442

HEAT EVOLVED IN FORMATION OF SALTS.

[211.

stantially similar, though the absolute quantities of heat which they obtained, in many cases differ considerably from those published by Andrews. The table which is given at the bottom of the two preceding pages indicates the amount of heat obtained by both observers by acting on 1 equivalent of each of the different bases with each acid, the acid being always very slightly in

excess.

In the present stage of our knowledge upon this subject, it appears safest to state, that the quantity of heat emitted during the act of combination of equivalent quantities of different acids with a given base, although nearly the same, is not rigidly so. They might probably be arranged in isothermic groups, as follows, -those which stand first evolving most heat:

1. Sulphuric and sulphurous acids

With 1 Eqt. of Potash. 16700-16300

2. Oxalic, nitric, phosphoric, arsenic, hydro-)
chloric, hydriodic

3. Chromic, formic, acetic

4. Tartaric, citric, succinic.

15100-14700

1440C-13800

13600-13500

The bases also differ in the amounts of heat which they evolve in acting on the same acid; but as only a very few of the bases are soluble in water, their thermic powers cannot be compared in so simple a manner as those of the acids. Potash, soda, and baryta seem to be nearly isothermic, whilst ammonia is decidedly inferior to these bases. Lime, magnesia, and the other insoluble oxides cannot be satisfactorily compared with each other, until the amount of heat which is absorbed during their passage into the liquid form is known.

(212) Estimate of Heat absorbed during Solution of Salts.During the solution of saline bodies in water, heat is generally absorbed; but in a few instances where anhydrous salts are dissolved, as in the case of the chlorides of zinc, iron, and copper, an evolution of heat occurs, owing to the preliminary formation of a solid hydrate. The following are experimental data furnished by Favre and Silbermann upon this point (Ann. Chim. Phys. 1853 [3], xxxvii. 414). The presence of very different quantities of water of crystallization in the various salts furnishes a partial explanation of the fact, that no relation is observed between the quantities of heat absorbed during the solution of the molecule of each compound :

Potassic nitrate

3...

KNO
NaNO
NHẠN,
Ca(NO3)2,4H2O
Sr(NO3)2,4H2O
Pb(NO3)2
AgNO,...

(213) Heat evolved during the Solution of Gases.-During the solution of gases in water, a considerable evolution of heat takes place, which has been estimated by Favre and Silbermann in the following instances (Ann. Chim. Phys. 1853 [3], xxxvii. 410).

Heat units

from I gramme.

Heat units from 1 eqt. in

IOI'I

70'5

7127

85

45'5

3867

80

65'9

5272

236

27'1

6396

283.6

41'2

11685

444

CHAPTER VI.

[214.

MAGNETISM AND ELECTRICITY.

I. Magnetism-II. Static Electricity.-III. Dynamic or Voltaic Electricity.-IV. Electro-Magnetism.-V. Magneto-Electricity.-VI. Thermo-Electricity.-VII. Animal Electricity. VIII. Diamagnetism.

(214) Magnetism and electricity are now found to be so intimately related, that it is hardly possible to study the operations of either separately.

The property of the loadstone to attract small pieces of iron was recognized as a remarkable natural phenomenon for centuries before the Christian era; and the pointing' of the magnetic needle north and south, was early applied to the purposes of navigation by the Chinese; but it was not employed for that purpose by European nations till the latter end of the fifteenth century. The property of temporarily attracting light objects, which amber acquires when rubbed, was also familiar to the Grecian philosophers; but it was not till about 280 years ago that Gilbert laid the foundation of electrical science, and that Otto de Guericke and Hauksbee contrived the first electrical machines. Nautical men, likewise, had often observed that after a ship had experienced a stroke of lightning,the compass was deranged or its poles were reversed; but it was not until the year 1819 that the true connexion between electricity and magnetism was pointed out by Oersted, when he published his memorable discovery, that a magnetic needle, if suspended freely at its centre, would place itself at right angles to a wire which was transmitting an electric current. After the publication of Oersted's discovery, the means of obtaining powerful temporary magnets by transmitting electrical currents through wires coiled round masses of soft iron, or in other words, the methods of preparing electro-magnets, were speedily devised; and thus the dependence of magnetism on electricity in motion was shown: whilst in 1831 the completion of this chain of discovery was effected by Faraday, who announced that a current of electricity might be obtained in a closed conducting wire from the magnet, by moving it across the line of the conductor.

