already seen that, when heat is added to a body, it first expands in bulk; if more heat be added, then LIQUEFACTION or fluidity takes place; that is to say, the body, from being a solid, passes into the form of a liquid. In some bodies the transition from the solid into the liquid state is immediate, as in ice, silver, copper. In others, it is gradual, as in wax, tallow, iron, which are first soft, and then viscid, before they become entirely fluid. If you reduce the temperature of the body thus liquefied, it resumes its solid form. The point of temperature, at which liquefaction takes place, is very different in different bodies. Some solid bodies, such as wax, become liquid at a temperature not much higher than that of the medium state of the atmosphere. Ice melts, as we have seen, at 32° of our thermometer. Frozen quicksilver melts at 40° below zero or 0. Because we are accustomed to see some bodies solid, when in the ordinary state of temperature, and others fluid in that state, they hence receive the name of solids or of fluids. Thus we call water a fluid, and iron a solid. But you will now understand, that every body is either solid or fluid, according to its state of temperature at the time. Some bodies, indeed, we have never seen in a solid state, and others we have never seen in a fluid state, because we are not able to produce the temperature, which will bring them into these states respectively. But they are all to be regarded as capable of being brought either into a solid or a fluid state, by production of the necessary temperature.

ON THE GENERAL EFFECTS OF HEAT-continued.

THE third general effect of heat is VAPORIZATION; or the conversion of a solid or liquid body into vapour or aëriform fluid, as is the case when water is, by addition of heat, made to boil. Most bodies pass first from the solid into the liquid, and from the liquid into the aëriform state but some pass directly from the solid state into vapour. The temperature, at which evaporation takes place, is very different in different bodies; water boils at 212°; quicksilver at 655°. Bodies, which pass into this state at a low temperature,

are called volatile; those, which require a very high temperature for this purpose, are said to be fixed. There are some, which are always seen in an aëriform shape, and cannot be condensed by any cold which we can produce: these are generally called airs or gases. Those substances, which may be easily restored to their condensed form, are generally distinguished by the name of vapours. There is, however, no difference between gases and vapours, except in so far as regards the temperature, at which they change their form. Both would be condensed, if we had it in our power to reduce them to the necessary temperature. There are some substances, on the other hand, such as iron, which we have never known in an aëriform shape: there can be as little doubt that they are convertible into vapours, if we could raise them to the necessary temperature. Every body therefore, is either solid or aëriform, just according to its temperature at the time. In passing from a liquid into an aëriform state, as every one knows, a movement generally takes place in the liquid, to which we give the name of boiling. This motion is now satisfactorily ascribed to the formation of vapour in the lower part of the fluid, which, ascending to the top, produces this agitation. Boiling however is by no means essential to vaporization: because, if the heat be applied above, in place of below, vapour will be produced without boiling.-The transition of bodies into vapour is much affected by pressure, which greatly impedes the repellent force of heat in overcoming the cohesive attraction. The same fluid will boil at very different temperatures, according to the pressure of the atmosphere at the time. If this pressure be entirely removed, boiling will take place at a temperature more than 100 degrees below the usual boiling point. A very amusing experiment connected with this subject is exhibited in the Chemistry Class. Water is put into a glass flask and held over the fire till it boils; the flask is then closely corked and removed from the fire, and the water still continues to boil for a long time: the cork is afterwards drawn, and the boiling immediately ceases. The reason of this is, that the vapour of the water expels the atmospheric air;

