The Boy's Playbook of Science Part 42

CHAPTER XXVII.

HEAT.

[Ill.u.s.tration: Fig. 342. James Watt.]

Throughout the greater number of the preceding chapters it will be evident that the active properties of matter may be summed up under one general head, and may be considered as varieties of attraction--such as the attraction of gravitation, cohesive attraction, adhesive attraction, attraction of composition (or chemical attraction), electrical attraction, magnetical attraction.

The absolute or autocratic system does not, however, prevail in the works of nature; and she seems ever anxious, whilst imparting great and peculiar powers to certain agents, to create other forces which may control and balance them. Thus, for instance, the great force of cohesive attraction is an ever-present power discernible, as has been shown, in solids and liquids; but if this agent [Page 352] were allowed to run riot in its full strength and intensity, it would tyrannically hold in subjection all liquid matter, and every drop of water which is at present kept in the liquid state, would succ.u.mb to its iron rule, and retain the solid state of ice. Hence, therefore, the wise creation of an antagonistic force--viz., heat; which is not provided in any n.i.g.g.ardly manner, but is liberally bestowed upon the globe from that all-sufficient and enormous source, the sun. And it is by the softening and liquifying influence of his rays that the greater proportion of the water on the surface of the globe is maintained in the fluid condition, and is enabled to resist the power of cohesion, that would otherwise turn it all, as it were, to stone.

Cohesion, electricity, and magnetism fully embody the notion of powers of attraction, or _a drawing together_; whilst heat stands almost alone in nature as the type of repulsion, or _a driving back_.

Mechanically, repulsion is demonstrated by the rebound of a ball from the ground; the parts which touch the earth are for the moment compressed, and it is the subsequent repulsion between the particles in those parts which causes them to expand again and throw off the ball.

The development of heat is produced from various causes, which may be regarded as at least four in number. Thus, it was shown by Sir Humphrey Davy, that even when two lumps of ice are rubbed together, sufficient heat is obtained to melt the two surfaces which are in contact with each other. Friction is therefore an important source of heat, and one of the most interesting machines at the Paris Exposition consisted of an apparatus by which many gallons of water were kept in the boiling state by means of the heat obtained from the friction of two copper discs against each other. The machine attracted a good deal of attention on its own merits, and especially because it supplied boiling water for the preparation of chocolate, which the public was duly informed was boiled by the heat _rubbed out_ of the otherwise cold discs of copper. When cannon made on the old system are bored with a drill, it is necessary that the latter should be kept quite cool with a constant supply of water, or else the hard steel might become red-hot, and would then lose its _temper_, and be no longer capable of performing its duty.

Count Rumford endeavoured to ascertain how much heat was actually generated by friction. When a blunt steel bore, three inches and a half in diameter, was driven against the bottom of a bra.s.s cannon seven inches and a half in diameter, with a pressure which was equal to the weight of ten thousand pounds, and made to revolve thirty-two times in a minute, in forty-one minutes 837 grains of dust were produced, and the heat generated was sufficient to raise 113 pounds of the metal 70 Fahrenheit--a quant.i.ty of heat which is capable of melting six pounds and a half of ice, or of raising five pounds of water from the freezing to the boiling point. When the experiment was repeated under water, two gallons and a half of water, at 60 Fah., were made to boil in two hours and a half.

Chemical affinity has been so often alluded to in these pages, that it [Page 354] may be sufficient to mention only one good instance of its almost magical power in evoking heat. When a bit of the metal sodium is placed on the tip of a knife, and thrust into some warm quicksilver, or if a pellet of sodium and a few globules of mercury are placed on a hot plate just taken from the oven, and then gently squeezed together, a vivid production of heat and light is apparent; and when the mixture of the two metals is cold, it will be found that the quicksilver has lost its fluidity, and a solid amalgam of sodium and mercury is obtained, which gradually, by exposure to the air, returns to the liquid state, the mercury being set free, whilst the sodium is oxidized, and forms soda. Just as an ordinary alloy of copper and gold used by jewellers would lose its colour and brilliancy by the oxidation of the copper; and when the rusty, dirty film is removed by rubbing and polishing, the surface is again brilliant, and remains so until another film of the exposed copper is attacked: in like manner the sodium is attacked and changed by the oxygen of the air, whilst the mercury being unaffected retains its brilliancy, and at the same time regains its fluidity. The evolution of heat in the above case indicates that a chemical union has taken place between the two metals.

