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such as was implied by the term electric affinity; for if it were so, why should oxygen unite with barium in one proportion at one temperature, and in another at a lower? Elements and compounds are found to behave differently under the action of the galvanic current, some going to the positive, others to the negative pole. When water is decomposed by electrolysis, the hydrogen is given off at the negative and the oxygen at the positive pole; also, when chloride of sodium is subjected to the same decomposing agent, chlorine is found at the positive and sodium at the negative pole; and from these observed properties oxygen and chlorine are called electro-negative, and hydrogen and sodium electro-positive. Other terms for these properties are employed, namely, chlorous and basylous chlorous, like chlorine or electro-negative; basylous, or acting as a base, like hydrogen or electro-positive. These different states, the chlorous or basylous, also determine the affinity which bodies have for one another. A substance which is electro-negative to others which are electro-positive to it, unites with that which is furthest removed from it in its electric condition. Iron is electropositive to copper, and if iron be placed in a solution of sulphate of copper, copper is precipitated, and iron takes its place in the solution; for iron, being further removed from SO, than copper, which is electro-negative to both, it combines with it to the exclusion of the copper. When substances are said to be electro-negative or electro-positive, it is not meant that they are so absolutely, but relatively to others which have a different condition; and even an element, is believed to be electro-negative to itself, that is, the atoms which form its molecule exist in these two different states.

When chlorine gas is passed into water, in the cold, only half of the chlorine can be precipitated by nitrate of silver; but on filtering off the precipitate, after sufficient nitrate of silver has been added to send down all that can be precipitated, the filtrate will contain the remaining half of the chlorine in a state of combination which cannot be disturbed by nitrate of silver. If, however, it be acted upon by some reducing agent, such for example as zinc and sulphuric acid, the remaining half of the chlorine can be precipitated by nitrate of silver. Here the chlorine passed into the water has decomposed a part of it, and has formed hydrochloric and hypochlorus acid

Cl2+ H2OH Cl + H CIO.

Half of the chlorine has combined with electro-positive hydrogen, and the remaining half with the electro-negative residue of the water, HO. It will be easily seen that HO must be electro-negative, as

the electro-negative oxygen is only half saturated by the basic or electro-positive hydrogen. We therefore conclude that the molecule of free chlorine contained atoms in different states of polarity (an objection has been raised to the use of this word, lest confusion should arise between the idea which it is intended to convey and magnetic polarity); it is true that no one has yet discovered the nature of this property of atoms, but on investigation it may be found to be of a kind analogous to it. When the hypochlorous acid is acted on by the zinc and sulphuric acid, chloride and sulphate of zinc are formed, the hydrogen taking the oxygen of the hypochlorous acid to form water

2 (H CI O) + 2 (Zn) + H, SO. Zn Cl, + Zn SO. + 2 (H2O). Here the chlorine has shown electro-negative properties, and has united itself to electro-positive zinc. When chlorine gas therefore is evolved in the free state, its atoms arrange themselves to form molecules, the one atom of the molecule having chlorous or electronegative, the other basylous or electro-positive properties, and these atoms become chlorous or basylous according to the properties of the bodies with which they are brought into contact at the moment they are set free from the combinations in which they existed. This is an example of the polarity of atoms afforded by the analysis of the molecule of chlorine, and we shall see that the synthesis of a molecule of oxygen shows that its atoms possess similar properties. Peroxide of hydrogen H, O,, which is made by dissolving peroxide of barium in dilute hydrochloric acid—

2

Ba H, O, + 2 (H Cl) == H2 02 + Ba C1, + H ̧ 0,

2

2

2

holds its second atom of oxygen very feebly, so that one would expect it to act as an oxidizing agent in all cases. This, however, is not the case; for, if it be added to a solution of bichromate of potash, it changes the liquid from bright yellow to green by reducing the chromic acid, Cr O,, to chromic oxide, Cr, Os. It also reduces the oxides of gold, silver, and platinum, leaving the metals, oxygen being set free, one-half coming from the peroxide of hydrogen, the other from the metallic oxide; and so violent is the action that if it be brought in contact, in a concentrated form, with oxide of silver, the decomposition takes place with considerable violence and rise of temperature. Here the oxygen is held feebly both by the peroxide of hydrogen and by the silver, and being in different electric states the two atoms are able to separate from their combinations and form a molecule of free oxygen. The action really is one of oxidation, for the atom of oxygen, in union with

the silver, oxidizes that of the peroxide, and oxide of oxygen, i.e., free oxygen, is the result. This strange reaction was not for a long time understood until Sir Benjamin Brodie, who was the first to investigate this subject, pointed out, in a paper on "The Condition of Elements at the Moment of Chemical Change," that the oxygen in the peroxide of hydrogen and that of the oxide of silver were in two different polar states, the former in the positive, the latter in the negative. He says, "Were it, for example, a hydride of silver which was thus decomposed by the peroxide, and the decomposition of the substances attended with the formation of water, the experiment would have attracted no attention. On the view here given, the formation of the oxygen is as truly a chemical synthesis as the formation of water itself, and may be substituted for it in a chemical change. The oxide of silver is here reduced by the oxygen of the peroxide of hydrogen, just as in other cases it might be by the hydrogen itself, the formation of the silver being the corresponding fact in the decomposition of the oxide of silver to the formation of water in the peroxide of hydrogen, so that the change may be represented thus"

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Another instance of synthesis is found in the formation of a molecule of hydrogen by the action of hydrochloric acid on hydride of copper, which was discovered by Wurtz, who found that when hypophosphorous acid acted on sulphate of copper, a brown hydrate of copper was formed, having the formula Cu2 H2; and that when this body was acted upon by hydrochloric acid, hydrogen was rapidly set free and subchloride of copper formed. This was very unexpected, because hydrochloric acid is generally believed to have no action on metallic copper.

