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(Journal of Science, No. XVI.) which he considers more convenient. One of the

fig. 7.

fig.8.

bulbs is made to stand higher than the other, and the included liquid is alcohol or ether coloured, which is made to boil for the purpose of excluding the air, previous to the closing of the instrument by the blow-pipe. (Fig. 8.) Several instruments have been made, to indicate changes of temperature, upon the principle of the unequal expansion of different metals. Mr. Crichton of Glasgow, has combined small oblong plates of steel and zinc: the compound bar thus produced, is firmly secured at one end to a board; the other end is applied to a moveable index, so that the whole of the bending occasioned by the superior expansibility of the zinc over that of the steel, is exercised in moving the arms of the index along a graduated arc, and leaves them at the greatest deviation to the right or left of any observed temperature.

"An exquisite instrument," says Dr. Ure, "on the same principle has been invented by M. Breguet, member of the Academy of Sciences, and Board of Longitude of France. It consists of a narrow metallic slip, about 1 of an inch thick, composed of silver and platina soldered together; and is coiled in a cylindrical form. The top of this spi

ral tube is suspended by a cross arm, and the bottom carries, in a horizontal position, a very delicate golden needle, which traverses as an index on a graduated circular plate. A steel stud rises in the centre of the tube, to prevent its oscillations from the central position. If the silver be on the outside of the spiral, then the influence of increased temperature will increase the curvature, and move the appended needle in the direction of the coil; while the action of cold will relax the coil, and move the needle in the opposite direction." The principle of these lastmentioned contrivances is clearly the same as that of Arnold's compensation balance, already alluded to.

Various modifications of the thermometer have been introduced, for the purpose of adapting it to particular purposes, which cannot here be described.

Of the contrivances for measuring high degrees of temperature, that of Wedgewood has been the most in use: its indications depend upon the contraction of pure clay when much heated. This reduction of bulk is first observed when the clay acquires a red heat, and continues to increase until vitrification ensues; the contraction of volume being permanent, and amounts, in the whole, to about one fourth. In order to take advantage of this property of clay, Mr. Wedgewood constructed a guage of brass, consisting of two straight pieces, two feet long, fixed upon a plate, a little nearer to each other at one end than at the other; the space between them at the widest end being five-tenths of an inch, and at the narrowest three-tenths. The converging pieces were divided into inches and tenths of inches. The pieces of clay, the contractions of which were to be measured, were of a cylindrical form, flattened on one side, and of such a size as to be exactly adapted to the wider end of the guage, so that it might slide farther in, in proportion, to the degree of heat applied to it.

The indications of this instrument, which he called the Pyrometer, from two Greek words signifying measure of fire, gave the comparative degrees of heat produced in different processes; but to obtain the utmost information which the instrument was capable of affording, it seemed absolutely necessary to apply a scale to it, the degrees on which should bear some certain proportion to the degrees on the scale of Fahrenheit.

Mr. Wedgewood observed that the heat which raised the temperature of Fahrenheit's thermometer from 50° to 212°,

expanded a piece of silver from 0° to 8° of a certain scale, and that a heat which expanded the silver from 0° to 66° of its scale, corresponded to 21° of the clay or Wedgewood's scale. By these and similar experiments, he found that each degree of his Pyrometer is equal to 130° of Fahrenheit's scale. The temperature of a red heat, visible by daylight, which was found to correspond to 107710, was taken as the commencement of Wedge

wood's scale.

The following TABLE, pointing out the effects upon bodies of different degrees of heat according to this and Fahrenheit's scales is taken from Murray's System of Chemistry.

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A saturated solution of salt boils Water boils (the barometer being at 80 inches); also a compound of five of bismuth, three of tin, and two of lead melts....... A compound of three parts of tin, five of lead, and eight of bismuth, melts rather below..... Alcohol boils ...

20577

18627

17977

Bees' wax melts..

Spermaceti melts.

125

17327

Phosphorus melts..

Ether boils.....

