"In 1823 I was assistant in the Royal Grammar School at Montreal, under Alexander Skakel, M.A., LL.D. On January 28th, Mr. Skakel-having observed on the previous night the thermometer was very low-looked at his thermometer early in the morning, and found that the mercury had shrunk entirely into the bulb. He then referred to his spirit thermometer, and found that it indicated 42° below zero. He sent upstairs to myself and Messrs. McDonald and Randal (the other teachers sleeping in the house), and informed us that he purposed breaking one of his mercurial thermometers, that he might say he had handled solid mercury. We all descended to a back gallery, on which he broke one, and the mercury rolled away like a marble. Mr. Skakel took it up, and afterwards gave it to each one of us to handle. There was not a breath of wind, and I walked the whole length of Little St. James Street without feeling the weather to be otherwise than moderately cold.

"Montreal, Jan. 28th, 1873.

“(Signed), “F. H. ANDREWS."

In all schemes for ventilating and supplying heated air, circulation must, of course, be maintained, either to impart or carry off heat.

It is said that, if the hand is kept perfectly still in water heated to a temperature of 150° F., the nerves are not disagreeably affected; but directly the hand is moved, then the heat becomes painful, and cannot be borne.

As an illustration of convection, or the carrying of heat, on the grand scale, there are the trade winds and the Gulf Stream:

In the tropics the heated earth imparts some of its force to great volumes of air, which ascend and flow towards the poles; upper currents from the equator to the poles must be succeeded by under currents from the poles to the equator.

The constantly ascending warm air is thus a carrier of heat to colder climates, and vice versa. These currents are modified by the various physical conditions of the earth's surface.

In like manner, a great current of warm water, which leaves the Straits of Florida at a temperature of 83° F., passes across the Atlantic in a northeasterly direction. It washes the north-western shores of Europe, and makes itself, or rather its heat-giving power, apparent by flowing round the coast of Ireland. In mild winters in England it is the diffusion of heat by certain winds, and the good offices of the Gulf Stream, which mitigate the severity of the season; and these carriers of heat are only neutralized when similarly, but contrarily, enormous masses of ice, icebergs, are detached from the polar regions, and rob the water of its heat on its journey to our shores.

LATENT HEAT.

CAPACITY FOR HEAT-SPECIFIC HEAT-HEAT OF ATOMS-ATOMIC HEAT. These somewhat difficult terms or titles, referring to truths that the young student does not, perhaps, fully appreciate at first, nay, to speak plainer, which he never will comprehend without industrious application to study, are set forth in the following chapters.

In all the old standard works upon natural philosophy it is usual to state that there are two kinds of heat that may be resident in a body, viz., one kind

called “sensible heat," which is designated as temperature, and is capable of measurement by the thermometer and other kindred instruments; another ard more subtle condition, not apparent to our nervous system, called “latent heat," and incapable, whilst in that condition, of affecting any measurer or test of "sensible heat." The dynamical theory substitutes the terms "actual energy," or force, for that of "sensible heat," and "potential energy" for that of "latent heat."

The one, actual heat or energy, is in use; the other, potential heat or energy, is in store. A horse-shoe nail may be warmed by any convenient source of heat, and as long as it remains above the temperature of the air we have evidence of "actual heat."

When cold it may be hammered on an anvil, by an expert blacksmith, and then becomes so hot it will set fire to sulphur or phosphorus. The heat thus evoked was formerly called "latent heat," and was supposed to be combined with the material substance of the iron; the dynamical theory rejects the idea of its being a distinct subtle fluid, but ascribes the heat to the motion of the particles of the iron. It may be useful here to tabulate the new terms used by Clausius, Rankin, Tyndall, and others, in their exposition of the dynamical theory of heat.

ENERGY OR HEAT.

Defined to be the power of performing work. It may be latent or sensible.

Latent.

Possible energy, or work to be

done.

Sensible.

Actual energy, or work is being done.

Potential energy is energy in store. Dynamic energy is energy in action. One column of terms is the exact antithesis of the other. There is no mechanical machine by which we can tear asunder or separate the ultimate molecules of bodies. Cohesion, or molecular force, is too potent to be overcome by mechanical energy. Heat, another kind of energy, will, however, act where the former fails; therefore heat is the equivalent for mechanical

energy.

When a metal is expanded by heat, every molecule is separated or forced asunder; the energy of heat must be enormous to overcome the force of cohesion. When a mass of metal is heated, there is not only the motion imparted the vibratory power set up to produce sensible or actual energy (heat)- but the molecules or atoms of the metal are pushed asunder, as shown by their expansion. This work, which goes on inside and throughout the mass of the metal, is not visible, and therefore may be called "interior work."

