This p 1aken

tage

some

Re ter

by

wror: snowing that they are

eriormed with a large ir an excited glass tube units electricity; it will be pure-exciting the tube, and tracted, but retain its state with excited sealing-wax, it in contact with the wax till it wiil be repelled, and in that state the glass. In these experiments remains freely suspended in the air,

arrying off its electricity.

-Mous electricity were applied to these observing that the same electricity

y, but that glass sometimes exhibited ani vice versa, adopted another term, and, tomena as dependent upon two electric presence of one fluid, in excess in some ners. To represent these states he used Rastiive and negative. When glass is rubbed rcity leaves the silk and enters the glass; it we, and the silk negative; but when sealingthe wax loses and the flannel gains; the forand the latter positive. All bodies in nacontaining the electric fluid, and when its hey exhibit the phænomena just described. merated in the following table become posied with those which follow them in the list, recede them they become negatively electrical, *, tom. ii., p. 220.

Cat's skin
Polished glass
Woollen cloth

Feathers

Paper

Silk

Gum lac

Rough glass.

path-balls, or strips of gold leaf, are usually em. the presence of electricity; and,

their divergence is affected by wax, the kind or state of electricity is woperly suspended or mounted for they form an electrometer or elecshes purpose the slips of gold leaf are cap and wire in a glass cylinder; wy when unelectrified; but when verge, as in the marginal wood

by the cold, and a fresh quantity of vapour arises successively from the water in the other, by which so much heat is carried off as to cause it to congeal.-Phil. Trans. 1813.

93. In many natural operations the conversion of water into vapour, and the condensation of vapour in the form of dew and rain, is a process of the utmost importance, and tends considerably to the equalization of temperature over the globe.

94. Nothing is known of the nature or cause of heat. It has been by some considered as a peculiar fluid, to which the term Caloric has Caloric a been applied; and many phænomena are in favour of the existence of such a fluid. By others, the phænomena above described have been referred to a vibratory motion of the particles of matter, varying in ve- A vibratory locity with the perceived intensity of the heat. In fluids and gases the motion. particles are conceived to have a motion round their own axes. perature, therefore, would increase with the velocity of the vibrations; and increase of capacity would be produced by the motion being performed in greater space. The loss of temperature, during the change of solids into liquids and gases, would depend upon loss of vibratory motion, in consequence of the acquired rotatory motion.

Tem

Upon the other hypothesis, temperature is referred to the quantity of caloric present; and the loss of temperature, which happens when bodies change their state, depends upon the chemical combination of the caloric with the solid in the case of liquefaction, and with the liquid in the case of conversion into the aëriform state.

SECTION IV. Electricity.

95. Ir a piece of sealing-wax and of dry warm flannel be rubbed against each other, they both become capable of attracting and repelling light bodies. A dry and warm sheet of writing-paper, rubbed with In- Electrical exdia rubber, or a tube of glass rubbed upon silk, exhibit the same phæ-citement. nomena. In these cases the bodies are said to be electrically excited; and when in a dark room, they always appear luminous.

attraction.

96. If two pith-balls be electrified by touching them with the sealing-wax or with the flannel, they repel each other; but if one pith-ball Repulsion and be electrified by the wax, and the other by the flannel, they attract each other. The same applies to the glass and silk: it shows a difference in the electricities of the different bodies, and the experiment leads to the conclusion, that bodies similarly electrified repel each other, but that when dissimilarly electrified they attract each other.

The term electrical repulsion is here used merely to denote the ap- Repulsion. pearance of the phænomenon, the separation being probably referable to the new attractive power which they acquire, when electrified, for the air and other surrounding bodies.

If one ball be electrified by sealing-wax rubbed by flannel, and another by silk rubbed with glass, those balls will repel each other; which proves that the electricity of the silk is the same as that of the sealing-wax. But if one ball be electrified by the sealing-wax and the

F

Franklin's theory.

other by the glass, they then attract each other, showing that they are oppositely electrified.

These experiments are most conveniently performed with a large downy feather suspended by a silken thread. If an excited glass tube be brought near it, it will receive and retain its electricity; it will be first attracted, and then repelled, and upon re-exciting the tube, and again approaching it, it will not again be attracted, but retain its state of repulsion: but upon approaching it with excited sealing-wax, it will instantly be attracted, and remain in contact with the wax till it has acquired its electricity, when it will be repelled, and in that state of repulsion it will be attracted by the glass. In these experiments care must be taken that the feather remains freely suspended in the air, and touches nothing capable of carrying off its electricity.

97. The terms vitreous and resinous electricity were applied to these two phænomena; but Franklin, observing that the same electricity was not inherent in the same body, but that glass sometimes exhibited the same phænomena as wax, and vice versa, adopted another term, and, instead of regarding the phænomena as dependent upon two electric fluids, referred them to the presence of one fluid, in excess in some cases, and in deficiency in others. To represent these states he used the terms plus and minus, positive and negative. When glass is rubbed with silk, a portion of electricity leaves the silk and enters the glass; it becomes positive, therefore, and the silk negative; but when sealingwax is rubbed with flannel, the wax loses and the flannel gains; the former, therefore, is negative, and the latter positive. All bodies in nature are thus regarded as containing the electric fluid, and when its equilibrium is disturbed, they exhibit the phænomena just described.

98. The substances enumerated in the following table become positively electrified when rubbed with those which follow them in the list, but with those which precede them they become negatively electrical, -BIOT, Traité de Physique, tom. ii., p. 220.