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Plate glass furnace, strongest

Bow porcelain, vitrifies . 121 16807

smith's forge.......

..........

heat...

Chinese porcelain, softened,

inferior sort..

120 16677

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Medium temperature of the globe Ice melts....

Milk freezes..

Vinegar freezes at about.. Strong wine freezes at about... A mixture of one part alcohol and three parts water freezes...... A mixture of alcohol and water in equal quantities freezes.... A mixture of two parts alcohol and one part water freezes.... Melting point of quicksilver (Cavendish).

Liquid ammonia crystallizes (Vauquelin).

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....

Delft ware baked in..

41 6407

Fine gold melts

32 5237

Settling heat of flint glass

29 4847

Nitric acid sp. gr. about 1.42 freezes (Cavendish).... Sulphuric æther congeals (Vauquelin)....

-45

-47

Fine silver melts..

28

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4717 27 4587 22 3937 3807

Heat, by which enamel co

Natural temperature observed by
Mr. Hutchins at Hudson's Bay
Ammoniacal gas condenses into a
liquid (Guyton) ......
Nitrous acid freezes...

-54

-56

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Cold produced from diluted sul

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phuric acid and snow, the materials being at the temperature

Heat of a common fire

-73

790

of 57

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It has been asserted by Guyton de Morveau, that the indications of Wedgewood's pyrometer are not so high as they are made to appear; but he has certainly erred, in supposing that the red heat, at which Wedgewood commences his scale, is no higher than 517° of Fahrenheit; since oil and mercury are both capable of indicating higher degrees of heat, without exhibiting the least appearance of redness.

In Guyton de Morveau's pyrometert, platina is used to measure high degrees of heat. The instrument alluded to, is formed of a mass of highly-baked white clay, having a groove in it for the purpose of receiving a rod or plate of platina, which, resting on the clay at one end, at the other presses against it the end of a bended lever, the longest arm of which is made, by the expansion occasioned by increased heat, to traverse a graduated arc, and thus indicates the rise of temperature.

Dr. Ure (Chemical Dictionary, p. 657) is of opinion that high degrees of heat may be measured by the expansion of air." Since dry air augments in volume 3-8ths for 180 degrees, and since its progressive rate of expansion is probably uniform by uniform increments of heat, a pyrometer might easily be constructed on this principle. Form a bulb and tube of platinum of exactly the same form as a thermometer, and connect, with the extremity of the stem at right angles, a glass tube of uniform calibre, filled with mercury, and terminating below in a recurved bulb, like that of the Italian barometer. Graduate the glass tube into a series of spaces equivalent to 3-8ths of the total volume of the platina bulb, with 3-4ths of its stem. The other fourth may be supposed to be little influenced by the source of heat. On plunging the bulb, and 2-3rds of the stem into a furnace, the depression of the mercury will indicate the degree of heat. As the movement of the column will be very considerable, it will be scarcely worth while to introduce any correction for the change of the initial volume by barometric variation. Or the instrument might be made with the recurved bulb sealed, as in Professor Leslie's differential thermometers. The glass tube may be joined by fusion to the platinum tube. Care must be taken to let no

mercury enter the bulb. Should there be a mechanical difficulty in making a bulb of this metal, then a hollow cylinder half an inch in diameter, with a platinum stem, like that of a tobacco pipe, screwed into it, will suit equally well."

Having considered the expansion of bodies by heat, and the various means of measuring that expansion, it seems to be required, in order to give a complete view of the subject, that the effects capable of being produced by reducing bodies below their usual temperatures, and the artificial modes by which this is effected, should be noticed in this place; but it is believed that a still more appropriate opportunity of entering into this discussion will be found in a more advanced part of this treatise.

CHAPTER V.

of the different powers of bodies in conducting heat.

To prove in a simple and convincing way that heat passes through different bodies with very different degrees of velocity, it is only necessary to take slender cylinders of different substances, as, for example, silver, glass, and charcoal, and while holding one end of each in the hand, let the other end be held in the flame of a candle; the silver will soon become too hot to hold, the glass will be much longer in being heated, and the charcoal will be ignited (or red-hot) at one end, long before any sensation of heat is felt at the other. The substances that become hot soonest at the end farthest from the flame, are said to be the best conductors of caloric.