In its chemical bearings, particular importance is attached to Volta's invention of the voltaic pile or battery, which, in the hands of Davy, led to the discovery of the metallic bases of the

215.]

MAGNETISM.

445

alkalies and of the earths, and effected a complete change in the aspect of chemical science. In later years, the applications of the voltaic battery to the chemical arts of gilding, silvering, zincing, &c., have rendered it an instrument of great importance in the industrial arts.

I. MAGNETISM.

(215) It will not be necessary to enter fully into the subject of magnetism, but a few remarks upon its more important peculiarities will materially aid in fixing upon the mind clear ideas of polarity and attraction.

Electricity is, like magnetism, polar, and the phenomena of chemical attraction also fall into the class of polar actions.

The most obvious character of magnetism is seen in the power of attracting masses of iron, which is displayed to a greater or less extent by magnetized bodies. This property of attracting iron was first observed by the ancients in an iron ore obtained from Magnesia, in Asia Minor: hence the property was termed magnetism, and when in more recent times its directive property was observed, the mineral itself was named the lead-stone or loadstone. A steel bar if rubbed in one direction with the loadstone acquires similar properties; when poised horizontally, as may be done by supporting it upon a point, such a bar will take up a fixed position with regard to the poles of the earth; in this country it will point nearly north and south. The end of a magnetic bar which points towards the north is distinguished by a mark, and is hence often termed the marked end of the magnet. This peculiarity in the magnet of taking a fixed direction, renders it invaluable to the navigator. A magnetized needle attached to a card marked with the cardinal points, and properly poised, constitutes the mariner's compass.

FIG. 160.

If a sheet of paper be laid over a magnetized steel bar, and iron filings be evenly sifted upon the paper, it will be found, on gently tapping the paper, that the particles of iron accumulate in two groups, one around each extremity of the bar, and that from these poles the filings arrange themselves in curved lines, somewhat resembling those shown in fig. 160, sextending from one end of the bar to the other. This experiment shows that the attractions are concentrated near the two extremities of such a bar.

A soft iron wire freely

446

MAGNETIC POLARITY-INDUCTION.

[215.

suspended at its centre, in a horizontal direction, will be attracted indifferently at both ends by either end of the magnetic bar; but if a second magnetic bar be poised in the same way as the iron wire, it will be found that one end of this bar will be attracted when the magnet is brought near it in one direction, whilst the same end will be repelled if the opposite end of the magnet be presented to it. Further examination shows that this repulsion takes place when the ends presented to each other are those which would naturally point in the same direction; two north ends repel each other, and similar repulsion ensues when two south ends are presented to each other; whereas, if the extremities presented naturally point in opposite directions, attraction ensues between them the north end of one bar attracts the south end of the other. Hence it appears that there are two kinds of magnetism, endowed with qualities analogous, but opposite to each other. The two kinds are always developed simultaneously, are always equal in amount, but are opposite in their tendencies; and thus are capable of exactly neutralising each other. They accumulate at opposite ends of the bar. These ends are termed the poles of the magnet.

FIG. 161.

(216) Magnetic Induction.-Magnetism acts through considerable intervals of non-magnetic matter upon bodies such as iron, which are susceptible of magnetism, and it produces a temporary development of magnetism in such magnetizable substances. A piece of soft iron brought near to a magnet immediately assumes the magnetic state. This influence of the magnet operating at a distance is termed magnetic induction, and it is in consequence of this action that the iron is attracted. If the north end, N, of a magnet, L (fig. 161), be presented to a piece of soft iron, the latter becomes a magnet with its poles similarly arranged; that is to say, the soft iron acquires in the extremity, s, presented to the permanent magnet, magnetism of the opposite kind to that of the end, N, of the magnet, L, which it is made to approach. The soft iron will now attract other pieces of iron, s n, s n, and they in turn will act upon others, by a continuation of the induction. On gradually removing the permanent magnet, the effects diminish as the distance increases, and at length disappear altogether. This diminution in the effect takes place much more rapidly than in the ratio of the squares of the dis

tance from the maguetic pole, but the exact law has not as yet