Examples of the production of heat by electricity and magnetism have been abundantly shown in the chapters on these subjects; and one of the best ill.u.s.trations of this fact has been shown on the occasion of the opening of the telegraphic communication between France and England by means of the submarine cable, when cannon were fired alternately at both ends of the conducting cable by means of electricity, and the event thus inaugurated in both countries.

That heat is a product of living animal organization is shown, as it were, visibly by the marvellous phenomena that proceed in our own bodies. People do not very often trouble themselves to ask where the heat comes from, or even to think that this invisible power must be maintained in the body, and that slow combustion, or, as Liebig terms it, _eremacausis_, must continually go on inside our frail mortal tenements; and more than this, that we cannot afford to waste our heat.

If the body is deprived of heat faster than it can be generated, death must inevitably occur; and a very melancholy instance of this remarkable mode of death has lately occurred in Switzerland to a Russian gentleman.

Such another instance of a man being slowly frozen to death within sight and sound of other beings, through whose veins the blood was flowing at its accustomed temperature (about 90 Fahr.), it would be difficult to find, and it stands forth, therefore, as a marked example and ill.u.s.tration of the statement already made, that living animal organisms are truly a source of heat, which is as essential to the well-being of the body as meat, drink, and air.

Heat is of two kinds, and may be either apparent to our senses, and therefore called _sensible_ heat; or it may be entirely concealed, although present in solids, liquids, and gases, and is then termed _insensible_ or _latent_ heat.

[Page 355]

_Sensible Heat._

The first effect of this force is a demonstration of its repulsive agency, and the dilatation or expansion of the three forms of matter whilst under the influence of heat, admits of very simple ill.u.s.trations.

The expansion of a solid substance, as, for instance, a metal, on the application of heat, is apparent by fitting a solid bra.s.s cylinder into a proper metal gauge, which is accurately filed so as to admit the former when perfectly cold. If the bra.s.s rod is then heated, either by plunging it into boiling water or by the application of the flame of a spirit lamp, its particles are separated from each other; they now occupy a larger s.p.a.ce, and expansion is the result, and this is clearly proved by the application of the gauge, which is no longer capable of receiving it. (Fig. 343.) When, however, the latter is cooled, the opposite result occurs, the particles of bra.s.s return to their old position, and _contraction_ takes place; hence it is stated that "Bodies expand by heat and contract by cold;" and it is proper to state here that the term "_cold_" is of a negative character, and simply means the absence of heat.

[Ill.u.s.tration: Fig. 343. A B. Cylinder of bra.s.s. C D. Iron gauge, admitting A B longitudinally, and also in the hole E when cold, but excluding A B when the latter is heated and expanded.]

Solid bodies do not expand equally on the application of the same amount of heat; thus, a bar of gla.s.s one inch square and one thousand inches long would only expand one inch whilst heated from the freezing to the boiling point of water. A bar of iron one inch square and eight hundred inches long would expand one inch in length, through the same degrees of heat; and a bar of lead one inch square and three hundred and fifty inches long would also dilate one inch in length. Hence,

Lead expands in volume 1/350th.

Iron 1/800th.

Gla.s.s 1/1000th.

The unequal expansion of the metals is well ill.u.s.trated by an experiment devised by Dr. Tyndal, the respected Professor of Natural Philosophy in the Royal Inst.i.tution of Great Britain, and is arranged as follows:--A long bar of bra.s.s and another of iron are supported on the [Page 356]

edges of two pieces of wood placed at an angle, and resting against the sides of a mahogany framework. The metallic bars only touch one end of the frame, and are in metallic communication with a piece of bra.s.s inserted there, and forming part of a conducting chain connected with a voltaic battery; when heat is applied to both bars they expand unequally; the bra.s.s bar dilates first, and filling up the minute s.p.a.ce left between the two ends of the frame, touches another bra.s.s plate and instantly completes the voltaic circuit, when a coil of platinum wire becomes ignited, showing the fact of expansion; and secondly, the difference in the power of dilatation possessed by each is clearly shown by removing the two angular supports of wood, when the iron falls away, whilst the bra.s.s remains and still completes the voltaic circuit. (Fig.

344.)

