The tube is now taken from the gla.s.s containing the mercury, and simply inverted; but in consequence of the very narrow diameter of the bore the air will not pa.s.s out of the first bulb until heat is applied, when the air expands, and the [Page 361] mercury, first stationary in the second bulb, will now displace the air, and fall into the first bulb when the tube is again cool.

The ball, No. 1 (Fig. 349 _a_), is now full of mercury, and there is also some left in No. 2; in the next place, the tube is supported by a wire, and held over a charcoal fire, when it is heated throughout its entire length, and the mercury being boiled expels the _whole of the air_, so that there is nothing inside the bulbs and capillary bore but mercury and its vapour. (No. 1, Fig. 350.) The open end of the intended thermometer is now temporarily closed with sealing-wax, and the whole allowed again to cool with the sealed end uppermost, so that the ball No. 2, Fig. 350, and the tube above it, are quite filled with quicksilver.

After cooling, the tube is placed at an angle with the sealed end uppermost, and, guided by experience, the operator heats the lower bulb so as to expand enough mercury into the upper one to leave s.p.a.ce for the future expansion and contraction of the mercury in the tube, which has now to be hermetically sealed. This is done by dexterously heating the tube at the cross whilst the mercury in the first bulb is still expanded; and by drawing it out rapidly with the help of the heat obtained from the lamp and blowpipe, the second bulb is separated from the first at the little cross (B, No. 3, Fig. 350), and the thermometer tube at last properly filled with quicksilver, and hermetically closed.

(No. 4, Fig. 350.)

[Ill.u.s.tration: Fig. 350.--No. 1. Boiling quicksilver in the tube with two bulbs.--No. 2. Tube cooled, with the sealed end uppermost.--No. 3.

Mercury in first bulb expanded by lamp A, and at the proper moment hermetically sealed by the flame urged by the blowpipe at B. The upper bulb and tube to the cross being drawn away and separated.--No. 4.

Thermometer tube containing the requisite quant.i.ty of mercury, hermetically sealed, and now ready for graduation.]

[Page 362]

In order to procure a fixed starting-point, the thermometer tube is placed in ice, with a scale attached; the temperature of ice never varies, it is always at 32 degrees. When, therefore, the mercury has sunk to the lowest point it can do by exposure to this degree of cold, the place is marked off in the scale, and represents that position in the graduated scale where the freezing point of water is indicated.

The tube is placed in the next place in a vessel of boiling water, care being taken that the whole tube is subject to the heat of the water and the steam issuing from it, and when the mercury has risen to the highest position attainable by the heat of boiling water, another graduation is made which indicates 212 degrees--viz., the boiling point of water. This graduation should be made when the barometer stands at 30 inches, because the boiling point of water varies according to the weight of the superinc.u.mbent air pressing upon it.

Between the graduation of the freezing and the boiling point of water the s.p.a.ce is divided into 180 parts, which added to 32 make up the boiling point of water to 212 degrees, being the graduation of Fahrenheit, who was an instrument-maker of Hamburg. Why he divided the s.p.a.ce between the freezing and boiling point of water n.o.body appears to know, unless he took a half circle of 180 degrees as the best division of s.p.a.ce. If the thermometer contains air the mercury divides itself frequently into two or three slender threads, each separated from the other in the capillary bore, and thus the instrument is rendered useless until the threads again coalesce. If the thermometer has been well made, and is quite free from air, it may be tied to a string and swung violently round, when the centrifugal force drives the slender threads of mercury to their common source--viz., the bulb containing the quicksilver, and the whole is again united. The string must be attached, of course, to the top of the thermometer scale.

When travelling on the Continent it is sometimes desirable to be able to read the thermometers which are graduated in a different manner to that of Fahrenheit. In France the Centigrade scale is preferred, and in many parts of Germany Reaumur's graduation. The difference of the graduation is seen at a glance.

In the Centigrade the freezing point is 0, the boiling point 100.

" Reaumur " 0, " 80.

" Fahrenheit " 32, " 212.

The number of degrees, therefore, between boiling and freezing is 100 in the Centigrade, 80 in Reaumur, and (212-32, that is) 180 in Fahrenheit.

