For ordinary purposes, mercury is the fluid best adapted for thermometers, its expansion being most equable; but alcohol is used when great degrees of cold are to be measured.

The thermometer commonly employed in this country is that of Fahrenheit2; but as three other thermometers are used on the Continent, it may be proper to notice all of them, and point out the circumstances in which their scales differ.

Fahrenheit, in forming his thermometer, ascertained two fixed points; that of freezing water, and that of boiling water: the space between these points he divided into 180 equal parts or degrees, and carried the scale downwards 32 degrees, and there fixed his zero. The part of the scale indicated by the freezing of water he made to be 32 degrees from its beginning : therefore 32° is marked as the freezing point: and the space between it and the boiling point, which is 212°, is equal to 180°. The scale may be extended above this point, and also below the commencement of the scale, the descending degrees being marked inversely with the same numbers as the ascending.

1 Thermometers of great accuracy may be purchased. Persons who may wish to construct them for themselves will find ample instructions for their guidance in the third chapter of Henry's Elements of Experimental Chymistry.

2 Fahrenheit was an artist of Amsterdam.

The scale of the thermometer of Celsius, which has been used in France since the Revolution, begins at the freezing point of water, which is consequently marked 0, and the space between that and the boiling point is divided into 100 equal degrees; hence it has been named the Centigrade Thermometer. Each degree of the scale is 4/9ths more than a degree of Fahrenheit's, or one of the latter is equal to 5/9ths of a degree of the centigrade scale. To find, therefore, the degrees of Fahrenheit's scale, corresponding to those of the centigrade, the given number of the latter must be multiplied by 9, and divided by 5, adding 32 to the quotient.1 The sum expresses the degree on the scale of Fahrenheit.

Reaumur's thermometer, which is still used in Italy and Spain, also commences at the freezing point, which is marked 0; and between this and the boiling point, it is divided into 80 degrees. Each degree is, therefore, 4/9ths more than one of Fahrenheit's; and to reduce the scale of Reaumur to that of Fahrenheit, the given number of degrees of the former must be multiplied by 9, and divided by 4, adding 32 to the quotient.

In De Lisle's, thermometer, which is used only in Russia, the space between the boiling and freezing points is divided into 150°, the gradation beginning at the boiling point, which is marked 0; and increasing inversely to the freezing point, which is marked 150°. It is seldom mentioned by authors.

To reduce 212° Fahrenheit to centigrade, by common arithmetic 212° Fahr. 32

180 5 9 ) 900 ( 100° centigrade.

Table Showing The Degrees Of Reaumur.s And Fahrenheit's Thermometers Corresponding With Those Of The Centigrade Thermometer

These instruments are well adapted for determining the variations of temperature which bodies undergo: but a certain degree of fallacy attends the observations made with them, chiefly owing to the expansion of mercury increasing with the temperature. Thus, the medium degree of heat between the freezing and boiling points, although marked on the scale 122°, yet is actually 118.8° 1. only; the temperature which is equal to raise the mercury in the tube 86° in the first instance, being sufficient, by increased expansion, to raise it 94° in the second.

Table Exhibiting The Degrees Of The Centigrade And Fahrenheit's Thermometers Corresponding To Those Of Reaumur's Thermometer

Thermometers are instruments for measuring moderate degrees of heat; for those beyond a certain point, or intense degrees, the Pyrometer is employed. This instrument consists of a metallic bar of little fusibility and uniform expansion. Professor Daniell has ascertained, that both platinum and wrought iron answer. The expansion of the bar indicates the degree of heat.1

1 Another pyrometer is that invented by Mr. Wedgwood. It depends on the degrees of contraction which pure argil suffers when exposed to high temperatures; and for this purpose small cylinders of pure clay are made in a mould, flattened on one side, and fitted exactly to the wider end of a gauge, consisting of two straight pieces of brass, 24 inches long, fixed on a brass plate so as to converge, and divided into inches and tenths. The length to which the pyrometrical pieces can be slid in the converging groove, indicates the heat to which they have been previously exposed; and, as they do not expand again when cold, no fallacy can result from the action of heat on the gauge. Each degree of this scale is equal to 130° of Fahrenheit; and the 0, or commencement of it, corresponds with 1077 1/2° of Fahrenheit's scale. The highest temperature that has been measured by it is 160°, or 21,877° of Fahrenheit, which is 30° above the point at which cast iron melts. But, as much higher temperatures than this must exist, so, also, there are temperatures much lower than can be measured by any thermometer.