[Ill.u.s.tration: Fig. 344. A A. The bra.s.s bar which has expanded by the heat from the gas jet B, and making the contact between the bra.s.s plates in connexion with the binding screws C C, the voltaic circuit is completed, and a coil of platinum wire in the gla.s.s tube D, is immediately ignited. The iron bar at E E has not expanded sufficiently, which is shown afterwards by removing the angular wooden supports K K, when the iron falls off, and the bra.s.s remains on the two ledges of the mahogany framework L L L.]

The force exerted by the expansion of solids is enormous, and reminds us again of the amazing power of all the imponderable agents; and it is truly wonderful to notice how the entry of a certain amount of heat into and between the particles of metals, or other solids, endues them with a mechanical force which is almost irresistible, and is capable of working much harm. Kussne made an experiment with an iron sphere, which he heated from a temperature of 32 Fahr. to 212 Fahr., and he found that the expansion of the ball exerted a force equal to 4000 atmospheres--_i.e._ 4000 15--on every square inch of surface, or a pressure equal to thirty millions of pounds; the entry of only 180 of heat into the iron sphere produced this remarkable result, just as Faraday has calculated that a single drop of water contains a sufficient quant.i.ty of electricity to produce a result equal to the most powerful flash of lightning, provided the electricity of quant.i.ty in the drop of water is converted into electricity of high tension or intensity.

The practical applications of this well-known property of solids with respect to heat are very numerous; thus, the iron bullet-moulds are always made a little larger than the requisite size, in order to allow for the expansion of the hot liquid lead, and the contraction of the cold metal. The tires of wheels and the hoops of casks are usually placed on whilst hot, in order that the subsequent contraction may bind the spokes [Pg 357] and fellies, or the staves, closely together. If an allowance was not made for the expansion and contraction of the iron rails on the permanent ways of railroads, the regularity of the level would be constantly destroyed, and the position of the rails, chairs, and sleepers would be most seriously deranged; indeed it is calculated that the railway bars between London and Manchester are five hundred feet longer in the summer than in the winter.

The walls of the Cathedral of Armagh, as also those of the Conservatoire des Art et Metiers, were brought back to a nearly perpendicular position, by the insertion (through the opposite walls) of great bars of iron, which being alternately heated, expanded, and screwed up tight, then cooled and contracted, gradually corrected the bulging out of the walls or main supports of these buildings. The principle of these famous practical experiments is neatly ill.u.s.trated by means of an iron framework with a bar of iron placed through both its uprights, and screwed tight when hot; on cooling, contraction occurs, which is shown by a simple index. (Fig. 345.)

[Ill.u.s.tration: Fig. 345. The iron frame, with C C, wrought-iron bar heated by putting on the semicircular piece of iron E E, which is first made red-hot, and as the heat is communicated to the wrought iron rod C C, it is screwed up tight by the nut K. G G. The index attached to the iron frame screwed up when hot; the arms come together at P, and separate further to H H as the contraction takes place by cooling the bar C D.]

It has often been remarked that there is no rule without an exception, and this applies in a particular instance to the law that "bodies expand by heat and contract by cold"--viz., in the case of Rose's fusible metal, which consists of

Two parts by weight of bis.m.u.th, One part " lead, One part " tin.

To make the alloy properly, the lead is first melted in an iron ladle, and to this are added first the tin, and secondly the bis.m.u.th; the whole is then well stirred with a wooden rod, and cast into the shape of a bar.

[Page 358]

When placed in the pyrometer and heated, the bar expands progressively till it reaches a temperature of 111 Fahr.; it then begins to _contract_, and is rapidly _shortened_, until it arrives at 156 Fahr., when it attains a maximum density, and occupies no more s.p.a.ce than it would do at the freezing-point of water. The bar, after pa.s.sing 156, again expands, and finally melts at about 201, which is 11 below the boiling-point of water. Fusible metal is sometimes made into teaspoons, which soften and melt down when stirred in a cup of hot tea or basin of soup, to the great surprise and bewilderment of the victim of the practical joke.

Unequal expansion is familiarly demonstrated with a bit of toasted bread, which curls up in consequence of the surface exposed to the fire contracting more rapidly than the other; and the same fact is ill.u.s.trated with compound flat and thin bars of iron and bra.s.s, which are fixed and rivetted together; when heated, the compound bar curves, because the iron does not expand so rapidly as the bra.s.s, and of course forms the interior of the curve, whilst the bra.s.s is on the exterior.

