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means of a bit of enamel or glass. When the mercury expands, it does so with sufficient force to push its way past this obstruction; but when it contracts, that part of the column past the obstruction is kept there, and the contraction takes

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place by the mercury to the left of A withdrawing into the bulb. It does not matter if the column past the obstruction go down to the bottom of the tube, for when the instrument is read it is gently tilted up until this detached column flows back to the obstruction, where it is arrested, and the end of the column will then denote the maximum temperature. resetting the instrument it is necessary to shake the detached column past the obstruction, in order to fill up the vacancy left by the contraction of the fluid after the maximum had been reached.

In

28. Minimum. In Rutherford's minimum thermometer alcohol is used, and a small glass index is immersed in the column of this fluid. When the instrument has been set this index is at the termination of the column which is kept in a horizontal position. Now should the temperature rise and the alcohol expand, it will flow past the index; but should the alcohol contract, it carries the index with it, for the fluid does not readily permit of its concave capillary surface being broken. The minimum temperature is thus registered.

In order to overcome the objection attached to the use of

alcohol, L. Casella has lately proposed a mercurial minimum thermometer. The principle of this instrument will be understood from Fig. 5. Its peculiarity consists in a side chamber, AB, the bore of which at A becomes smaller very abruptly, and afterwards swells into a pear-shaped termination.

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the side chamber as far as the abrupt termination A, but the pear-shaped vessel is left empty. Suppose now the temperature to rise-instead of the column in the main tube moving, the rise will take place by the mercury at A flowing into the pear-shaped vessel: the reason of this probably being that the bore is here wider than that of the main tube, and there is consequently less resistance to the movement of the fluid. Suppose, next, that the temperature falls the fluid in the pear-shaped vessel will first contract, but when the mercury has reached A, or the point at which the instrument was originally set, the effect produced by the flat surface at A will prevent the mercury receding farther, and the contraction will now take place in the main tube. Thus any fall below the temperature of setting takes place in the main tube, while any rise takes place in the side chamber; and hence the instrument serves as a minimum thermometer. A comparison at Kew Observatory has shewn that the indication of such an instrument agrees very nearly with that of a Rutherford's minimum thermometer when the latter is carefully used.

29. Leslie's differential Thermometer. Sir John

Leslie has constructed an instrument for shewing the difference in temperature between two neighbouring substances or places, and which is hence called the differential thermometer. In this instrument two bulbs, A and B, filled with air are connected together

A

B

C

D

by means of a bent tube, as in Fig. 6 a little coloured liquid fills the lower part of this tube, and rises to the levels C and D when both bulbs are of the same temperature. But should A become warmer than B, since air expands very much for an increase of temperature, the column of liquid will be pushed down at C and made to rise at D; and this motion will be reversed when B becomes warmer than A. Such an instrument will therefore indicate any difference of temperature with great delicacy. The fluid in the tube ought to be one which is not volatile-sulphuric acid is frequently used. We shall find afterwards (Art. 165) that the thermo-pile registers any difference of temperature with still greater delicacy than this instrument.

Fig. 6.

The author of this

30. Fluctuation Thermometer. work has proposed an instrument for summing up fluctuations of temperature. If a bulb be blown connecting together two horizontal glass tubes of different bores, and if this instrument be nearly filled with mercury, it will be found that this fluid will expand in the tube of wide bore when the temperature rises, and contract in the other when it falls. Thus the mercury will gradually travel toward the extremity of the tube of wide bore, and its position from time to time will indicate the amount of fluctuation which

the temperature undergoes. This instrument is however difficult of construction.

31. Other instruments for measuring temperature. Wedgwood's pyrometer is an instrument for measuring high temperatures, and its action depends on the contraction which takes place in baked clay when heated. An air thermometer furnishes, however, a much more accurate means of obtaining the same result, and this will be afterwards described.

Breguet's metallic thermometer is another instrument which may be used in measuring temperature, but a description of it must be deferred to a future occasion.

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CHAPTER II.

Dilatation of Solids.

32. In the present chapter the relation between the temperature and the volume of a solid will be investigated.

It is a general, though not a universal law, that when such a body increases in temperature it also expands in volume, or dilates, and that when it diminishes in temperature its volume contracts, so that when restored to its original temperature it resumes its original volume.

The subjoined apparatus (Fig. 7) is used to illustrate the expansion of solids through heat. A rod A is fixed at one end by a screw B, while the other end presses against the short arm of a lever, whose long arm P forms a pointer. This pointer exhibits by its movement along a graduated scale any change of length in the rod-thus, were the rod to expand, the pointer would be pushed upwards; and were

it to contract, the pointer would fall downwards. Any small change in the length of the rod is thus rendered visible.

B

Fig. 7.

But the way in which a solid expands is different according as the substance is crystalline in its structure or amorphous, and hence the subject naturally divides itself into two parts. In the first of these the expansion of uncrystallized solids will be considered, while in the second the behaviour of crystals under change of temperature will be shortly described.

DILATATION OF UNCRYSTALLIZED SOLIDS.

33. In some cases it is the increment of the volume of a body that we wish to estimate, while in others, as for instance when we are considering a substance, such as a bar, of which the length is the important element, it is change of length and not change of volume with which we concern ourselves. The former of these is called linear and the latter cubical dilatation or expansion. We shall commence with linear expansion: but let us first proceed to define what is meant by "the coefficient of expansion," whether linear or cubical. The coefficient of expansion of a substance is the expansion for one degree of temperature of that quantity of the substance whose length or volume (as

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