The densest bodies are generally the best conductors; but there is no invariable relation existing between the density of a body and its conducting power; as the densest of the metals, platinum, is one of the worst of metallic conductors. Earthy substances are much inferior to metals in their conducting power; wood is still more so; but the solid substances that have the least conducting power, are those which constitute the coverings of animals, as wool, hair, and feathers. Hence the great use of even small portions of such substances in preventing the heat of animals from being carried off by the cold air; in other words, keeping them warm.

Ex. The difference between the conGuyton on Wedgewood's Thermometer, Ann. de ducting powers of metal and wood may Chimie, xxxi. 171. Guyton's Metalline Thermometer of Platina, cylindrical tube, or still better a solid be strikingly shown, by taking a smooth

Repertory, . III. 459.

piece of metal, about one and a half inch in diameter, and eight inches long; wrapping a piece of clean writing paper round the metal, so as to be in close contact with its surface, and then holding the paper in the flame of a spirit lamp: it may be held there for a considerable time, without being in the least affected. Wrap a similar piece of paper round a cylindrical piece of wood of the same diameter, and hold it in the flame; it will very speedily burn. When the paper is in close contact with the metal, the heat which is applied to it in one particular part cannot accumulate there; but enters into the metal, and is equally diffused through its substance, so that the paper cannot be burned or scorched until the metal becomes very hot: but when paper is wrapped round wood, the heat that is applied in one particular part, not being able to enter into the wood with facility, accumulates, in a short time, in sufficient quantity to burn the paper.

Sand conducts heat so slowly, that the red hot balls used at Gibraltar in repelling the attack of the Spaniards, were conveyed from the furnaces to the bastions, in wooden wheelbarrows, having only a layer of sand between them and the balls.

Solid substances conduct heat in all directions, upwards, downwards, and sideways, with nearly equal facility.

A set of experiments was made by Richman, with a view to ascertain if any relation existed between the conducting powers of bodies and their other properties. He took hollow balls of the metals, equal in size to each other, and having the bulb of a mercurial thermometer inclosed in each. The balls having been immersed in boiling water until each thermometer attained the same temperature, they were then exposed to the air, and the times of their cooling observed: the differences in this respect were considered as marking their differences of conducting power. The metals which appeared to have the greatest power of retaining heat were brass and copper; then iron, tin; and lead the least of all. The decrements of temperature in a given time, in the metals above mentioned, being as follows: lead, 25; tin, 17; iron, 11; copper, 10; brass, 10,-he considered himself justified in inferring, from his experiments, that the increments and decrements (or increases and decreases) of temperature in the bodies upon which he experimented, are not in the inverse ratio of their

density, of their hardness, of their cohesion, nor in any compound ratio of these.

Rods of different metals, of the same length and diameter, were dipped by Ingenhouz into melted wax, by which they acquired a coating of that substance. When cold, they were plunged to the depth of about two inches into heated oil, and the conducting power was inferred from the length of wax coating melted in a given time. Silver, according to these experiments, is the best conductor; then gold, tin, copper, platinum, steel, iron, and lead. These experiments, however, are not considered as perfectly accurate. The experiments of Meyer, of Erlangen, by which he endeavoured to ascertain the conducting powers of different kinds of wood, appear to be subject to so many causes of error, that the results obtained by them can scarcely be depended upon.

The following TABLE gives the results which he obtained; the conducting power of water being made the standard. Conducting Specifie Power. Gravity. .10 ......1.000

Water
Ebony wood
Apple tree .......

Ash
Beech
Hornbeam
Plum tree
Elm

Oak
Pear tree .
Birch
Silver fir
Alder
Scotch fir..
Norway spruce

Lime

...

