tion, but acids and bases enter into combination | is generally used to mean quicklime-the anwith water in another and special way. We have hydrous base, but not unfrequently for what is pointed out before that there are substances which properly called slaked lime, the compound of lime may be said to be intermediate between acids and and water (hydrate of lime). bases, which act as bases to strong acids, and as acids to strong bases. Now, water is such a sub-acters of acids and bases, we shall now shortly stance. Water unites with many bases, forming substances called 'hydrates,' or 'hydrated bases' (thus, hydrate of lime,' 'hydrated oxide of lead,' &c.); it also unites with many acids, forming substances called 'hydric salts,' or 'hydrated acids' (thus, hydric sulphate,' or 'hydrated sulphuric acid,' &c.). This kind of combination of water with acids and bases was long confounded with one or other of the two kinds mentioned just before, and from this there has arisen a confusion in the nomenclature of acids, bases, hydrates, and hydric salts, which is very likely to prove perplexing to the beginner.

Thus, the term 'sulphuric acid' is used to express two different things-1. Oil of vitriol, which, when as strong as it can be made, consists of nearly pure 'hydric sulphate,' or 'sulphate of water;' and 2. A white, solid substance, which is oil of vitriol without the water. This white solid, when put into water, unites with it, giving out a great deal of heat, and forming oil of vitriol, or 'hydric sulphate.'

In the case of many other acids, we have the same double meaning of the word; we have the 'anhydrous' (that is, waterless) acid, and the compound of that with water-the hydrated acid, or hydric salt. Some chemists restrict the word acid to the hydric salt, and call the anhydrous acid the anhydride;' others restrict the name acid to the anhydrous substance, and call the hydrated acid a hydric salt. Thus, we have the name sulphuric acid applied to two substances anhydrous sulphuric acid, or sulphuric anhydride; and hydrated sulphuric acid, or hydric sulphate. And these differ from one another in that the latter is a compound of the former with

water.

As all the processes described in the preceding pages in illustration of the action of acids on other substances, were assumed to take place in water, it was not necessary to allude to this ambiguity, and the acids taken as examples (sulphuric, nitric, hydrochloric, and acetic) were supposed to be in the state of hydric salts. The question, indeed, would be one of only theoretical interest, if every acid existed in both forms. But there are some acids which occur only as hydric salts, others which occur only as anhydrous acids. Thus, carbonic acid gas dissolves in water, but does not combine with it to form a hydrated acid, or hydric carbonate. Again, oxalic acid cannot be obtained in the anhydrous form. If we try to drive off the water from hydrated oxalic acid (hydric oxalate), we find that the acid itself decomposes and breaks up into other substances. We shall return to the consideration of these relations when we come to speak of saltradicals.

As acids (that is, anhydrous acids) unite with water to form hydrated acids, or hydric salts, compounds in which water acts as a base, so anhydrous bases unite with water also, forming hydrated bases or hydrates in which water acts as an acid; and here also we are apt to get into confusion with the names. Thus, the word lime

Having considered in a general way the charexamine their composition, for they are all compounds-that is, they can all be produced by the union of elements, and can all be decomposed or broken up by separating these elements from each other. And first, we shall investigate the composition of water, which is, as we have seen, both an acid and a base—an acid in relation to the strong bases; a base in relation to the strong acids.

Water, as is well known, was long regarded as a simple or elementary substance. It is one of the four elements' of the ancients-fire, air, earth, and water; and although we now know that water is decomposed in a great many chemical changes constantly taking place, and produced from its elements in some of the most familiar and frequent cases of chemical action, the real nature of these changes escaped the observation of chemists until a comparatively recent time. The discovery of the composition of water was made by Cavendish about the year 1781. He shewed that when hydrogen gas is burned in the air, water is formed; and that if, instead of common air (which contains about 20 per cent. of oxygen), pure oxygen is used, the hydrogen and oxygen disappear, and water is produced, and that the weight of the water formed is the same as that of the hydrogen and oxygen which have disappeared. He thus proved that water is formed by the union of hydrogen and oxygen-that it is a compound of these two gases. By measuring the quantity of each gas, he ascertained the proportion in which they unite, and shewed that one volume of oxygen unites with two volumes of hydrogen. As oxygen weighs bulk for bulk 16 times as much as hydrogen, the proportion by weight is 8 parts of oxygen to 1 of hydrogen; that is, I oz. of hydrogen in burning will use up and unite with 8 oz. of oxygen, and produce 9 oz. of water; or 9 oz. of water can be decomposed into 8 oz. of oxygen and I oz. of hydrogen; and the 1 oz. of hydrogen will occupy a space twice as great as the 8 oz. of oxygen. The composition of water was discovered by the method of synthesis-that is, by forming it from its constituents. We can also prove it by analysisthat is, by decomposing it into its constituents. This may be done by passing through water a current of electricity, from a galvanic battery for instance, when the hydrogen will bubble up from the one wire (that connected with the zinc of the battery), and the oxygen from the other (that connected with the copper of the battery). The two gases can thus be collected separately, and measured, when it will be found that the hydrogen produced fills just twice the space filled by the oxygen.

