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constitutes so large a proportion of the compound ; and it is found to play the part of a base almost as energetically as potassium or sodium, combining with the acid-radicals to form very stable salts, the salts of ammonium.

From the capability of the basic elements of combining with every acid element, and vice versa, whether the opposing elements are unequal or equal in intensity of antagonism, results that large class of chemical changes known as “ double decompositions.” They arise in many instances from bringing together a pair of salts the constituents of which are not well apportioned to each other, but which, by an interchange of their basic and acid elements, would become exactly, or at least more nearly balanced in antagonism: let us take an extremely simple instance :

-oxide of sodium or soda (Na,0) is a salt in which the basic properties of the sodium preponderate vastly over the acid tendency of the oxygen; chloride of hydrogen or hydrochloric acid (HCI) is a salt in which the precise reverse obtains, as we have before seen ; now, if these compounds be mixed, the opposed elements of each being much more nearly the exact counterpart of one another, unite, forming two new compounds which are far more stable than those formerly existing, since the mutual antagonism of their components is more perfect :

Na, 0+2HCI=H,O+2NaCl. This apparent election, which bodies make when they are, as it were, at liberty to choose with what they will combine, has been variously termed “ elective affinity," “ chemical affinity,” and “ chemical attraction.” In reality, it is in every case the effect of many causes,--the resultant of many forces; we cannot, however, enter now into the details of their action, except so far as to state that in many double decompositions much depends upon insolubility, much upon volatility, and much upon mass.

It has been lately shown that double decompositions are of far more frequent occurrence in chemistry than was formerly imagined; it was thought that in many cases, especially where the elements were concerned, direct combination took place when bodies of opposite chemical characters were presented to each

other: thus, when chlorine and hydrogen are mixed in the sunlight, and hydrochloric acid (HCI) results from their union, it was imagined that a kind of combination occurred, differing from that mentioned just now, inasmuch as it consisted in a direct union of the molecules of hydrogen and chlorine. Recent experiments, however, appear to show that the molecular arrangement of the elements is not expressed truly when written H and Cl, but that their constitution is that of a binary molecule, a true salt, in fact, in which the hydrogen or chlorine, or whatever the element may be, is both base and acid ; and that thus when HCl is formed by H and Cl uniting, the action is as follows:

HH+C1 C1=HCI+HCI. Numerous instances might be adduced and proofs given, but are here unnecessary; let it suffice to say that this view seems to give one reason why elements in the nascent state are more powerful agents than yben liberated. It is argued since the element has not yet united with itself to form a twin atom, therefore its combining tendency is still wholly unsatisfied.

It will at once be seen that if this law of binary combination and double decomposition be one of those most deeply impressed upon matter, it is likely to prevail in the more complex chemical compounds. By chemical analysis it is found, that though among the large number of bodies possessing the general characters of salts many consist of but two elements, and those of the opposed classes, yet that there are many which have by no means so simple a constitution; these, however, participate in the same double decompositions as their congeners of a simpler form; and so, when their “elective affinity” is allowed full play, we find that they transfer to each other their basic and acid compounds, just as we have seen the simpler types of salts mutually interchange their basic and acid elements. The occurrence of basic compounds is rare; the bodies of this constitution belong almost exclusively to the domain of organic chemistry, and the student will for the present have little or nothing to do with them; with one, however, the compound base ammonium (NH), he will

speedily become acquainted; it is a substance of great importance
analytically, and is, moreover, a remarkable instance of the
manner in which the basic element hydrogen overcomes the
feebly acid properties of nitrogen, and thus confers on their com-
bination a basic character. The acid compounds are far more
numerous and important, and are either produced by the union
of the acid elements among themselves, or by their combination
with basic elements in such proportion as to allow of the pre-
ponderance of their own properties over those of the basic ele-
ments. It is singular also that many of these acid compounds
result from the combination of the two acid elements, sulphur and
oxygen, with the remaining bodies of their own class; the com-
position of a few such compounds is given below, with the names
which, by long usage, have become attached to them and cannot
be desirably removed, although they must be regarded as wholly
apart from any theoretical considerations touching the nature and
constitution of the bodies which they designate; the examples are
given as existing in saline combination with hydrogen :-
HNO,
HCIO

HC10,
Nitric acid. Hypochlorous acid. Chloric acid.
H, SO,
H, CO,

H, C, O,
Sulphuric acid. Carbonic acid. Oxalic acid.
H, PO,
H, Cfdy*

H, Cits Phosphoric acid. Hydroferricyanic acid. Citric acid. With regard to the nomenclature of these compounds, it may be as well remembered that (with one or two exceptions, as in the case of sulphocyanogen (CyS)), when the compound acid portion of a binary combination contains no oxygen or sulphur, the termination of its own name is altered into -ide when we wish to designate any compound which it forms with a basic element, thus:CN CN CI CI

PP Cyanogen. Chlorine. Sulphur. Phosphorus. KCN

NH, CI Fe, s Ag; P Cyanide of potas Chloride of Sulphide of Phosphide of sium.

ammonium. iron. silver. * Cfdy=Fe, Cg Ng.

