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ON PHYSICS, OR NATURAL PHILOSOPHY.

No. XXXIV.

(Continued from page 95.)

EXPANSION OF LIQUIDS.

Apparent and Absolute Expansion.-In liquids, it is only cubic expansion which is to be considered; and of this there are two kinds, apparent and absolute expansion. Apparent expansion is the increase in volume which a heated liquid assumes when it is contained in a vessel which expands less than the liquid does under the same degree of heat; as the expansion of mercury or alcohol in thermometers. The absolute expansion is the real expansion which the volume of the liquid alone undergoes. The apparent expansion of a liquid is less than the absolute expansion by the amount of the expansion of the vessel in which it is contained. The expansion of the vessel is rendered evident in the case of the thermometer, by immersing it in boiling water, provided the bulb is large and filled with coloured spirits of wine up to the half of the stem. At the instant when the bulb enters the hot water, the spirits of wine sink in the tube, which evidently arises from the expansion of the sides of the bulb; but if the latter is kept immersed, the spirits of wine become heated, and rise in the tube by a quantity equal to its absolute expansion, diminished by that of the vessel.

As in the case of solids, the increase in a unit of the volume of a body when its temperature is raised from 0° to 1° Centigrade, is called the co-efficient of expansion; but then a distinction is to be made between the co-efficient of the apparent expansion and the co-efficient of absolute expansion. Various methods have been employed in order to determine these two co-efficients of expansion. The following is that employed by MM. Dulong and Petit.

Co-efficient of the Absolute Expansion of Mercury.-In order to determine the co-efficient of the absolute expansion of mercury, the influence of the expansion of the vessel containing it must be avoided. MM. Dulong and Petit effected this by the application of the hydrostatic principle, that in two communicating vessels the heights of two liquids are in the inverse ratio of their densities; a principle which is independent of the diameters of the vessels, and consequently of their expansion. Their apparatus was composed of two glass tubes A and B, fig. 181, supported in a vertical position and connected by a

d

I'-h ht

Now let hand d be the height and the density of the mercury in the branch A, at the temperature of 0° Centigrade; and h' and d' the same quantities in the branch B, at the temperature ; then, according to the hydrostatic principle just referred to, we have h' d' hd. But d'= = But d't by our former lesson, k being the co-efficient of the absolute expansion of mercury; therefore replacing d' by its value just referred kd to, we have 1+kt hd; whence, we find that = This formula shows how to find the co-efficient of the absolute expansion of mercury, when the heights h and h' of this liquid in the two tubes have been measured, and the temperature t of the bath in which the tube is immersed, are given. In the experiment of Dulong and Petit, this temperature was measured by a thermometer and capsule, as explained in the next paragraph. As to the heights h' and h, they were measured by means of a cathetometer (see fig. 18, vol. iv., p. 100). By this process, these experimenters found that the co-efficient of the absolute expansion of mercury, between 0° and 100° Centigrade, was 550. But they observed that this co-efficient increased with the temperature; for between 100° and 200° Centigrade the mean co-efficient was ; and between 200° and 300 Centigrade it was 300. The same phenomenon was observed in other liquids; thus we see that these bodies do not expand regularly. It was found that their expansion was more irregular the nearer that their temperates pproached those of congelation and ebullition. As to mercury, the experimenters found that its expansion was very sensibiy regular between -36° and 100° Centigrade.

Fig. 182.

Co-efficient of the Apparent Expansion of Mercury.-The coefficient of the apparent expansion of a liquid varies with the nature of the vessel which contains it. That of mercury, in a glass vessel, was determined by MM. Dulong and Petit by means of the apparatus represented in fig. 182.

