together or mixed up with one another, they will always retain unchanged their state with respect to heat. All such bodies are said to be of the same temperature.

But if when the water and the mercury are brought together the water parts with some of its heat to the mercury, then it is said to be of a higher temperature than the mercury; and if, on the other hand, the water receives heat from the mercury, then it is said to be of a lower temperature than the mercury.

13. It will be inferred from what we have said that if two bodies of different temperature be intimately associated with one another they will both at length attain one common temperature.

Let us now suppose that we have a large mass of liquid whose temperature we wish to measure by means of a thermometer. Strictly speaking, when the thermometer and the liquid are brought intimately together, a common temperature will be attained, but if the mass of the liquid be very much greater than that of the thermometer the former will not be appreciably changed in temperature by the immersion in it of the latter, and the result will be that the thermometer will denote with sufficient accuracy the temperature of the liquid-only it is necessary that this instrument be very small. But if the temperature of the liquid be kept constantly recruited by some natural process, as for instance that of boiling, the thermometer will in such a case, whatever be its size, at length attain the temperature of the boiling liquid in which it is immersed; but a small thermometer will attain this temperature sooner than a large one.

14. Requirements to be fulfilled by a good Thermometer. Having stated these facts, let us now point out the requirements which an instrument for measuring temperature may be expected to fulfil. 1st, it ought to be of small size and easily portable. 2nd, it ought always to give the same

indication for the same temperature, or to be capable of doing so by a simple correction. 3rd, it ought to do something more than merely denote that one body is hotter or colder than another. For the difference between two temperatures, such as the freezing and the boiling points of water, is one which we conceive to be capable of accurate subdivision into any number of equal parts, which form as it were successive equal steps by which we may mount from the lower to the higher temperature. This last requirement is the most difficult one, for it implies not only a knowledge of the agent Heat, but also of the changes which it produces upon bodies, since we must evidently make use of some one of these changes in the construction of our instrument for measuring temperature.

While a mercurial thermometer may probably be so made as to fulfil the first and second of these requirements, it is an air thermometer which will best satisfy the third; but the reasons which lead us to suppose that this instrument gives us the means of measuring temperature with great accuracy cannot well be discussed at the outset of this work. These will be given hereafter: in the meantime, let us take it for granted that an air thermometer does really fulfil this requirement, and refer our readers for a description of this instrument to our chapter on the dilatation of gases.

15. But while an air thermometer gives a very correct indication of temperature, it is nevertheless an instrument difficult of construction and awkward in use: it ought therefore to be employed rather as a standard of reference, by means of which the errors peculiar to some other instrument of easy construction and simple form may be determined.

MERCURIAL THERMOMETER.

16. An instrument of this kind is found in the mercurial thermometer, which is constructed upon the principle

that mercury when heated expands very much more than glass.

In making a mercurial thermometer a bulb is first blown at one extremity of a tube of glass having a capillary bore, the other end of the tube being open to the atmosphere. The bulb is then heated so as to drive out some of the air which it contains, and the open end of the tube is inserted into a basin of pure mercury. As the bulb begins to cool and the pressure of the air within it diminishes, part of this mercury will be driven up the bore into the bulb. The mercury is then boiled, and the mercurial vapour drives away any air or moisture that may have adhered to the tube. While the instrument is hot and full of the vapour of mercury its extremity is once more plunged into the basin, by which means, on condensation of the vapour, the bulb and tube will be filled with mercury. When the tube is full of mercury it is hermetically sealed, and when the instrument has cooled the mercury ought to fill the bulb and part of the stem, the other part being empty. If now the bulb of this instrument be heated the glass envelope will expand, and also the mercury with which it is filled; but the mercury will expand much more than the glass envelope, and in consequence the mercurial column will rise in the capillary tube. If the bore be fine enough, a considerable rise may thus be produced even when there is only a small expansion of the mercury, and by this means a very great amount of delicacy may be given to the instrument.

17. Calibration of the tube. If the instrument is to be as accurate as possible, it is necessary to know the relative diameter of the bore at different parts of its length, since this is always variable even in the best tubes.

To accomplish this the tube is made by the glass-blower in such a way that by a simple mechanical contrivance a small column of mercury, occupying the length of about

one-third of an inch in the bore, may be detached from the main body of the fluid. This column is then made to travel from the one extremity of the tube to the other, and its length is measured by a microscope at each short stage of its progress. It is obvious that this column will be long where the bore is narrow and short where it is wide, and that by this means we may obtain the relative. diameter of the bore in different parts of the tube. The detached column having served its purpose is now reunited to the main body of the mercury. It will be afterwards seen (Art. 20) in what manner the information thus obtained is made use of.

18. Determination of the fixed points. The freezing point. Our object in constructing a thermometer presupposes the existence of at least two fixed points of temperature. The two universally adopted are the freezing and the boiling points of water. Suppose that a substance ascends from the lower to the upper of these temperatures through a certain number of equal stages or degrees, it is the office of the thermometer to indicate these. In order to determine the lower fixed point, a wooden box is perforated in the bottom with a few holes to permit drainage, and placed in a room whose temperature is above the freezing point of water. It is then filled with snow or pounded ice in a melting state, and it has been ascertained that the temperature of this melting ice is under ordinary circumstances absolutely constant. The thermometer is now placed vertically in this mixture, the ice being heaped about the stem, and is so left for a quarter of an hour, or until the mercury has become stationary. The tube is then marked with a scratch at the termination of the mercurial column, and the lowest of the two points is thus determined. Presuming that Fahrenheit's scale is to be employed, this point will denote 32°.

19. The boiling point. The next process consists in determining the boiling point of water. This, unlike the freezing point, is not strictly constant, for the temperature of steam in contact with water depends upon the pressure under which it exists.

It had been observed by Gay Lussac that water boils under the same pressure at slightly different temperatures in different vessels; but Rudberg afterwards found that the nature of the containing vessel altered only the temperature of the water and not that of the steam. It is therefore in steam, not water, that a thermometer ought to be plunged in order to have its upper point determined. Hence also if steam escape from an open vessel containing water into the air, the temperature of the steam, which depends upon the pressure under which the steam exists, will therefore depend upon the atmospheric pressure, since this must be the same as that of the steam. The temperature of the steam of water boiling in an open vessel will therefore vary with the barometer; but if we know the law of this variation we can make allowance for it in graduating our thermometer.

The Commissioners appointed by the British Government to construct standard weights and measures, and the Kew Committee of the British Association, have both agreed that the upper point of a thermometer graduated according to Fahrenheit's scale, or that adopted in this country, shall be taken to represent at London the temperature of steam, at the pressure of 29.905 inches of mercury reduced to the freezing point. This is therefore the true meaning of 212° Fahrenheit, and the temperature of steam at other pressures may be found from the following table.