If, then, the letters C, R, F, be taken to denote the _number_ of degrees from the freezing point at which the mercury stands in the Centigrade, Reaumur, and Fahrenheit thermometers, we have the following proportions:--

(1.) 100: 80 :: C: R, whence C = 5/4 of R, or R = 4/5 of C.

(2.) 180:100 :: F: C, whence F = 9/5 of C, or C = 5/9 of F.

(3.) 180: 80 :: F: R, whence F = 9/4 of R, or R = 4/9 of F.

[Page 363]

The following examples will show how to apply these formulae:--

(1).--Suppose the Reaumur stands at 28, at what height does the Centigrade stand? We have C = 5/4 of R (in this case), 5/4 of 28 = 35: that is, the Centigrade stands at 35.

(2).--Suppose Fahrenheit to stand at 41, what will Reaumur stand at? R = 4/9 of (41-32) (that is, the number above freezing in Fahr.) = 4/9 of 9 = 4. Reaumur stands at 4.

(3).--Suppose Fahrenheit stands at 23, what will the Centigrade stand at? C = 5/9 of F = 5/9 of (32-23) = 5/9 of 9 = 5 below freezing (or-5).

(4).--If Fahrenheit stands at 4 below 0, what will Reaumur indicate? R = 4/9 of F = 4/9 of (32 + 4) = 4/9 of 36 = 16 below 0 (or-16).

The only liquid which has the exceptional property of expanding by cold is water, and it will be seen presently that this curious anomaly is of the greatest importance in the economy of nature.

If a box containing a mixture of ice and salt is placed round the top of a long cylindrical gla.s.s containing water at a temperature of 60 Fahr., the intense cold of the freezing mixture, which is zero--that is to say, 32 below the freezing point of water--very soon reduces the temperature of the water contained in the gla.s.s, and as it becomes colder it contracts, is rendered heavier, and sinks to the bottom of the vessel, and its place is taken by other and warmer water. This circulation commencing downwards, proceeds till the water has attained a temperature of about 40 Fahr., when the maximum density is obtained and the circulation stops, because after sinking below 40 the cold water becomes lighter, and continues to be so until it freezes, and of course, being of a less specific gravity than the warmer water, it floats (like oil on water) upon its surface; so that a small thermometer placed at the bottom of the jar indicates only 40 Fahr., whilst the solid ice enveloping the other or second thermometer placed at the top may be as low as 29, or even lower, according to the quant.i.ty of ice and salt used in the box surrounding the top of the gla.s.s. (Fig. 351.)

[Ill.u.s.tration: Fig. 351. A B. Long cylindrical gla.s.s containing water and two thermometers; the one at the bottom shows a temperature of 40; the other at the top 32, or even lower, C C C C. Section of box containing the ice and salt, and standing on four legs, two of which are shown at D D.]

[Page 364]

The importance of this curious anomaly cannot be overrated. If water did not possess this rare property, all the seas, rivers, ca.n.a.ls, lakes, &c., would _gradually_ become impa.s.sable from the presence of enormous blocks of ice formed during the winter. The whole bulk of water contained in them would have to sink below 32 before it could solidify provided water increased in density or continued to contract by cold.

Having once solidified, the warmth of the rays from a summer's sun would certainly melt a great deal of the ice, but not the whole, and winter would come again before the solid ma.s.ses had disappeared. The ocean could not be navigated in safety even near our own sh.o.r.es, in consequence of the vast icebergs that would be formed, and float about and jostle each other even in the British Channel.

The earth has been wonderfully prepared for G.o.d's highest work--Man, and in nothing is this supreme wisdom more apparent than in the fact that water offers the only known exception to the law "that bodies expand by heat and contract by cold."

The expansion of gases by heat and contraction by cold take place in obedience to a law to which there is no exception, except in degree. It was discovered in 1801 by M. Gay Lussac, of Paris, and also about the same period by the famous English philosopher who established the atomic theory--viz., by Dr. Dalton. Since these experiments and calculations Rudberg, Magnus, and Regnault have made other researches, and their successive experiments give the following results:--

Vols. of air. Volumes.

