happens that two moles make use of the same high-road, each having distinct citadels and hunting-grounds. Should they chance to meet, however, the weaker must retire into some side-alley, and let his neighbour pass on, otherwise a desperate encounter, ending only with the death of one or other combatant, is inevitable.

There are many more details of interest concerning the private life of the mole, to be discovered by any one who takes pleasure in investigating common things,-that strange world of wonders which is around us, and about us, undreamt of and unknown, save by a very few. Enough has at least been said to refute the libels which that most flippant of pseudo-naturalists, Buffon, has cast upon our little engineer. According to his account, the mole is "a miserable and imperfect creature, compelled to never-ending toil in gloomy holes in the earth," and so forth. We might as well pity the ant-bear for its lack of teeth, or ourselves for the want of wings. Had this brilliant compiler been as careful in authenticating his statements by personal observation, as he was in rounding his periods, he would have bequeathed us a work of perhaps less eloquence, but one certainly of far greater value and trustworthiness; and would have learnt that the all-wise Creator, in pronouncing everything to be very good, did not except even such "imperfect and unfortunate creatures" as the sloth, the woodpecker, and the mole.

Woodhouse-Grove.

HEAT.

A Lecture, by the late Robert Ripley, Esq., M.D.,
of Whitby.

(Continued from page 454.)

WE now enter on the consideration of HEAT. I shall not at present make any remarks on the important part which this agent performs in the economy of nature. A few reflections of this kind will come more appropriately at the conclusion of this division of our subject.

As to the NATURE OF HEAT, there are two views. According to one, it has an abstract existence; in which case, we say that it is a subtle, imponderable fluid, like light and electricity. Or we may assume an undulatory theory of heat, corresponding with the undulatory theory of light; about which more will be said in a future lecture.

It is convenient to adopt the material theory, in considering the accumulation of heat in bodies, and in expressing quantities of heat. Everything relating to the absorption of heat, suggests also the idea of its substantial existence for heat, unlike light, is never extinguished when it falls upon a body; but is either reflected, and may be further traced, or is absorbed, and may again be derived from the body without loss. In speaking of heat, however, as having a material existence, we must remember that this existence is merely ideal or hypothetical. I said before, it was a subtle, imponderable fluid; so subtle, indeed, that it eludes our most refined attempts to isolate it; and so imponderable, that, however much of it may accumulate in a body, causing its form and bulk to vary, still its weight is unaffected.

With reference to the undulatory theory, a peculiar imponderable medium or ether is supposed to pervade all space, differing from air, but co-existing in the same space; through this undulations are propagated, which produce the impression of heat. A hot body radiating heat, possesses the faculty of originating or exciting undulations in this ether, which spread on all sides around it, like the waves from a pebble thrown into still water; the different properties of heat being referred to differences in the size of the waves, as differences of colour are accounted for in light.

Without deciding between these two views, I will next pass on to explain several terms which you will meet with in works on natural philosophy, when treating of heat, and about which it is necessary to have precise ideas. You will often find the terms heat and caloric used synonymously: but, strictly speaking, they bear the relation of cause and effect to each other; caloric being the principle or cause of heat, and heat the effect of caloric.-Then, again, you find mention made of sensible heat, latent heat, and specific

heat. Sensible heat is that which is appreciable to the touch and the thermometer. Latent heat does not affect either of these, but exists combined with bodies, keeping them in their normal or natural state of existence. This will be best illustrated by an example or two. If you mix one pound of ice, the temperature of which is 32° of the thermometer, and one pound of water at 172°, every particle of ice will be dissolved, and you will have two pounds of water, the temperature of which, instead of being a mean between 32° and 172°, will only be 32°. What, then, has become of the 140° of sensible heat existing in the one pound of water? It has all become latent in the one pound of ice, now converted into one pound of water, and may be derived from it, by causing it to become solid again: 140° is, therefore, the latent heat of water, or its heat of liquefaction; that is, water must have this combined with it, to preserve it in a liquid form; and the whole of this heat disappears in melting ice, before its sensible temperature is raised a single degree. In the same way, though steam and boiling water both indicate the same temperature, namely, 212°, yet it can be proved that the steam contains 980° more latent heat than water: in other words, this quantity of sensible heat disappears, or becomes latent, before water is converted into steam; and all this may be derived from it, by making it pass from the form of vapour into that of a liquid. For if we convert one pound of water into steam, and then re-condense it, by passing it through ice, we shall find seven pounds of ice dissolved. As before stated, 140° is the heat of liquefaction of water, or, in other words, its latent heat; and 7 times 140° gives us 980°, which is the latent heat of steam. It is the latent heat which enters into the composition of bodies that causes the variations in their forms or modes of existence, of which we have already spoken; that is, it is the quantity of heat which exists in combination with a body, that determines whether it is a solid, liquid, or gas. In this way heat is the cause of fluidity, both with reference to liquids and aeriform fluids or gases.