Expansion, or increase of bulk, is the most general effect of caloric, and, with very few exceptions, may be regarded as the general law of its operation. When caloric flows into a body, it separates its integrant particles from one another, and hence augments its volume. This change is smallest in solids, more considerable in liquids, and most in gaseous bodies; or the expansibility is in the reverse ratio of the force of aggregation. Thus the expansion of air is 8 times greater than that of water; and the expansion of this 45 times greater than that of iron.

The expansion of solid bodies is, in general, so very inconsiderable as not to be easily ascertained by measurement; but, as far as it can be known, it is nearly equable. The degree of expansion, however, is not the same in all solids: thus, for example, the metals expand in the following order, commencing with the least expansible: platina, antimony, iron, bismuth, copper, tin, lead, zinc. Argil is an exception to the law of expansion in solids; for the bulk of pure clay diminishes, when heated, in the ratio of the intensity of the heat to which it is exposed. The cause of this anomaly has not been discovered. The expansion of liquids is more evident than that of solids, but not uniform - the differences apparently depending on the greater or less volatility of the liquids; those expanding the most, the boiling point of which is lowest, and which, consequently, most readily assume the gaseous form. The degree of their expansion, also, increases with the augmentation of the temperature; or, the nearer a liquid approaches to the boiling point, the greater is the expansion produced by each degree of caloric; and the further it is from this point, the more equable is the expansion. Liquids, in the same manner as solids, suffer a difference of expansion from a given change of temperature.

The following table, by Dr. Dalton, shows the expansion of the more common liquids, from 32° to 212° Fah., the volume at 32° being denoted by 1.

To the general law of the expansion of liquids by heat, water furnishes an exception. Thus, from the lowest temperature at which water can remain liquid, to 40°, or 39° * 39 Fahrenheit, heat diminishes the bulk of water, instead of expanding it; but above 40° to 212°, expands it.

All gaseous bodies suffer the same expansion by the same additions of caloric, supposing the circumstances to be equal. Their expansion is almost perfectly equable, or the same augmentation takes place by the same addition of caloric at every degree of temperature between the freezing and the boiling point of Fahrenheit's thermometer. By the experiments of Gay Lussac, 100 parts of atmospheric air, heated from 32° to 212°, expand 137.5 parts, or 1/480th for every degree of the thermometer; and the other gases, the steam of water, and the vapour of ether, undergo the same expansions by the same augmentations of temperature. The cause of the equable expansion of gaseous bodies appears to be the absence of cohesion; so that, at a low temperature, there is no more resistance made to the expansive power of the caloric thrown into the gas, than at a high temperature.

But, besides the change in bulk produced by the introduction of caloric into substances in different quantities, they are changed in state, assuming the fluid form and that of vapour; or, they are ignited.

Fluidity is an effect of caloric, arising from the repulsive force of the caloric, which enters into any substance fitted to take on the fluid form, separating the particles from one another to such a distance as to render them easily moveable on one another in every direction. All solids, with a very few exceptions, are susceptible of the fluid form, when exposed to a sufficient degree of heat; and all liquids, with the exception of alcohol and ether, become solid when exposed to very low temperatures. The particular temperatures necessary for the production of these changes, however, are exceedingly various, but for the same bodies they are always the same.1 In some cases, the change is sudden, or the body instantly passes from the solid to the liquid state; in other cases, it passes through several degrees of softness before it be perfectly liquefied: the conversion of ice into water is an example of the first; the melting of glass, of wax, and other unctuous matters, are instances of the second.