Tyndall compares this interior work to the raising of a weight from the earth -the overcoming of the force of gravity, which attracts all things, and keeps all terrestrial bodies in their places. The raising of a weight by a cord from the earth, it is clear, confers a motion-producing power." The weight can fall, and in its descent can perform work. Whilst hanging in the air, it represents possible energy, or "potential" energy.

The pull, or attraction of gravity, causes this possible or "potential" energy. If there were no attraction between the substance and the earth, there would be no "possible" energy.

Substitute the ultimate atoms of bodies for the weight and the earth:

remember that the atoms of solid bodies are held together with molecular force (cohesion), and it must be evident that whenever they are separated, although the distance to which they are separated cannot be measured-it is too minute-still the fact remains, and when the atoms come together it is like the fall of the weight to the earth, and the result must be the production of actual energy, or heat.

This is what Tyndall means when he speaks of the clashing together of the atoms.

The heating of the cold horse-shoe nail by hammering, or the heating of cold bars by rolling, is simply the conversion of mechanical energy into molecular motion; if the approach of the molecules of a body will produce actual energy, a still nearer approach must increase that energy, or heat. Indeed, the experiment already quoted, of heat produced by hammering and bringing the atoms nearer together, is a good illustration of the above argument.

The "specific heat" (a term that must be carefully considered presently) of a metal like copper is altered when a nice, soft, well-annealed piece is hammered: heat is produced, and the specific heat changes from o'09501, 0'09455, to o'09360, 009330; and its specific gravity or density becomes higher. When again heated red hot and allowed to cool slowly, as is done in the process of annealing, its specific heat returned to o'09493, 009479, or very nearly the same that it was at first. Thus by alternately hammering and then heating or annealing a metal, the atoms are brought more closely together or pushed further apart. When the atoms are pushed further apart, the heat becomes potential or latent; when advanced nearer to each other, the heat is actual or sensible. Nearly every philosopher selects a particular subject to which he devotes his special attention. Let us read what Dr. Tyndall says of latent heat in his standard work, “Heat a Mode of Motion." "We shall now direct our attention to the phenomena which accompany changes of the state of aggregation. When sufficiently heated, a solid melts; and when sufficiently heated, a liquid assumes the form of gas. Let us take the case of ice, and trace it through the entire cycle. This block of ice has now a temperature of 10° C. below zero. I warm it; a thermometer fixed in it rises to o°, and at this point the ice begins to melt; the thermometric column, which rose previously, is now arrested in its march, and becomes perfectly stationary. I continue to apply warmth, but there is no augmentation of temperature; and not until the last film of ice has been removed from the bulb of the thermometer, does the mercury resume its motion. It is now again ascending; it reaches 30°, 60°, 100° C.; here steam-bubbles appear in the liquid; it boils, and, from this point upwards, the thermometer remains stationary at 100°. But during the melting of the ice, and during the evaporation of the water, heat is incessantly communicated. To simply liquefy the ice, as much heat is imparted as would raise the same weight of water 79'4° C., or as would raise 79°4 times the weight one degree in temperature; and to convert a pound of water at 100° C. into a pound of steam at the same temperature, 537 2 times as much heat is required as would raise a pound of water one degree in temperature. The former number, 79'4° C. (or 143° F.), represents what has been hitherto called the latent heat of water; and the latter number, 537.2° C. (or 967° F.), represents the latent heat of steam.

"It was manifest to those who first used these terms, that throughout the entire time of melting, and throughout the entire time of boiling, heat was

communicated; but inasmuch as this heat was not revealed by the thermometer, the fiction was invented that it was rendered latent. The fluid of heat was supposed to hide itself in some unknown way in the interstitial spaces of the water and the steam.

According to our present theory (the dynamical), the heat expended in melting is consumed in conferring potential energy upon the atoms: it is virtually the lifting of a weight. So likewise as regards steam, the heat is consumed in pulling the liquid molecules asunder-conferring upon them a still greater amount of potential energy.

"When the heat is withdrawn, the vapour condenses, the molecules again clash with a dynamic energy equal to that which was employed to separate them, and the precise quantity of heat then consumed now re-appears.

"The act of liquefaction consists of interior work expended in moving the atoms into new positions. The act of vaporization is also, for the most part, interior work; to which, however, must be added the exterior work of forcing back the atmosphere, when the liquid becomes vapour.

Let us then fix our attention upon this wonderful substance, water, and trace it through the various stages of its existence. First, we have its constituents as free atoms of oxygen and hydrogen, which attract each other, fall or clash together. The mechanical value of this atomic act is easily determined. The heating of 1 lb. of water 1° C. is equivalent to 1,390 foot-pounds; hence the heating of 34,000 lbs. of water 1° C. is equivalent to 34,000X1,390 footpounds.

