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of cubical expansion of glass, and x the expansion of unit volume of the given metal, we have the equation

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214. Convection of Heat in Liquids. — When different parts of a liquid are heated to different temperatures, corresponding differences of density arise, leading usually to the formation of currents which tend to produce equality of temperature as far as their presence extends. To this phenomenon, which is altogether distinct from conduction, the name of convection is given.

Thus, for instance, if we apply heat to the bottom of a vessel containing water, the parts immediately subjected to the action of the heat expand and rise to the surface; they are replaced by colder layers, which in their turn are heated and ascend; and thus the process continues indefinitely. The two currents can be very well shown by throwing some oak saw-dust into the water. By the movements of this substance, which has nearly the same density as water, it will be seen that the ascending current occupies the centre of the vessel, while the descending current passes along the sides.

215. Heating of Buildings by Hot Water.-This is a simple application of the principle just stated. One of the most common arrangements for this purpose is shown in Fig. 215; it is called the highpressure system, because the water in the boiler can acquire a temperature considerably above 100° C. The boiler C is heated by a fire below it, and the products of combustion escape through the chimney AB. At the top of the house is a reservoir D, communicating with the boiler by a tube. From this reservoir the liquid flows into another reservoir E in the story immediately below, thence into another reservoir F, and so on. Finally, the last of these reservoirs communicates with the bottom of the boiler. The boiler, tubes, and reservoirs are all completely filled with water, with the exception of a small space left above in order to give room for the expansion of the liquid. An ascending current flows through the left-hand tube, and the circulation continues with remarkable regularity, so long as the temperature of the water in the boiler remains constant.

216. Currents in the Sea. In the production of these currents convection plays an important, though perhaps not the principal part. In fact, the sea is an enormous mass of liquid whose temperature varies from point to point. Equilibrium is consequently impossible,

CURRENTS IN THE SEA.

285

and the different parts must therefore be in a state of continual motion with regard to each other. The waters of the tropical seas should, by reason of

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their excess of tempera

ture, have a higher level

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than those of the polar seas; and the result is, a continual kind kind of overflowing of the waters about the equator, and consequently a vast current setting towards the poles. But to this current evidently corresponds a lower current of cold water flowing towards the equator, there to become heated, to overflow again, and so on. One of the most remarkable of oceanic currents is that which is known as the Gulf Stream. This current of warm water forms a kind of immense river in the midst of the sea, differing in the tem perature, saltness, and colour of its waters from the medium in which it flows. Its origin is in the Gulf of Mexico, whence it issues through the straits between the Bahamas and Florida, turns to the northwest, and splits into two branches, one of which goes to warm the coasts of Ireland and Norway, the other gradually turns southwards, traverses the Atlantic from north to south, and finally loses itself in the regions of the equator.

Fig. 215.-Heating by Hot Water.

"The Gulf Stream is a river in the ocean; in the severest droughts it never fails, and in the mightiest floods it never overflows; its banks and its bottom are of cold water, while its current is of warm; it takes its rise in the Gulf of Mexico, and empties into Arctic seas. There is on earth no other such majestic flow of waters. Its current

is more rapid than the Mississippi or the Amazon, and its volume more than a thousand times greater. Its waters, as far out from the Gulf as the Carolina coasts, are of indigo blue. They are so distinctly marked that their line of junction with the common sea-water may be traced by the eye. Often one-half of the vessel may be perceived floating in Gulf Stream water, while the other half is in common water of the sea, so sharp is the line."—(Maury, Physical Geography of the Sea.)

Another cause of oceanic currents is to be found in the winds, which again are themselves examples of convective currents in the atmosphere. In the case of the Gulf Stream, it would appear that an accumulation of water is produced in the Gulf of Mexico by the trade-wind which blows steadily towards it over the South Atlantic. The elevation of level occasioned by this accumulation is probably to be regarded as the principal cause of the Gulf Stream. We shall discuss the origin of winds in a later chapter (Chap. xxxiv.)

CHAPTER XXIII.

EXPANSION OF GASES.

217. Experiments of Gay-Lussac.-Gay-Lussac conducted a series of researches on the expansion of gases, the results of which were long regarded as classical. He employed a thermometer with a large reservoir A, containing the gas to be operated on; an index of mercury mn separated the gas from the external air, while leaving it full liberty to expand. The gas had previously been dried by pass

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ing it through a tube containing chloride of calcium, or some other desiccating substance. The thermometer was fixed in a vessel which was first filled with melting ice, and when the gas had thus been brought to 0° C., the tube was so adjusted that the index coincided with the opening through which the thermometer passed.

The tube being divided into parts of equal capacity, and the reservoir having been previously gauged, the volume V is known which is occupied by the gas at an external pressure H indicated by a barometer; the apparatus is then raised to a given temperature T by

means of the furnace below the vessel, and the stem of the thermometer is moved until the index reaches the edge of the opening; at this new temperature the gas occupies a volume V' expressed in divisions of the tube: at the same time the pressure may have varied; suppose it to have become H'. From these data it is easy to deduce the expansion of unit volume of the gas from 0° to T at constant pressure. If D denote this expansion, the volume of the gas at T at the original pressure would be V (1+D). But the gas occupies a volume V' at the temperature T and pressure H'. At the pressure

H

H the volume would therefore be V' H But the divisions of the thermometer have expanded in the ratio 1+KT, K being the coefficient of expansion of glass: the true expression therefore for the new volume of the gas at the pressure H is have the equation

H

V (1+D) = V′(1+KT) H ’

D

V′(1 + KT) H′

H

; whence we

from which we can find the value of D, and consequently that of the mean coefficient of expansion By means of this method GayLussac arrived at the following results:

1st.-All gases expand by the same amount between the same limits of temperature.

2d. The coefficient of expansion is independent of the pressure. He also found that the coefficient of expansion of air between 0° and 100° was '00375.

These laws, which, together with Boyle's law, were long regarded as defining the fundamental properties of the gaseous state, are not rigorously exact, but are subject to restrictions similar to those which apply to Boyle's law. The absolute value of the co-efficient: of expansion of air as laid down by Gay-Lussac is very sensibly erroneous. The true value has been determined, by subsequent experiments conducted with greater precision, to be 003665, or 273

1

218. Regnault's Experiments. The apparatus employed by GayLussac had one serious imperfection. The mercurial index did not constitute a sufficient barrier between the gas under investigation and the external air; so that a portion of the gas was able to escape, while at the same time some of the external air became mixed with the gas; either of which circumstances would impair the accuracy of the experiment. It also appears that the means employed by Gay-Lussac

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