The term specific heat refers to the capacities of bodies for

heat; in other words, to the quantity of heat which bodies require to raise their sensible temperatures. This quantity varies considerably in different bodies. Thus, if you take one pound of quicksilver and one pound of water, each at 40°, and introduce the same quantity of sensible heat into them, for every degree that the temperature of the water is raised, that of the quicksilver will be raised 23°; and in cooling down the same bodies, a given quantity of water will melt twenty-three times as much ice as the same quantity of quicksilver, or give out twenty-three times as much heat. The specific heat, therefore of quicksilver, or its capacity for heat, as compared with that of water, is as 23 to 1. That of other bodies may be ascertained in the same way, and be expressed by the same standard.

The principal SOURCES OF HEAT are the sun, and mechanical and chemical condensation. Several others might be named; but these are the most important. I need not say anything about the sun as a source of heat; but there are several interesting points connected with the other sources which I have named. It is a law in physics, that as the density of a body becomes increased, that is, as its bulk diminishes, its capacity for caloric diminishes also; and thus heat previously latent becomes sensible. Thus there is condensation in the experiment already mentioned of liquefying ice, by passing steam through it: the molecules, which were widely separated in the steam, owing to there being a large quantity of caloric combined with them, become pressed into a more compact form when the steam is condensed into a liquid; a large portion of the heat which exists as an atmosphere or halo around the particles being in fact pressed out, as the atoms are brought into closer contact, and this serves to liquefy the ice. The converse of this is also true as a body changes its state for one of less density; that is, as its bulk increases, its capacity for caloric also increases; heat previously sensible becomes latent in it; and when this heat is abstracted from surrounding bodies, cold is produced. In this way we account for the heat evolved when the steam is condensed into

water. In the experiment just referred to, it enters into the ice, and increases its bulk or density by liquefying it. The perspiration is an admirable provision for cooling the body in hot weather, and it produces its effects on the same principle. Ordinarily it escapes as an insensible vapour; but it reaches the skin in the form of a liquid, and, in assuming an aeriform condition, it abstracts heat from the body, producing a grateful sense of coolness, and thus counteracting the effects of external heat.

Several familiar examples may be named of heat being evolved as the result of mechanical condensation. The blacksmith lights his fire, or at any rate used to do, by hammering a piece of soft iron, until it becomes nearly red hot: he puts this amongst his coals, and ignites these with a few blasts from his bellows: the particles of iron are mechanically condensed by the hammering, and a portion of heat is thus squeezed out from between the particles. In the old flint and steel, heat was in the same way evolved from the metal by percussion. The Indian lights his fire by rubbing a hard and a soft piece of wood together; and forests have been set on fire by the friction of dry branches on each other, produced by the wind: here also the friction produces condensation, the particles are heaped closer together, and heat existing in the interstices between them is squeezed out in sufficient quantity to ignite the whole.

I may mention also an example of the evolution of heat through chemical action. If a few drops of cold water be poured upon a piece of newly-burnt lime, a portion of the water enters into chemical combination with the lime, forming what is called a hydrate. In doing so, it changes its state suddenly from a liquid to a solid; and thus, on the principle already explained, its capacity for caloric is diminished: it parts, therefore, with its latent heat; that is, its heat of liquefaction; and this combines with another portion of the water, and becomes latent in it, as it converts it into steam. Heat is also evolved when oil of vitriol and cold water are mixed together; but it is a disputed point whether this is the result of mechanical or of chemical condensation.