Water is thus a compound of hydrogen and oxygen, or, in chemical language, an oxide of hydrogen; the compounds of oxygen with other elements being termed oxides.' Now, a very

*When the bulks of two or more gases have to be compared,

they must be measured under the same pressure and at the same temperature, as a change of pressure or of temperature produces a change in the bulk of a gas.

large number of the anhydrous acids and anhy- | metal as an element which forms in combination drous bases are oxides, and the anhydrous bases which are oxides are all oxides of metals. This brings us to the consideration of the fourth group or family of substances mentioned in p. 307-namely, Metals.

with oxygen at least one base. There are two general principles which may be mentioned here: I. The more readily a metal unites with oxygen, the more basic is the oxide; thus, sodium is more easily oxidised than magnesium, magnesium than zinc, zinc than copper, copper than silver; and soda (oxide of sodium) is a stronger base than magnesia (oxide of magnesium), magnesia than oxide of zinc; and so on. 2. When a metal forms two basic oxides, we usually find that that which contains the least oxygen is the stronger base. Thus, two of the oxides of iron are bases; one (ferrous oxide) contains iron and oxygen in the proportion of 7 to 2; the other (ferric oxide) in the proportion of 7 to 3; and the former is the stronger base.

Like all the other groups which we have examined, the metals possess certain characters in common, by which they may be recognised as belonging to the group; some of these characters are physical, some are chemical. The most marked physical characters of the metals are, the 'metallic lustre,' and the readiness with which they conduct heat and electricity. The metallic lustre is the name given to the peculiar brilliancy of a polished metallic surface. All metals, when polished, shew this lustre; but some substances which are not metals shew it also. Thus, 'black- We shall now examine the action of acids and lead,' or plumbago, which is not a metal, and is salts upon metals. The anhydrous acids, as a rule, in no way related to lead, but is a form of carbon, do not act at all upon metals. There are some has a brilliant metallic lustre ; and a considerable exceptions to this rule, which we may have occasion number of compounds shew it also, such as iron to allude to farther on. The action of the hydrated pyrites and galena. The power of conducting acids or hydric salts on metals is quite analogous heat and electricity is possessed to a greater or to that of other salts. We shall illustrate this less extent by all substances; by some, however, action by one or two examples. If we take a clear to such a small extent, that they are usually called colourless solution of nitrate of silver, and place in 'non-conductors;' as instances, we may mention it a piece of bright clean copper, we see in a few glass, resin, gutta-percha. The metals differ minutes that the copper is covered over with a greatly from one another in conductivity,' or the growth of what looks like soft white moss; this power of conducting heat and electricity; but they gradually increases, the liquid at the same time are, as a rule, much better conductors than non- becoming blue. The white covering on the copper metallic substances. In other physical properties, consists of small scales of silver, and the blue metals shew great variety-in fusibility, ranging colour of the liquid is due to nitrate of copper. If from mercury, which is liquid at ordinary tem- we take enough copper, we can in this way decomperatures, and only freezes at about -40° Fahr. pose the whole of the nitrate of silver, separating to platinum, which requires the highest tempera- the silver in the metallic state. Again, if we place ture we can produce for its fusion; and osmium, a piece of clean iron in a solution of nitrate (or which has not yet been fused. Metals differ sulphate) of copper, metallic copper is deposited greatly in density; the lightest known simple on the iron, the blue colour of the solution dissolid and the heaviest-lithium and osmium— appears, and we have a solution of nitrate (or being both metals. Some metals are malleable sulphate) of iron.* Again, if we place a piece of and ductile-that is, they can be beaten into thin iron in a solution of hydrated sulphuric acid (oil of plates or leaves, and drawn into wire, as is the vitriol), that is, hydric sulphate, we have hydrogen case with gold, silver, copper, and iron; others gas given off, and sulphate of iron remains in are brittle, and can be pounded into a powder in solution. In each of these cases we have one a mortar. This is the case with antimony and metal driving out another, and taking its place.† bismuth. Most metals are white or gray, but we Copper drives out silver, iron drives out copper, have yellow gold and red copper. Some are and iron drives out hydrogen. In each such case, very hard, others very soft. As examples among the metal which drives the other out is said to be common metals, we may compare the hard-positive;' that which is driven out is said to be ness of iron with that of silver, and that of 'negative.' Thus, silver is negative to copper; lead.