There are some bodies, nevertheless, which cannot be melted or fused, owing to their suffering chemical decomposition at a lower temperature than is required for their fusion under the ordinary pressure of the atmosphere: - a piece of wood, for instance, cannot be melted by the application of any degree of heat.

1 Hallstrom, Ann. de Chimie et Phys. t. xxviii. p. 90.

Although the melting point, in most cases, is always the same in the same bodies, yet circumstances may vary it; and the admixture of other substances may alter it very considerably. Thus, the melting point of ice, or, what is the same thing, the freezing point of water, is 32°; but by exposing water slowly to the action of freezing mixtures, it may be cooled down to 22° before it freezes. The addition of salts renders this point still lower, as may be seen by the following table.2

1 Table Showing The Degree Of Temperature, According To Fahrenheit's Thermometer, At Which Several Solid Bodies Melt

2 Phil. Trans. 1788, 27., quoted by Dr. Thomson, Syst. Chymistry, 4th edit. i. 520.

When solids pass to the liquid state they receive an additional quantity of caloric, which combines with them, but does not sensibly elevate their temperature; and this caloric of fluidity or latent heat, as it has been named, is again given out in a sensible form, when the body returns to a solid state. If water, for example, be exposed to a great degree of cold, and kept free from agitation, it may be cooled several degrees below the freezing point, namely, to 21°, and yet remain fluid; but if it be then agitated, it instantly congeals, and at the moment of its congelation its temperature rises to 32.. All fluids, therefore, are combinations of solids and certain doses of caloric. Thus, if snow at 32° be mixed with an equal weight of water at 172°, the snow instantly melts, but the temperature of the mixture is only 32°; so that 140° of caloric have disappeared, or rather have entered the ice, and as the caloric is no longer sensible to the thermometer, it is justly said to have become latent: thence the quantity of caloric necessary to give fluidity to ice is 140°. These facts were first ascertained by Dr. Black, in 1762; and fluidity in general has been proved to depend on a similar cause.

Softness, plasticity, malleability, and ductility, probably depend also upon the repulsive force of the latent heat which combines with bodies.

Vapour, which is another effect of caloric, is that state into which all fluids and some solids pass when their temperature is raised to a certain point, or caloric is thrown into them in sufficient quantity to separate their integrant particles to distances beyond the sphere of the attraction of cohesion. The fluid passes to the state of vapour, becoming invisible and elastic, and possessing the other mechanical properties of air.

Evaporation, however, is also spontaneously produced, partly by the agency of caloric alone, partly by the solvent power of atmospheric air, forming a solution of the body in the aerial fluid. By spontaneous evaporation, the fluid is gradually converted into the aeriform state at every temperature. Water, alcohol, ether, and volatile oils, are susceptible of spontaneous evaporation, so that a portion of any of them exposed to the air, in a flat vessel, soon altogether disappears: "but sulphuric acid and the fixed oils never assume the form of vapour till they are raised to a certain temperature."

All fluids have a fixed point of temperature at which their vaporization, or conversion into vapour, commences, which is denominated their boiling point; and beyond this point fluids cannot be heated, if freely exposed to the air so as to allow the vapour to escape as it forms. Thus water at 212° boils, and is progressively converted into steam at the bottom of the vessel, which, rising in bubbles through the water, produces the ebullition that characterises boiling; but although the fire be raised ever so much, yet the temperature of the water never exceeds 212°, the vapour carrying off every additional increment of heat in a latent form. The boiling point, as this degree of temperature is termed, varies in different bodies; and in the same body also, if it be placed under different circumstances, particularly with regard to pressure. Thus, at 29.92 of the barometer, the boiling point of ether is 96°; of alcohol, 176°; of pure water, 212°; of water saturated with, common salt, 225°; of oil of turpentine, 316°; of mercury, 662°; and so on.