"We thus find that the concussion of our 1 lb. of hydrogen with 8 lbs. of oxygen is equal, in mechanical value, to the raising of forty-seven million pounds one foot high.

"I think I did not overstate matters when I stated that the force of gravity, as exerted near the earth, is almost a vanishing quality, in comparison with these molecular forces.

"The distances which separate the atoms before combination are so small as to be utterly immeasurable; still it is in passing over these spaces that the atoms acquire a velocity sufficient to cause them to clash with the tremendous energy indicated by the above numbers. After combination, it is in a state of a vapour, which sinks to 100° C., and afterwards condenses into water. In the first instance the atoms fall together to form the compound; in the next instance the molecules of the compound fall together to form a liquid. The mechanical value of this act is also easily calculated. 9lbs. of steam, in falling to water, generate an amount of heat sufficient to raise 537 2X9=4,835 lbs. of water 1°C., or 967 X9=8,703 lbs. 1° F. Multiplying the former number by 1,390, or the latter by 772, we have in round numbers a product of 6,720,000 lbs. as the mechanical value of the mere act of condensation.

"The next great fall is from the state of liquid to that of ice, and the mechanical value of this act is equal to 993,564 foot-pounds. Thus our 9 lbs. of water, at its origin and during its progress, falls down three great precipices; the first fall is equivalent in energy to the descent of a ton weight down a precipice 22,320 feet high; the second fall is equal to that of a ton down a precipice 22,900 feet high; and the third is equal to the fall of a ton down a precipice 433 feet high.

"I have seen the wild stone-avalanches of the Alps, which smoke and thunder down the declivities with a vehemence almost sufficient to stun the

observer. I have also seen snow-flakes descending so softly as not to hurt the fragile spangles of which they were composed; yet to produce from aqueous vapour a quantity, which a child could carry, of that tender material, demands an exertion of energy competent to gather up the shattered blocks of the largest stone-avalanche I have ever seen, and pitch them to twice the height from which they fell."

CAPACITY FOR HEAT.

This term, which is most simple and useful, expresses a fact that has been forced upon observers by numerous experiments made with the thermometer. The thermometer is usefully applied to determine the temperature of any solid, fluid, or gaseous matter; but it will not tell the observer how much heat or actual energy is contained in different measures of the same fluid. A gallon of water in one vessel, and a pint of water in another, may be shown by the thermometer to have a temperature of 212° F.; but the quantity of energy or heat must be much greater in the larger measure-the one gallon-than in the single pint. The thermometer fails to show the quantity of energy, whilst it gives relatively the "relative actual heat"-the "temperature." A photometer, or measurer of light, will demonstrate the relative illuminating power of any given source of light; but it cannot give the number of vibrations per second producing the light. A thermometer can tell us truthfully how mach hotter or colder than 32° or 212° F. a substance may be ; but it cannot inform us what may be the amount of vibratory power given, and the molecular force detached, which, according to the dynamical theory, must be the equivalent for the expression or quantity of heat. There are certain facts, explain them how we will, which are indisputable. If 10 lbs. of water (one gallon) at 100° F. are mixed with the same weight of oil at 50° F., the resulting temperature will not be the mean, 75° F., but 83 F. The water, therefore, has lost "actual energy” equal to 16}; but the same energy has caused the oil to rise 333.

33

If the experiment is reversed, and 10 lbs. of oil at 100° are mixed with 10 lbs. of water at 50°, the mean will be 6630: the 333° actual heat or energy given out from the oil is only able to raise the temperature of the water 1630.

The actual energy which will raise the temperature of oil 2° will raise an equal quantity of water only 1°. The heat that will raise any given substance from o° C. to 1° C., compared with the amount of "energy" required to heat an equal weight of water to the same point, is called its "specific heat." Therefore the specific or potential heat of oil will be a half, 5, as compared with the unit or one-viz., water.

As the oil has been quickly heated, so it will rapidly cool; it has only half the "energy of heat" possessed by water to give up. If the water require one hour to cool to any given temperature, the oil would reach the same point in half-an-hour.

Hence “time” is the test used sometimes to determine the specific heat of bodies the time required by a substance to cool. Or the process may be reversed by ascertaining the quantity of ice which exactly equal weights of other bodies can melt in falling from one temperature to another, say from the boiling-point to the freezing-point of water. As the process of mixture already described with the oil and water may be employed, there are therefore three methods by which the specific heat of bodies may be determined :

: