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