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It is composed of a cylindric glass reservoir or bulb, to which is cemented a capillary glass tube hent at a right angle and open at its extremity. In order to make the experiment, the instrument is weighed when empty, and also when filled with mercury at 0° Centigrade; the difference between the two weights gives the weight P of the mercury cont. ined in the apparatus. Raising it, then, to a temperature denoted by t, the mercury expands, and a certain quantity of it comes cut, which is received in a small capsule, and weighed. If the weight of the mercury which has come out be represented by P, that of the mercury which remains in the apparatus is denoted by P-p. Now, let v' be the volume of the mercury at 0°Centigrade, whose weight is P, and the volume, also at 0° 112

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sins in the apparatus, and of these two quantities, mperature, are therefore proportional But is A ve hare

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tion. Several processes have been employed in order to deter mine the temperature of the maximum density of water. M Hällstrom, by weighing, in water at different temperatures, glass ball ballasted with sand, found, taking into consideratio the expansion of the glass, that it was in water of the tempera ture of 4°1 Centigrade or 39° 38 Fahrenheit, that the ball los with rises, when heated from the most of its weight; whence he concluded that at this tem Legos 14 therefore, the co-effi-perature the maximum density of water takes place. MM tyson of mercury by d, we Munke and Stampfer have stated the temperature of th maximum density of water, at 3°.75 Centigrade, or 38.7 1 leng te mrease which the unit Fahrenheit, according to their experiments. But M. Despret ne man passing from 0 to to has ascertained, by numerous experiments, that the tempera ture of the maximum density of water is really that of Centigrade, as above stated. By gradually cooling a water thermometer (that is, a thermometer with water instead mercury) in a bath whose temperature was given by a me curial thermometer, he found that it was at 4° Centigrade th the maximum contraction of the water in the water-therm meter took place.

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of expansion varies at the different forms im not pures tubes used in Chemistry, Lot 18 0 0000254. By beaten te expansion of mercury we omat trus officient for 1° Fahend by multiplying by 4, kumur; thus, 350 X

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Chlorine as a Supporter of Combustion.-In determin whether any medium be or be not a supporter of combust treating of the it is only natural that we should begin with the example to render combustible in the ordinary sense of the term. seasons of therefore, take a candle or small taper mounted upon a sary always wire, and furnished with disc and cork, as frequently to a constant cribed already, and as represented in our subjoined diagn

that of the zig point of fig. 36.

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after a fashion, but a strange sort of burning it is; only a small | denly extinguished, and the copper support will vehemently pale powerless flame appearing, whilst copious black fumes burn. are liberated on all sides. Now, why is this? Charcoal is a very combustible material under all ordinary circumstances: it is indeed the combustible par excellence of man; wherefore, then, does it not burn in our present experiment? The young chemist will perhaps jump to the conclusion, that chlorine, although a supporter of combustion, is a very bad supporter. Conclusions, however, can seldom be justly arrived at from the consideration of one isolated experiment; let us therefore try a

few others.

Exp. Dry a small piece of phosphorus, not larger than a pea in size; place it in a copper deflagrating ladle properly mounted, and immerse it in a jar of chlorine. These directions having been followed, the phosphorus will spontaneously burst into flame, but the flame will not possess any great illuminating power.

How shall we now designate chlorine in relation to combustion, after having witnessed the evidence of our last experiment? We surely must not term it a bad supporter of combustion, seeing that it will accomplish a result quite out of the power of even oxygen gas to accomplish; namely, it will cause the spontaneous ignition of phosphorus.

Exp. Procure some Dutch leaf, and hanging a few pieces of this on a properly-mounted hooked wire, as represented in fig. 37,

Fig. 37.

The functions of chlorine, as developed by our preceding experiments, considerably enlarge the sphere of our notions relative to combustion, and we arrive at the remarkable conclusion that, if in place of our own atmosphere of oxygen and nitrogen, we were surrounded with an atmosphere of chlorine, carbon, including charcoal, coke, and coal, would be absolutely incombustible, and carbon holding materials such as oils, fats, coal-gas, etc., would only be combustible to the extent of

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plunge it into a jar of chlorine gas. The metallic leaf will generally take fire, again proving that under certain circumstances, and for certain bodies, chlorine is a remarkably good supporter of combustion..

Erp. Having procured some powdered antimony-real metallic antimony, not the sulphuret-throw a little of the powder into a jar of chlorine gas. Immediately the antimony will take fire. In all these cases, be careful to confine as much as possible the results of combustion, as they are very injurious when taken into the lungs. They may be confined by dexterously sliding over the mouth of the bottle or jar a greased glass plate.