Dalton, Gay Lussac 1000 heated from 32 to 212 became 1375 Rudberg 1000 " " " 1365 Magnus, Regnault 1000 " " " 1366.5

As a natural result, air at 32 Fahr, expands 1/491 part of its volume for every degree of heat on the scale of Fahrenheit; and a volume of air which measures 491 cubic inches at 32 will measure 492 at 33, 493 at 34, and so on. The exception is only in degree, and Magnus and Regnault discovered by their searching experiments that the gases easily liquified are more expansible by heat than air and those gases (such as oxygen, hydrogen, and nitrogen) which have never been liquified.

The expansion of air is easily shown by placing the open end of a tube with a large bulb blown at the other extremity, under the surface of a little coloured water; on the application of heat the air expands and escapes, and its place is taken, when cool, by the coloured liquid. Such an arrangement represents the first thermometer constructed by Sanctorio about A.D. 1600, which might certainly answer for rough purposes, but as the ascent and descent of the fluid depend on the bulk of air contained in the bulb, and as this is affected by every change of the height of the barometer, no satisfactory indication of an increase or decrease of temperature could be obtained with it, although the instrument itself is interesting in an historical point of view, and in a [Page 365]

modified form as an air thermometer has been employed by Sir John Leslie, under the name of the "Differential Thermometer," in his refined and delicate experiments with heat.

[Ill.u.s.tration: Fig. 352 A. Sanctorio's original air thermometer; the expansion and contraction of the air in the bulb indicate the rise or fall of the temperature. The cork is merely a support, and is not fitted into the bottle air-tight. B C. The differential thermometer. When both bulbs are subjected to a uniform temperature, no movement of the fluid shown at D occurs; but if the bulb B is put into any place warmer than the position of the bulb C, then the air expands in B, and drives the coloured liquid, which consists of carmine dissolved in oil of vitriol, up the scale attached to the stem of the bulb C.]

Fire balloons are a good example of the expansion of gases, and the levity of the air thus increases in bulk was taken advantage of by Montgolfier in the construction of his famous balloon, which, with a cage containing various animals, ascended, in the presence of the King and royal family of France, at Versailles; and in spite of huge rents in two places, it rose to a height of 1440 feet, and after remaining in the air for eight minutes, fell to the ground at the distance of 10,200 feet from the place whence it started, without injury to the animals. When it is considered that a volume of air heated from 32 to 491 is doubled, and tripled when heated to 982, it will at once be understood how great must be the ascending power of such balloons, provided the air within them is kept sufficiently hot.

That gallant aeronaut, Pilate de Rozier, offered himself to be the first aerial navigator; and having joined Montgolfier, they made three successful ascents and descents with a large oval-shaped balloon, forty-eight feet in diameter, and seventy-four feet high. On the fourth occasion he ascended to a height of 262 feet, but in the descent a gust of wind having blown the machine over some large trees of an adjoining garden, the situation of the brave aeronaut was extremely dangerous, and if he had not possessed the strongest presence of mind, and at once [Page 366] given the balloon a greater ascending power, by rapidly supplying his stove with some straw and chipped wood, he might on this occasion have met with that untimely end which subsequently, in another rash aeronautic adventure, befell this brave but foolhardy Frenchman.

On descending again, he once more, and without the slightest fear, raised himself to a considerable height by feeding his fire with chopped straw. Some time after he ascended, in company with M. Giroud de Vilette, to the height of 330 feet, hovering over Paris at least nine minutes, in sight of all the inhabitants, and the machine keeping all the while perfectly steady.

The danger in using this method of inflating the balloon arises from the possibility of generating gas, which escaping unburnt into the body of the balloon, may acc.u.mulate and blow up, or burn afterwards.

[Ill.u.s.tration: Fig. 353. A B. Wessel's gas stove, with ring of gas jets lighted inside; the air rushes in the direction of the arrows, C C, and escaping at the top of the chimney, D D, soon fills the air or fire balloon, which is usually made of paper.]

Fire balloons, as usually made, are very dangerous toys, and may sometimes prove rather costly to the person who may send them off, in consequence of their being blown by the wind on a hay or corn rick, or other combustible substances. The safest mode of using fire balloons is to fill them with hot air from a lighted gas stove (Wessel's, for instance); the balloons may then be used in large rooms, or out in the air, without fear of doing any harm to neighbouring property, as of course the stove and the fire remain behind, and will fill any number of air balloons. (Fig. 353.)