Cat's skin

Polished glass

Woollen cloth

Feathers

Paper

Silk

Gum lac

Rough glass.

99. Very delicate pith-balls, or strips of gold leaf, are usually em ployed in ascertaining the presence of electricity; and, by the way in which their divergence is affected by glass or sealing wax, the kind or state of electricity is judged of. When properly suspended or mounted for Electrometer. delicate experiments, they form an electrometer or electroscope. For this purpose the slips of gold leaf are suspended by a brass cap and wire in a glass cylinder; they hang in contact when unelectrified; but when electrified they diverge, as in the marginal wood

When this instrument, as usually constructed, becomes in a small degree damp, its delicacy is much diminished, and it is rendered nearly useless. The following improvement in its construction by the late Mr. Singer renders it a much more sensible and useful instrument. It is constructed as usual, with a glass cylinder, surmounted by a brass cap; but the insulation is made to depend upon a glass tube, about four inches long, and one-fourth of an inch internal diameter, covered both on the inside and outside with sealing-wax, and having a brass wire of a sixteenth or twelfth of an inch thick and five inches long, passing through its axis, so as to be perfectly free from contact with any part of the tube, in the middle of which it is fixed by a plug of silk which keeps it concentric with the internal diameter of the tube. A, is a brass cap screwed upon the upper part of this wire; it serves to limit the atmosphere from free contact with the outside of the tube, and also defends its inside from dust; to the lower part of the wire the gold leaves are attached, and the whole mounted as usual, and as represented above.

Singer's im-
provement.

termining the

100. The kind of electricity by which the gold leaves are diverged may be judged of by approaching the cap of the instrument with a Method of destick of excited sealing-wax; if it be negative the divergence will in-kind of elec crease; if positive, the leaves will collapse, upon the principle of the tricity. mutual annihilation of the opposite electricities, or that bodies similarly electrified repel each other, but that when dissimilarly electrified they become mutually attractive. (96.)

do

a

Coulomb's electrical balance.

101. To ascertain the actual repulsive and attractive powers appertaining to weakly-electrified bodies, M. Coulomb has ingeniously availed himself of the principle of torsion, and has thus constructed his electrical balance. It consists of a fine metallic wire, a, one end of which is attached to the screw b, and to the other is suspended the horizontal needle c, composed of gum-lac or other nonconductor, and armed at one extremity with a gilt pith-ball, counterpoised at the other end by an index. The conductor d, is a small wire with a ball at each end passing through the glass receiver in which the needle is suspended, and having its lower ball opposed to that of the needle. By the screw b, the two balls are brought into contact, and the index then points to ⚫, on the divided scale of degrees. On communicating a very feeble electrical power to the conductor, it transfers it to the moveable pith-ball, and repels it a certain number of degrees, proportional to the intensity of the acquired electricity, and measured by the power of torsion which it exerts upon the fine wire. By expe- Electrical riments made with this electrometer, it would appear that the electri-powers are incal powers follow the law of gravitation, in being in the inverse ratio of squsly of their the squares of the distances of the acting bodies. In the most deli- distances i. e. cate construction of the instrument a single silk-worm's thread is used powers of gra

1 nonconductors.

102. Some bodies suffer electricity to pass through their substance, and are called conductors. Others only receive it upon the spot Conductors & touched, and are called nonconductors. The former do not, in general become electric by friction, and are called nonelectrics: the latter, on the contrary, are electrics, or acquire electricity by friction. They are also called insulators. The metals are all conductors; dry air, glass, sulphur, and resins, are nonconductors. Water, damp wood, spirit of wine, damp air, and some oils, are imperfect conductors.

103. Rarefied air admits of the passage of electricity; so does the Electricity Torricellian vacuum: hence if an electrified body be placed under passes through the receiver of the air-pump, it loses its electricity during exhaustion. So that the air, independent of its nonconducting power, appears to influence the retentive properties of bodies in respect to electricity, by its pressure.

a vacuum.

No constant

104. There appears to be no constant relation between the state of relation be- bodies and their conducting powers: among sólids, metals are conducstate of bodies tors, but gums and resins are nonconductors; among liquids, strong aland their con- caline, acid, and saline solutions, are good conductors; pure water is

tween the

ducting pow

ers.

Some snbstances become electric

an imperfect conductor, and oils are nonconductors; solid wax is almost a nonconductor, but when melted, a good one. Conducting powers belong to bodies in the most opposite states; thus the flame of alcohol, and ice, are equally good conductors. (BIOT, Traité de Physique, tom. ii., p. 213.) Glass is a nonconductor when cold, but conducts when red-hot; the diamond is a nonconductor, but pure and wellburned charcoal is among the best conductors.

105. There are many mineral substances which show signs of electricity when heated, as the tourmalin, topaz, diamond, boracite, &c.; by being heat- and in these bodies the different surfaces exhibit different electrical

ed.

tricities at op

states.

106. Whenever one part of a body, or system of bodies, is positive, Opposite elec- another part is invariably negative; and these opposite electrical states posite sides of are always such as exactly to neutralize each other. Thus, in the a body. common electrical machine, one conductor receives the electricity of of the glass cylinder, and the other that of the silk rubber, and the former conductor is positive and the latter negative; but if they be connected, all electrical phænomena cease.

107. The best electrical machine for experimental purposes is represented in the annexed sketch.