Let us now turn to the chemical characters of metals. We have just mentioned that all the anhydrous bases which are oxides are oxides of metals. We may now add further, that every metal has an oxide which is a base. It is possible, directly or indirectly, to obtain a compound of each metal with oxygen. Some metals, such as zinc, aluminium, and magnesium, unite with oxygen in only one proportion; others, such as iron, copper, lead, mercury, form each more than one oxide, the various oxides containing the metal and oxygen in different proportions. In the former case, the oxide is a base; in the latter case, at least one of the oxides is a base. Thus, the oxide of zinc, the oxide of aluminium, and the oxide of magnesium, are bases; two of the three oxides of iron, both of the oxides of copper, one of the three oxides of lead, and both of the oxides of mercury, are bases. We may therefore define a

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copper is positive to silver, but negative to iron; iron is positive to copper; and so on. By means of a number of experiments of this kind, we can arrange the metals in a series, beginning with the most negative, and ending with the most positive. If we examine this series, we find that the more 'positive' a metal is, the more basic is its most basic oxide. So that just as a strong base drives out a weak one, so the metal of a strong base drives out the metal of a weak

one.

We have hitherto looked upon salts as compounds of acids and bases, but the reader will now see that they may also be represented in another

This action may be taken advantage of to detect copper in a when it is dipped in a jar of pickles which have been coloured green solution; thus, a steel fork is covered with a red deposit of copper by means of copper,

for water (the oxide of hydrogen) acts as a base, and substances ↑ We here speak of hydrogen as a metal. Chemically it is so, have been obtained which may be regarded as alloys of hydrogen.

way. Thus, we have spoken of sulphate of copper | represented as compounds of acid and base. This as a compound of has led to a modification of the old names of many salts; thus, we often use the names sulphate of

Oxide of Copper, that is, Copper and Oxygen,

Base.

and Anhydrous Sulphuric Acid. sodium (or sodium sulphate) instead of the old

Acid.

But just as a strong base, such as potash, will decompose this salt, driving out the oxide of copper, and forming sulphate of potash, so a more positive metal than copper-for instance, zinc-drives out the copper, and forms sulphate of zinc, in which the base is oxide of zinc. Here it is the metal which is changed; all the rest remains as it was. We may therefore represent the salt as a compound of metal with all the rest of the salt, thus: Copper and Oxygen and Anhydrous Sulphuric Acid.

sulphate of soda. It must not be supposed that sulphate of sodium means a compound of sodium and sulphuric acid; it is a compound of sodium and oxygen and sulphuric acid (anhydrous).

We may now place side by side the various names given to the same salt:

Sulphate of Soda.. Sodic Sulphate... Sulphate of Sodium.
Hydrated Sulphuric Acid.. Hydric Sulphate.. Sulphate of Hydrogen.
Hydrochloric Acid

Muriate of Soda....

Sodic Chloride.... Chloride of Sodium. Hydric Chloride.. Chloride of Hydrogen. Nitrate of Oxide of Lead.. Plumbic Nitrate.,Nitrate of Lead. &c. &c.

&c.

Sulphate of soda means the salt formed by the action of sulphuric acid on soda; sulphate of sodium means the compound of sodium and the salt-radical of the sulphates; while sodic sulphate may be used to indicate the same substance without referring to either view of its constitution. For this, as well as for other reasons, the names in the middle column are to be preferred, although this system sometimes involves us in the use of novel and awkward adjectives, such as zincic, or bismuthous.