After all the fuss made about the novelty of the American hot-air engine, it is somewhat amusing to look back to the records of civil engineering, and in the "Transactions of the Inst.i.tution of Civil Engineers," to read Mr. James Stirling's account of his improved air engine, in which the great expansion of air mentioned at p. 365 has been successfully applied. The engine was constructed about the year [Page 367] 1843, and the principle, discovered thirty years before by Mr. R.

Stirling, will be comprehended by reference to the cut. (Fig. 354.)

[Ill.u.s.tration: Fig. 354. Stirling's air engine.]

Two strong air-tight vessels are connected with the opposite ends of a cylinder, in which a piston works in the usual manner. About four-fifths of the interior s.p.a.ce in these vessels is occupied by two similar air-tight vessels or plungers, which are suspended to the opposite extremities of a beam, and capable of being alternately moved up and down to the extent of the remaining fifth. By the motion of these interior vessels, which are filled with non-conducting substances, the air to be operated upon is moved from one end of the exterior vessel to the other, and as one end is kept at a high temperature, and the other as cold as possible, when the air is brought to the hot end it becomes heated, and has its pressure increased; and when it is brought to the cold end, its heat and pressure are diminished. Now, as the interior vessels necessarily move in opposite directions, it follows that the pressure of the enclosed air in the one vessel is increased, while that of the other is diminished. A difference of pressure is thus produced upon the opposite sides of the piston, which is thereby made to move from the one end of the cylinder to the other, and by continually reversing the motion of the suspended bodies or plungers, the greater pressure is successively thrown upon a different side, and a reciprocating motion of [Page 368] the piston is kept up. The piston is connected with a fly-wheel in any of the usual modes; and the plungers, by whose motion the air is heated and cooled, are moved in the same manner, and nearly at the same relative time, with the valves of a steam engine.

The pressure is greatly increased and made more economical by using somewhat highly-compressed air, which is at first introduced, and is afterwards maintained, by the continued action of an air-pump. The pump is also employed in filling a separate magazine with compressed air, from which the engine can be at once charged to the working pressure.

Mr. Stirling's chief improvement consists _in saving all or nearly all the heat of the expanded air after it has done its work_, by pa.s.sing it from the hot to the cold end of the air vessel through a mult.i.tude of narrow pa.s.sages, whose temperature is at the beginning of the tubes nearly as great as that of the hot air, but gradually declines till it becomes nearly as low as the coldest part of the air vessel. The heat is therefore retained by these pa.s.sages, so that when the mechanism is reversed, the cold air returns again through these hot pipes, and is thus made nearly hot enough by the time it reaches the heating vessel to do its work. Thus, instead of being obliged to supply at every stroke of the engine as much heat as would be sufficient to raise the air from its lowest to its highest temperature, it is necessary to furnish only as much as will heat it the same number of degrees by which the hottest part of the air vessel exceeds the hottest part of the intermediate pa.s.sages. This portion of the engine may be called the _economical process_, and represents the foundation of all the success to which it has attained in producing power with a small expenditure of fuel. No boiler being required, of course the danger of explosions is much lessened. The higher the pressure under which the engine was worked the greater was the effect produced. A small engine on this principle was worked to a pressure of 360 pounds on the square inch; and perhaps the best popular notion of the novelty in the arrangement is that suggested by Mr. George Lowe, who compared the economical part of the machine to a "Jeffrey's Respirator" used by consumptive patients. The heat from the air _expired_ being retained by the laminae, and again used when cold air is inspired or drawn into the lungs. Mr. Stirling states that the consumption of fuel as compared to the steam engine which the air engine had replaced was as 6 to 26; the same amount of work being now performed by about six cwt. of coals which had formerly required about twenty-six cwt., though he ought to have stated that the steam engine removed was not of the best construction, nor had the boiler any close covering.

(Fig. 354.)

_Conduction of Heat._

This property of heat with reference to matter, and the consideration of the curious manner in which it creeps, as it were, through solid substances, brings the thoughtful mind at once to the bold question of What is heat? Is it to be regarded as something real or material? or [Page 369] must it be considered only as a property or state of matter?

These questions are not to be solved easily, and they demand a considerable amount of experiment and reasoning even to appreciate their meaning.