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Experiments were made by Count Rumford, for the purpose of inves tigating the fitness of various substances, as articles of warm clothing. That philosopher suspended a thermometer in a cylindrical glass tube, the end of which had been blown into a bulb, 1 inch in diameter, placing the bulb of the thermometer in the centre of the larger bulb, surrounded with the substance, the conducting power of which was to be ascertained. Prepared in this way, the apparatus was heated by being plunged into boiling water, and afterwards cooled by being plunged in a mixture of pounded ice and water; and the number of seconds was accurately marked, which the thermometer required in each experiment to cool from 70° to 10° of Reaumur.

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Seconds. and ice-houses are constructed upon this principle.

576

1284

1169

917

1118

1046 1032 1296

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1315 1305

937

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Charcoal

Lamp black. Wood-ashes.

......

........

The worst conductors, as hares' fur and eider-down, involve a large quantity of air among the parts of which they consist, to which, it is believed, they chiefly owe the power of resisting the passage of heat. The same substance is found to have different conducting powers, in proportion to the closeness or openness of its texture, as will be seen by reference to the experiments on silk, the twisted silk having the greatest conducting power.

The substances which form the warmest articles of clothing are those which have the longest nap, fur, or down, on account of the air which is involved resisting the escape of the natural warmth of the body. The imperfect conducting power of snow arises from the same cause; and is of the greatest utility in preventing the surface of the earth from being injuriously cooled in many parts of the world. It is affirmed, that while the temperature of the air in Siberia has been 70° below the freezing point, the surface of the earth, protected by its covering of snow, has seldom been older than 32°. Advantage is taken of the imperfect conducting powers of bodies for the purpose of confining heat: furnaces are frequently surrounded by a thick coating of clay and sand for some purposes; the interposition of a layer of charcoal, or of a stratum of air, is very effectual in preventing the escape of caloric. Double windows may be seen at Kensington Palace, and in many houses in and about London, upon the same principles. The air inclosed between the two windows opposes great resistance to the escape of the heat which is produced within the house in winter,

Loose clothing is warmer than such as fits close, on account of the quantity of imperfectly conducting air confined around the body, resisting the escape of heat. The same substances that prevent the escape of heat, will be equally effectual in preventing its admission;

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The very different sensations .hich we experience on touching substances of different kinds, as ivory, marble, glass, wood, are occasioned by the dif ferences of conducting powers in these bodies. A piece of wood, for example, being touched in cold weather, does not seem so cold by very much as a piece of iron in the same place, although they are exactly of the same temperature, as may be proved by the application of a thermometer to them. The iron feels colder, because, being a good conductor of caloric, the heat existing in the hand over that of the iron, has a tendency to enter into the iron, that an equality of temperature may be produced between them, and the rapid abstraction of caloric occasions the sensation alluded to; but wood, being a slow conductor, it does not take away heat from the hand so rapidly, and therefore does not feel so cold. For the same reason, when the iron and the wood are at high temperatures, the former seems the hottest, because it imparts heat most readily.

Operators who have frequently to touch substances hotter or colder than is agreeable, find it very convenient to wear gloves of worsted, that substance being a very bad conductor of heat.

Count Rumford illustrated, by numerous experiments, the very imperfect conducting power of fluids: indeed, he supposed it proved by his experiments that they are absolutely non-conductors of caloric. This opinion has been successfully controverted, and fluids are now generally admitted to have a very small degree of conducting power. It has been proved that water may be made to boil in the upper part of a tube, without imparting much heat to the lower portions: that water may be brought to the boiling point within one fourth of an inch of ice without producing immediate liquefaction; and that ice is melted eighty times slower, when it is fixed at the bottom of a cylindrical vessel, with warm water above it, than when it floats upon the surface of warm water.

Dr. Murray, who was the most successful opponent of Count Rumford's theory, selects the following as one of the most unobjectionable of the Count's experiments. Over a piece of ice, frozen in the bottom of a cylindrical glass jar, and having a small projection of ice rising from the centre of it, he poured olive oil, at 32°, to the height of three in

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