We have seen that many bases are oxides of metals, and we must now attend to the relation of the metals to their oxides. When a metal (or other substance) is made to unite with oxygen, it

Hydrogen and Oxygen and Anhydrous Sulphuric Acid. is said to be oxidised;' when the oxygen is

Salt-radical.

taken away from an oxide, and the original substance (whether metal or not) is reproduced, the oxide is said to be 'reduced.' We have thus two processes inverse to one another—that is, the one undoing what the other does, oxidation and reduction. Thus, there is an important ore of iron called the magnetic oxide of iron; when this is smelted, the oxygen is taken from it, and we obtain metallic iron-this is a process of reduction. If we heat iron strongly in the air, it becomes covered with a crust or scale of a black, brittle substance, which breaks off when the iron is hammered-this is an oxide of iron, and is identical in composition with the magnetic iron orethis is a process of oxidation. It is obvious that these two processes go in opposite directions.

There are, however, a number of substances known, which, in combination, play exactly the part of the salt-radicals we have just been considering that is, they unite with metals to form salts, and with hydrogen to form hydric salts, corresponding exactly in general properties to the hydrated acids. These substances are called 'halogens' (that is, salt-producers). We shall illustrate their characters by means of an example, and for this purpose we shall select the most important of them-namely, chlorine. Chlorine is a simple substance-in other words, it has not been decomposed-and thus differs entirely from the salt-radicals of the sulphates, nitrates, &c. which consist of anhydrous acid and oxygen, and have not been obtained in Most processes of oxidation are accompanied the free state. Chlorine, in the free state, is a by the giving out of heat, sometimes in a very greenish yellow gas with an acrid smell. It marked degree; thus, when charcoal is strongly readily unites with hydrogen, forming the sub-heated in air or oxygen (and it must be rememstance known as muriatic acid gas, or, from its composition, hydrochloric acid gas. This is a true hydric salt, possessing all the general properties of the hydrated acids. It acts upon bases, producing salts and water; it drives out weaker acids (in the hydrated form—that is, as hydric salts) from their salts. The salts thus formed we have in the former pages called 'muriates,' or hydrochlorates; but we now see what they are; they are compounds of metals and chlorine, the oxygen of the metallic oxide (or base) uniting with the hydrogen of the hydrochloric acid to form water. They are hence called 'chlorides.' Thus, instead of 'muriate of soda,' we say 'chloride of sodium;' and so on. They are, however, true salts, undergoing all the changes and reactions common to salts. It will be noticed that all salts can be represented as compounds of metal and salt-radical, while some salts (such as the chlorides) cannot be

bered that about one-fifth of the atmospheric air consists of oxygen), it unites with oxygen, forming an oxide (carbonic acid, Black's fixed air), and in doing so, gives out much heat. Such an oxidation we call a combustion, or burning. And although different processes of oxidation are accompanied by the giving out of very different quantities of heat, we may generalise the term, and call all such processes cases of 'burning.' Similarly, all processes of reduction may be grouped together as cases of unburning.*

As in a case of 'burning' or 'oxidation,' heat is given out, so in a case of unburning' or 'reduction,' heat disappears, or is used up; and we find that exactly as much heat is used in effecting the unburning of an oxide as was given out in producing it. It will thus be seen that the greater

*This is here stated generally; some of the few exceptions will be mentioned among the compounds of Chlorine and Oxygen.

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the quantity of heat given out during an oxidation, the greater is the amount of work required to be done, in order to effect the corresponding reduction. Now, the difficulty of undoing a union, or of breaking up a combination, may be taken as a measure of its stability, so that the most stable oxides are those during the formation of which the most heat is given out.

Many oxidations can be effected directly by the action of oxygen, and some reductions can be effected directly by means of heat or electricity; thus, hydrogen unites with oxygen, or is burnt, producing water; and water can be decomposed by heat or by electricity into hydrogen and oxygen. There are, however, many oxidations which cannot be effected by the direct action of oxygen, but require the use of an 'oxidising agent'—that is, a substance or mixture containing oxygen in combination, and ready to part with it to a body capable of being oxidised. Thus, nitric acid contains a large quantity of oxygen, and readily gives up part or all of this oxygen to oxidisable substances; accordingly, nitric acid is a powerful oxidising agent, and is used to oxidise bodies in cases where the action can either not be produced at all, or not so conveniently by oxygen alone. As an instance of an oxidising agent which is a mixture, we mention the mixture of chlorine and water. This mixture will oxidise in many cases in which water alone would be without action-the tendency of chlorine to unite with the hydrogen of the water increasing the readiness of the oxygen to leave the hydrogen, and unite with the substance to be oxidised.