In a vacuum all liquids boil at a temperature 140° lower than in the open air1; and in Papin's digester, in which water can be heated under a great pressure, the temperature may be raised to 300° without ebullition. Owing to this circumstance, highly volatile substances, as ammonia and ether, cannot easily be manufactured in elevated situations.

The elasticity of the vapour of liquids boiled in the open air is equal to that of the circumambient atmosphere; but, under such a pressure that the temperature of the vapour may be much augmented, the elasticity increases with the temperature. At low temperatures, on the contrary, vapours lose their elasticity, are condensed, and return to their fluid state. The conversion, therefore, of liquids into elastic fluids depends on the same cause as the conversion of solids into fluids; namely, "the combination of a certain dose of caloric with the liquid, without any increase of temperature."2 The vapour carries off all the caloric which enters a fluid after it arrives at its boiling point, and retains it in a latent form; for the vapour is not sensibly hotter than the boiling liquid: thus, steam, the temperature of which is indicated by the thermometer to be 212°, is water combined with 967° of caloric, which remain latent as long as the temperature of the steam is maintained at 212°, but is again given out when a lower temperature changes that vapour to the state of a liquid.

The vapour of alcohol contains 442° of latent heat, that of ether 302.379, of oil of turpentine 177.87, of nitric acid 531.99, and of vinegar, 875.3 Bodies which resist the greatest known heat without vaporization are said to be fixed in the fire.

1 Professor Robison, Black's Lectures, p. 151.

2 The important discovery of the causes which produce the changes of bodies from the solid to the liquid and aeriform state was made by Dr. Black, in 1760.

3 Ure, Phil. Trans. 1818.

Gases resemble vapours in their constitution, but differ from them in the greater reduction of temperature which is re-' quired for their condensation, some of them not being reducible by ordinary pressure, or by any known reduced temperature, to the fluid or solid state. Are they compounds of solid or of liquid substances and caloric? Ammoniacal gas condenses into a fluid at 45°; and many of the other gases have been condensed into fluids by Dr. Faraday.

Ignition is another effect of caloric, but differing altogether from expansion, fluidity, and vaporization, which may in some measure be regarded as different degrees of one general effect. It implies an emission of light from bodies which are much heated, or combined with a large portion of caloric, without their suffering any change of composition. It is totally independent of the presence of air, and is a simple effect of caloric. Aeriform substances are not susceptible of ignition.

The degree of temperature at which all bodies capable of ignition begin to be ignited, or become red-hot, is nearly the same, - about the 1,000th degree of Fahrenheit in broad day, but between the 600th and 700th in the dark: by raising the temperature the illumination increases, until a perfectly white light is produced, which is the highest point of ignition. Ignition is supposed to arise from the extrication of the light, which is regarded as a constituent of the" ignited body, by the repulsive agency of the additional caloric: but this explanation of the phenomenon is liable to some objections, and the real cause remains still undetermined.

As a pharmaceutical agent, Caloric is of the first importance. In the majority of cases it produces decomposition; but in some it favours combination. The decomposition most easily effected by it is the separation of the more volatile from the more fixed ingredients of compounds. Thus, in the process of distillation (see Operations), when weak spirits are heated, the alcohol separates from the water, owing to its superior volatility, and, by condensation in a different vessel, is obtained as a distinct substance. Almost all compounds into which oxygen has entered without having occasioned combustion, as nitric acid, and some metallic oxides, suffer likewise decomposition by caloric. All compound bodies containing combustibles are also decomposable by it; as are also compounds consisting of two or more combustible ingredients, in combination with oxygen, as almost all animal and vegetable matters. On the contrary, the compounds which are little or not at all affected by caloric, as far as regards their composition, are those which have been formed by combustion; such as water, phosphoric acid, and carbonic acid.1 The proper application of caloric for the purpose of obtaining new combinations by lessening the force of aggregation, and thus favouring the attraction of affinity; or for producing decompositions by weakening or destroying altogether the force of these attractions so as to obtain the principles of bodies in a distinct state, constitutes the most important feature of operative pharmacy. (See Operations.)