then, upon the removal of the flask from the fire, the vapour itself condenses, by which the water is deprived of its pressure, and thus boils at a low degree of temperature; when the cork is drawn, the atmospheric pressure is restored, which puts an end to the boiling. If, while the flask is corked, it be immersed in cold water, the water within it boils violently; take it out of the cold water, and immerse it in warm water, the boiling ceases. This apparently strange phenomenon is easily explained: the cold water condenses the vapour and thus removes the pressure: the warm water retains the steam in its aëriform state, and thus there is a pressure on the fluid, which prevents it from boiling. As the diminution of pressure causes a fluid to boil at a lower temperature, so its increase produces the opposite effect. Thus water has been heated in a close metallic vessel to the temperature of 400° without passing into vapour.-There is a species of evaporation, commonly known by the name of spontaneous evaporation, which many substances, solid as well as fluid, undergo when exposed to the air, though at a temperature very far indeed below the boiling point, nay even in hard frost. Thus every one knows, that, if a little water be at any time spilt upon the ground, it very soon disappears; and the ink, with which we write, quickly dries up. Those substances, which have the lowest boiling point, are also most speedily converted by spontaneous evaporation. A quantity of spirit of wine, thrown upon a table, will disappear much more quickly, than the same quantity of water. The rapidity of this evaporation depends much upon the extent of surface exposed to the air: the same quantity of water will evaporate much more quickly when spread over a large surface, than when heaped upon a smaller one. This evaporation is also much promoted by causing a free circulation of the air: thus almost every one has blown upon a small quantity of fluid for the purpose of drying it up; and ground, which has been wet with rain, becomes much sooner dry in windy than in calm weather. This is easily accounted for. A certain quantity of air can only absorb a certain quantity of vapour; by blowing, new portions of air are continually introduced,

which successively take in the vapour, till the whole is absorbed. This evaporation is increased by an increase of temperature; and hence it is, that the rain dries up so much more quickly in summer than in winter. By a reduction of temperature, the air will deposit some of the vapour, which it formerly received: this it frequently does in the form of dew, hoar-frost, &c. During spontaneous evaporation much heat is absorbed, or, in other words, a great degree of cold is produced. Hence, in warm climates, water is sprinkled about the rooms to render them more comfortable. There is a difference of opinion among philosophers about the cause of spontaneous evaporation. By some it is ascribed merely to the agency of heat; according to others, it is produced by the air dissolving the body.-IV. The last general effect of heat which we have mentioned is INCANDESCENCE, by which we mean that red-heat, to which bodies are brought by a great increase of temperature, and which is unaccompanied with any other chemical change; as, for example, when a bar of iron is rendered red-hot by being kept long in a fire. You must be careful to distinguish this incandescence from combustion or inflammation, such as takes place when you kindle a piece of paper. Incandescence is the effect of high temperature alone; whereas combustion depends upon the action of the air: incandescence takes place in many bodies which are not susceptible of combustion: after incandescence has ceased, the body returns to its former state; whereas by combustion a permanent change is effected upon it: incandescence may be renewed; whereas combustion can never take place again in the visible matter which it leaves behind. The point of temperature at which incandescence takes place is supposed to be the same in all bodies; but what that point is, it has been found difficult to fix with precision. It is supposed to be about 800°. Incandescence may be produced by friction or percussion, as well as by communication from a heated body. What we have here called incandescence is frequently known by the name of ignition; but this term is no less frequently employed, in common language at least, to denote inflammation.

ON CHEMICAL ATTRACTION.

CHEMICAL ATTRACTION (which is known also by the name of Chemical Affinity, or the Attraction of Compo sition), is that force, by which the particles of different bodies are intimately united, so as to form a new substance. It differs from the attraction of gravitation in this, that, while gravity acts on large masses, though at a great distance from each other, chemical attraction unites only small particles when brought so near as to be apparently in actual contact. It differs from the

attraction of cohesion in this, that, while the cohesive attraction acts only on similar particles, the chemical attraction operates on particles of a different description from each other. The union produced by chemical attraction is called chemical combination or synthesis. Combination differs from mere mixture, in which last the particles, however intimately blended, still exist apart, so as to be capable of being recognised and separated by mechanical means. The substance produced by combination is called a compound substance. The sub stances which are combined are called the constituent or component parts of this new compound. The chemical process, by which these constituent parts are again separated, is called decomposition or chemical analysis. As combination differs from mixture, so decomposition is also quite different from mere division. Division only makes a separation between what are called the integrant parts of a body, that is to say parts which continue quite similar in their properties to each other, and to the whole body in its undivided state: whereas decomposition separates the constituent parts; which are quite different, both from each other, and from the substance which they formerly composed. Those substances which cannot be resolved, or at least never have been resolved, into any simpler parts, are called elements. Of these Aristotle enumerated four, fire, water, earth, and air; which still, in ordinary language, retain the name of elements, and were so accounted even by philosophers till about fifty years ago. It is now however quite certain that this opinion was erra»