The experiment with the compound bar is made more conclusive and interesting by arranging it with a voltaic battery and platinum lamp.

One of the wires from the battery is connected with the extremity of the compound bar, and as long as it remains cold, no curve or arch is produced, but when heat is applied, the bar curves upwards, and touching the other wire of the battery, the circuit is completed, and the platinum lamp is immediately ignited. (Fig. 346.)

[Ill.u.s.tration: Fig. 346. A B. Compound bar resting on two blocks of wood. The end A is connected with one of the wires from the battery. The circuit is completed and the platinum lamp D ignited directly the bar curves _upwards_ by the heat of the spirit lamp, and touches the wire C C connected with the opposite pole of the battery.]

The expansion and contraction of liquids by heat and cold is also another elementary truth which admits of ample ill.u.s.tration, and indeed introduces us to that most useful instrument called the thermometer.

If a flask is fitted with a cork through which a long gla.s.s tube, open [Page 359] at both ends, is pa.s.sed, and then carefully filled with water coloured with a little solution of indigo, so that when the cork and tube are placed in the neck, all the air is excluded, a rough thermometer is thus constructed, which, if placed in boiling water, quickly indicates the increased temperature by the rising or expansion of the coloured water inside the flask. (Fig. 347.)

[Ill.u.s.tration: Fig. 347. Expansion of liquids shown at A by the coloured water rising in the tube from the flask, which is quite full of liquid, and heated by boiling water. B. The expansion of the water heated by the spirit-lamp is shown by the rising of the piston and rod C C. D represents a retort filled up like A to show the expansion of a liquid by heat.]

The thermometer embraces precisely the same principle as that already described in Fig. 347, with this difference only, that the tube is of a much finer bore, and the liquid employed, whether alcohol or mercury, is boiled and hermetically sealed in the tube, so that the air is entirely excluded. To make a thermometer, a tube with a capillary bore is selected of the proper length; it is then dipped into a gla.s.s containing mercury, so that the tube is filled to the length of half an inch with that metal. The half-inch is carefully measured on a scale, and the place the mercury fills in the tube marked with a scratching diamond; the mercury is then shaken half an inch higher, and again marked, and this proceeding is continued until the whole tube is divided into half inches. The object of doing this is to correct any inequalities [Page 360]in the diameter of the bore of the gla.s.s tube, because if wider at one part than another, the s.p.a.ces filled with the mercury are not equal; as the bore is usually conical, the careful measurement of the tube with the half inch of mercury in the first place gives the operator at once a view of the interior of his tube, and enables him to graduate it correctly afterwards. (Fig. 348.)

[Ill.u.s.tration: Fig. 348. A B. Magnified view of the bore of one of the thermometer tubes which are made by rapidly drawing out a hollow ma.s.s of hot gla.s.s whilst soft and ductile, consequently the bore must be conical, and larger at one end than the other.]

The next step is to heat one extremity by the lamp and blowpipe, and whilst hot, to blow out a ball upon it; if this operation were performed with the mouth, moisture from the breath would deposit inside the fine bore of the gla.s.s tube, and injure the perfection of the thermometer afterwards. In order to prevent any deposit of water, the bulb is blown out, whilst red-hot, with the air from a small caoutchouc bag fitted on to the other extremity of the tube. The operator now marks off the intended length of his thermometer, and above that point the tube is again softened with the flame and blowpipe, and a second bulb blown out.

(Fig. 349 _a_.)

[Ill.u.s.tration: Fig. 349 _a_.--No. 1. First bulb. The intended length of the thermometer is shown at the little cross.--No. 2 is the second bulb placed above the cross.]

The open end of the tube is now placed under the surface of some pure, clean, dry quicksilver, and heat being applied to the upper bulb, the air expands and escapes through the mercury, and as the tube cools a vacuum is produced, into which the mercury pa.s.ses. By this simple method, the mercury is easily forced into the tube, as otherwise it would be impossible to _pour_ the quicksilver into the capillary bore of the intended thermometer. (Fig. 349 _b_.)

[Ill.u.s.tration: Fig. 349 _b_. Heating and expanding the air in the top bulb, so that when cool the mercury in the gla.s.s A, may rise into the tube and fill the bulb B.]