Exp. Our next experiment shall have reference to the curious phenomenon already noticed in the instance of the burning taper, of the development and evolution of carbonaceous fumes, but it shall manifest that phenomenon in a still higher degree. Dip a slip of bibulous or absorptive paper into oil of turpentine, then plunge it into a bottle of chlorine, and close the bottle by means of a greased glass plate. Most probably, under these circumstances, the paper will take fire, and the peculiar black carbonaceous fumes will be still more evident than before when we employed a taper. But a still more expressive demonstration of the incombustibility of carbon in chlorine gas will be afforded by the ensuing experiment, for the performance of which we made preparations in our last

lesson.

Exp. Take the piece of charcoal, properly mounted on copper wire as described in that lesson, and light the charcoal thoroughly, by directing against it the point of a blowpipeflame, fig. 38.

When thus thoroughly ignited, plunge the whole into a bottle containing chlorine gas. A very curious result will follow, although one in strict accordance with previous deductions. The charcoal, though fully ignited, will become sud

their contained hydrogen. Combustion, in point of fact, is one consequence and direct evidence of intense chemical action. Without chemical action resulting in combination there can be no combustion; and inasmuch as chemical union between chlorine and carbon does not ensue, for this reason, charcoal and carbon generally are not able to burn in an atmosphere of chlorine gas. There is, however, a combination of chlorine and hydrogen, the result being hydrochloric acid gas-the gas which, when absorbed by water, constitutes the well-known muriatic acid, or spirit of salt. Hence it follows, that when we ignite a material holding both carbon and nitrogen, such for example as a taper, or oil of turpentine, and dip either thus ignited into a jar of chlorine, hydrogen is alone consumed and carbon deposited.

It may be well in this place to say a few words relative to an insufficient definition formerly offered, and during many years retained, of the function of combustion, which was defined to be a rapid combination of bodies with oxygen. If this definition be accepted, then it follows that the experiments we have just performed, and the phenomena we have just witnessed, are not tho-e of combustion; indeed certain systematic writers have excluded them from the category, seeing they did not fall under the definition. Such an expulsion, however, is unphilosophical. No definition should do violence to a natural and well-established idea. Surely, then, violent chemical action, attended with the evolution of light and heat, ought to be regarded as an instance of combustion.

The definition of combustion as the rapid chemical union of a body with oxygen originated in a remarkable period, of which it bears the stamp. It originated during the early part of the first or great French revolution: a period when the genius of innovation was active, and human intellect unbridled in its pride. Then it was that the chemical nomenclature of Lavoisier appeared, a nomenclature sufficiently expressive of the chemical facts then known, but totally incapable of expansion. In order, indeed, that a system of nomenclature in any science should be so, either all its facts should be well-known, or should be deducible from such simple elements, as in Geometry,

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Co-efficient of the Expansion of Glass.-The sion of a liquid being equ d to its apparent expansion of the vessel which contains it, w expansion of glass in the preceding experi difference between the co-efficient of the of mercury and that of its apparent expar have the co-efficient of the cubic expansi 0.00002586; for 30 · 0100 = JATT = Regnault has found that the co-efficien with different kinds of glass, and even wi of the vessels. For the common glass tub he found that the co-efficient of expans multiplying the co-efficient of the absolute for 19 Centigrade by, we obtain this c renheit; thus X ; and we obtain this co-efficient for 1° Rea =140.

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The co-efficient of the absolute expar can be found in the game manner a determined; and thence by adding to expansion of glass, the co-efficient of t can also be found.

Correction of the Height of the Baro. barometer, p. 258, col. 2, we remarke its indications in different places an the year comparable with each oth to refer the height of the column or fixed temperature, such as that water. This correction is made in t the height of the barometer at t° C and its height at 0° Centigrade b mercury being compared to a m heated from 0° to to Centigrade t ht -; whence h 5550

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icient of the absolute expans tficient of its apparent expa value of H is the same as if .ght of the mercury in the ba Je diameter of the tube, and ec Maximum Density of Water.phenomer on, that when its te contracts as far as 4° Centigra this point, although the loweri not only does the contraction even to the freezing point, w Centigrade or 32° Fahrenhe 399-2 Fahrenheit, water expe

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