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Cu, Cl + H..

2

In this case the hydrogen of the cupreous hydride is evidently electro-negative, and that of the hydrochloric acid electro-positive; and as the compound Cu, H, is very unstable, being decomposed at 92° C., the atoms of hydrogen being in different polar states, unite to form free hydrogen.

2

A further example is afforded by the behaviour of iodine, when an iodide and an iodate are treated with a dilute acid. Iodide of potas

* Phil. Trans., II., 1850, 759.

In his paper, Sir B. Brodie uses the old notation thus

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sium, when pure, is not decomposed by hydrochloric acid, which is free from chlorine; nor is pure iodate of potassium decomposed by an acid, but when iodide and iodate are mixed, acetic acid sets iodine free, and the free iodine is built up of the atoms existing in the two different potash salts. This is just the reverse of the action which we before considered in the case of chlorine, with this difference, that when iodine is added to a solution of hydrate of potash, hydriodic acid, H I, is formed along with iodic, H I Os, there being no compound of iodine with oxygen similar to hypochlorous acid, iodine having, under similar conditions, a more powerful affinity for oxygen than chlorine has. The molecules of iodine, as those of chlorine did, break up into their atoms, some of which exhibit basylous, while others show chlorous properties. It would occupy too much of our space to cite more of the many examples which might be brought forward to illustrate this point. Those who desire to follow up this very interesting and important subject, are referred to Sir B. Brodie's very able paper. The molecules of the elements, then, are composed of atoms with electro-positive and electro-negative properties, so that chlorine is C1 Cl, chloride of chlorine, one atom positive, the other negative to it, and the same. is the case with the other elements. HH is hydride of hydrogen, and the molecule is, so to speak, as regards these properties, in a state of equilibrium. The electro-positive atom, H, cannot exist alone, nor can that which is electro-negative; so that when the atoms are set free from any combination they instantly unite together to form the free element, or with some other body in contact with them, and this is what is meant by the "nascent state," which is, as Sir B. Brodie says, really a polar state; for when zinc and sulphuric acid act together in the presence of water, hydrogen is evolved, and if any substance be present with which hydrogen forms compounds, for example, common arsenic or arsenious acid, As, O,, arsenuretted hydrogen and water are formed

2

3

6 Zn +6 (H, SO) + As ̧ O ̧ =6 (Zn SO1) + 2 As H, + (H, O). Whereas, no arsenuretted hydrogen is produced if the hydrogen be passed over arsenious acid in the cold; in fact, no reaction takes place, and simply metallic arsenic and water are the products if the arsenious acid be heated while submitted to the action of free hydrogen.

LYELL'S GEOLOGICAL PHILOSOPHY.

SIR CHARLES LYELL may well be congratulated on bringing out the tenth edition of his "Principles of Geology." In his preface to this edition he tells us that it is now thirteen years and a-half since the ninth edition made its appearance, and he has found it necessary to re-write some chapters, to modify others, and to omit passages no longer conforming with the best views of the day. The "Principles " made their first appearance in 1830-1833, and in 1834 a re-publication of the whole with new matter was issued in four volumes. Looking back to these dates, the science which Sir Charles has so ably expounded, and to which his own researches and methods of thought have contributed so much, appears in a widely different condition to what it is at present. Only a few bold thinkers ventured to reject Archbishop Usher's Chronology, or to differ from the orthodox views of the very recent creation of our globe and "all that it inherit," and the consequence was that geological theories were all founded upon the belief that changes, which are now known to have resulted from the slow operation of ordinary causes, were produced by violent catastrophes and convulsions, sweeping many plants and animals from our globe, as a sponge cleans the diagrams from a blackboard, and leaving the ground clear for any number of fresh creations that seemed to be required.

It is easy and especially so with the help which Lyell's own researches into the history of geological speculation affords-to show that some previous thinkers had been impressed with the erroneous character of such spasmodic conceptions of the order of nature, but to him more than to any other philosopher in our own country, or on the continent, belongs the distinguished merit of marshalling facts and arguments so as to give a sound direction both to the theoretical and the practical part of the science. So long as observers were content with irrational endeavours to account for all the facts they met with, by ascribing them to forces of tremendous violence operating in very brief spaces of time, the supposed geological past bore only a very faint resemblance to the actual present, and the different portions of terrestrial history conformed to no known or intelligible plan. We now know that although various sciences may be made to throw combined light on each other's paths, that each natural group of facts must be studied independently, and no generalizations admitted which they do not fairly warrant. The known facts of geology, fifty or a hundred

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