And in the same way it may be shewn, by means of other replacements, that an equivalent of any basic oxide contains the same quantity of oxygen. By an extension of the meaning of the word equivalent, this quantity of oxygen is called an equivalent of oxygen, and the quantity of metal united to it is called an equivalent of the metal. Those metals which form more than one basic oxide have, of course, more than one equivalent; and when we speak of the equivalent of a metal of this kind, we must state which basic oxide or which set of salts we are referring to. Thus, the black oxide of mercury contains twice as much mercury as the red oxide for the same quantity of oxygen; and therefore the equivalent of mercury in the black oxide and its salts is twice as great as the equivalent of mercury in the red oxide and its salts. The phenomena of 'electrolysis' furnish an excellent illustration of the equivalence of metals in their salts. When an electric current from a galvanic battery is passed through a solution of a salt, the salt is decomposed, the metal being Just as we have oxidising agents, so we have deposited at the end of the wire coming from also reducing agents-that is, substances or mix- the zinc of the battery, and the salt-radical at tures which remove oxygen: these are generally the end of the wire coming from the copper of bodies having a tendency to combine with oxygen, the battery. This decomposition, by means of in other words, they are oxidisable; and the most an electric current, is called 'electrolysis.' Now, powerful reducing agents are, as might be ex- a very remarkable law of electrolysis is, that a pected, those which give out most heat in uniting given amount of electricity decomposes (or electrowith oxygen. By far the most practically import-lyses) equivalent quantities of different salts. Thus, ant reducing agents are hydrogen and carbon, if a current from a battery be made to pass through either singly as hydrogen gas, and as charcoal or coke, or combined, as they are in coals, coal-gas, wood, and other ordinary combustibles. A large number of metallurgical operations are cases of reduction, in which coal, or coke or charcoal, is used to take away the oxygen of a metallic oxide, and leave the metal uncombined. It is in this way, as is more fully explained in the article METALLURGY, that iron, tin, zinc, and some other metals are reduced from such of their ores as contain the metals as oxides.

a series of vessels, each vessel containing a solution of a salt, the quantity decomposed of any one salt will be equivalent to the quantity decomposed of any other. If, now, one of these vessels contain a solution of a salt of the red, oxide of mercury, and another a solution of a salt of the black oxide of mercury, we shall find that twice as much mercury is separated in the second as in the first.

In order to express these equivalent quantities in numbers, it is necessary, in the first place, to fix upon a number which shall, by convention,

THE DOCTRINE OF EQUIVALENTS, AND THE represent one of them, and then, by experiment,

ATOMIC THEORY AND NOTATION.

We have already seen what is meant by an 'equivalent' of an acid, or of a base; we shall now consider the question of 'equivalence' somewhat more fully. As has been already stated, zinc can turn copper out of a salt, such as sulphate of copper, and take its place; this is technically called 'replacement,' and zinc is said to 'replace' copper. As the salts may be considered **Replace' is here used in the sense of the French remplacer,

and not in its ordinary English sense; but as we have no one English word which can be used to mean 'to be a substitute for,' seems better to keep the word at present in use, than to try other, such as 'displace' (as has been suggested), which exses the meaning no better.

find out the relation between the equivalent of that substance and the equivalent of every other element, radical, or substance. For this purpose, it has been found most convenient to select hydrogen as the element with which to compare all others, and to fix the equivalent of hydrogen as unity. The equivalent of hydrogen is thus settled by convention to be 1, and by experiment we determine the equivalents of other elements or radicals. Thus, when hydric sulphate (hydrated sulphuric acid) acts on metallic zinc, for every

* Where the metal is one which acts on water (such as sodium), we have, of course, not the metal itself separated, but the products of its action on water; and similarly if the salt-radical is incapable of separate existence, we obtain the products of its decomposition.