† Ci=CpHOz.

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An exception is here made when these acid bodies unite with the basic element hydrogen; and though it is chemically correct to call such a compound as HCl “ chloride of hydrogen,” yet conventionality demands that it be also termed “hydrochloric or chlorhydric acid ;” and thus with all such compounds.

Now, if oxygen be present in the acid compound, a totally distinct nomenclature is adopted; these bodies combined with hydrogen constitute a very large section of the so-called acids, and receive either the termination -ic, as nitric acid (HNO3), or -ous, as hypochlorous acid (HC10). When the hydrogen in one of these groups has been replaced by another basic element, the designation that previously terminated in -ic now ends in -ate, and that in -ous in -ite; as nitrate of potassium (KNO2), and hypochlorite of calcium (CaC10). From the following Table of the hydrogen and potassium salts of some compound acid-radicals which contain sulphur and oxygen, the student will perceive the usage and changes of these terms:-

Hyposulphurous acid H, 8,02 Hyposulphite of potassium K, S, 0 Sulphurous acid ......H, SOg Sulphite of potassium ......K, SO Sulphuric acid .........H, SO Sulphate of potassium ......K, SO,

Here is a similar list, containing the hydrogen and potassium salts of a series of compound acid-radicals formed by the union of chlorine and oxygen :

Hypochlorous acid ...HCIO Hypochlorite of potassium...K CIO Chlorous acid............H CIO, Chlorite of potassium.........KCIO, Chloric acid ............H CIOZ Chlorate of potassium.........KCIO Perchloric acid .........H CIO Perchlorate of potassium ...KCIO,

It will have been observed, that in the Table of acids given at page 45 there are three classes of those bodies, distinguished by containing a different amount of hydrogen, that is, of basic element: these classes are the groups of monobasic, bibasic, and tribasic acids which contain acid-radicals requiring these different amounts of basic element to satisfy their respective combining powers; and if the student will refer to what was said regarding the three series of acid elements at page 38, he will at once perceive that these three series of compound acid-radicals accu

SILVER SALTS OF

rately correspond with them. This relation may be exhibited
thus:-
Series I.
Series II.

Series III.
ACID Monatomic.

Biatomic.

Triatomic.
ELEMENTS ) Chloride of silver. Sulphide of silver. Phosphide of silver.
Ag Cl
Ag, s

Ag, P
Bibasic.

Tribasic.
{ Ag Clo

Ag, SO4

Ag,PO, | COMPOUNDS | Chlorate of Silver. Sulphate of silver. Phosphate of silver.

ACID

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Monobasic.

The difference between a ferrous and a ferric, between a cuprous and a cupric salt of an elementary acid-radical, has been already pointed out; it now only remains to transfer this distinction to the corresponding salts of the compound acid-radicals. We will take potassium, iron, and bismuth as the basic elements in our salts; and, for the compound acid-radicals, those existing in the nitric, sulphuric, and phosphoric acids. Now, potassium is monatomic, i.e. K=H,; and iron is sometimes sesquiatomic, i.e.Fe,=H14, or Fe,=H; while bismuth is triatomic, i.e. Bi =H. Taking these values, we arrive at the following formulæ for the different salts of the same acids :

Series I.
Monatomic.

KNO,
Potassic nitrate.

Fe(NO),
Ferric nitrate.

Bi(NO)
Bismuthic nitrate.

Series II.
Biatomic.

K. SO
Potassic sulphate.

(Fe,), (SO2).
Ferric sulphate.

Bi,(SO2)
Bismuthic sulphate.

Series III.
Triatomic.

K PO,
Potassic phosphate.

Fe, PO
Ferric phosphate.

Bi PO,
Bismuthic phosphate.

Here it is seen how the formula of the same description of salt varies, not only with the saturating power of the acid body, but also with the atomic function of the basic constituent.

A word in explanation of a term in general use, the term “ basic salt:”—when an absolutely perfect double decomposition does not occur, the precipitate produced, instead of being a pure salt, is a compound one; this may be best illustrated by a hypo

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