Page images

multiple of the original combining proportional, or to bear some simple relation to it.

But it must be observed that certain acid elements combine with 2, others again with 3 equivalents of basic element : such are the elements oxygen, sulphur, selenium and tellurium, which combine in the proportion of 1 volume or 1 equivalent with 2 volumes or 2 equivalents of hydrogen, sodium, &c., and are consequently sometimes called biatomic, from their being thus equal to two combining proportions of chlorine or its congeners; while the members of a third class of acid elements, as nitrogen, phosphorus and arsenic, unite in the proportion of 1 volume or 1 equivalent with 3 volumes or 3 equivalents of basic element, and are said to be triatomic, that is, equal to 3 equivalents of chlorine, bromine, &c. The nature of the combinations which the second or biatomic class of acid elements forms, is exemplified in the following Table :

Formula. | Proportion by Weight. | Proportion by Volume. Water, or oxide of hy- ) H, 0 H, = 2 0=16 H, =2 0=l

drogen ............... J Oxide of cadmium ...... cd, cd, =112 O=16 cd, =2 0=1 Hydrosulphuric acid ... H, S H, = 2 S=32 H, =2 S=1

Sulphide of mercury ... Hg, S Hg,=200 $ =32 Hg,=2 S=1. When these combinations take place, if the body produced remains a gas, it is found to be condensed to grds the volume of its gaseous constituents, i. e. the 3 volumes of which it was composed occupy, after combination, the space of only 2.

The combinations formed by the members of the third class of acid-radicals, those to which the name “ triatomic” has been assigned, are shown below; thus

Formula. | Proportion by Weight. | Proportion by Volume. Ammonia ............ H, N |H, = 3 N =14 H, =3 N =1 Phosphide of copper Cu, P Cug=96 P =31 Cuq=3 P =1

Arsenide of nickel ... Ni, As Ni, =88.5 As=75 | Nig=3 As=1. When it is possible to examine this third class of compound substances in the gaseous state, its members also are found to have suffered condensation, and that to one-half the gaseous bulk of

their components: thus the 2 volumes of the condensed compound contain the 4 volumes of which it is composed.

In this manner we find three series of bodies remarkable in their relations of weight and volume.

1st series : suffers no condensation in combination,

[merged small][ocr errors][merged small]

2nd series: suffers a condensation to two-thirds,

3 vols. become HH0

H, 01. 3rd series : suffers a condensation to one-half,— 4 vols.

become 2 vols. |HHHN

| H, N. The different saturating power possessed by the three classes of these acid elements, and shown by the formulæ given above (HCI, H, O, H, N, &c.), is not confined to them alone, but is manifested, as will be seen presently, in numerous compound molecules, which present analogous features, and are capable of replacing the simple bodies just alluded to.

The term “salt” is generally applied to those of the above combinations in which the properties of the constituents are completely, or nearly completely masked and merged in those of the new substance, which is thus in every way dissimilar from its components; thus the combination of potassium and chlorine, or sodium and iodine, gives rise to substances in which no property of the component elements is retained; the metal in each case has lost its metallic character, and has no longer any tendency to combine with a further quantity of the acid element, while the salt-radical has entirely parted with its odour and its distinguishing chemical properties, and cannot be expelled from the combination into which it has entered by the application of the highest temperature. The compounds formed in either instance being soluble, are also found to have acquired the peculiar taste which is well known as “saline,” and are no longer such powerful chemical agents as their uncombined constituents.

The remarks now about to be made apply chiefly to that class of bodies in which the opposed elements which constitute the salt are united simply in the relation of their saturating power, for to these the term “salt” or “saline combination” is almost exclusively applied; water (H,0), for example, is not the only oxide of hydrogen; there is another, the peroxide of hydrogen (H,02), in which it will be observed that the double biatomic molecule 0, (0,=H.) is only combined with the double monatomic molecule of hydrogen, H,; the compound H, 0, cannot therefore be said to contain the elements hydrogen and oxygen in the simple relation of their saturating power, at least as that has been determined by such observations as those just detailed. Cases of combination of this nature will be presently adverted to.

With regard, however, to this simplest class of compounds now under consideration, the perfect neutralization of the chemical properties of their constituents only holds good, as was just now stated, when the intensity of the antagonism of the two bodies is nicely balanced; but regarded theoretically, the constitution of a salt does not depend upon the nice adjustment of this equilibrium, it simply requires the union of an acid with a basic element; and viewed in this light, the combinations of oxygen with metals, or of hydrogen with salt-radicals, are true saline compounds, although with the more basic metals the feebly acid character of the oxygen is more than counterbalanced, and the compound remains basic, while the intensely acid character of the more powerful salt-radicals overcomes the weak basic properties of the hydrogen, and such combinations are therefore found to be acid. To illustrate these facts, and to exhibit the differences incident upon the varying apportioning of basic and acid element, the following cases are given :

Class I. Class II. Class III.
Basic ........ ?

Neutral ...... NaCl HO
Acid ........HCI

In the first of these classes the compound with preponderance of base is wanting, because the intensely powerful acid element is capable of completely neutralizing the most energetic basic element; in the second class, the acid compound is deficient, because the less powerful acid element is incapable of effecting more than the neutralization of a less energetic basic element, hydrogen; in the third class, the very weak acid element is incapable even of the neutralization of the basic constituent, hydrogen, and consequently both the acid and neutral compounds are absent; at least it is probable that such is the case, although the inertness or insolubility of the members of this series (Ag, P, H, As, &c.) does not allow accurate observations of their relations to neutrality to be made.

Hitherto we have been regarding the multiplication of saturating power on the part of the acid elements only, but the same thing occurs also in the case of many basic elements. Two instances, those of the metals iron and copper, will suffice to show this, the comparison being made in each case with salts of the same acid-radical, and in which the monatomic molecule of the metal is supposed to exist either singly or multiplied.


Ferrous*. ... Feci


Fe, P?
Ferric ...... Fe, C1, (Fe,),0, (Fe,)Pz.


Phosphide. Cuprous ....Cu, Cl (Cu),0 (Cu,), P Cupric...... Cuci

Cu, o

Cu, P. * It has been proposed by some chemists to assume, in the case of iron and of several other metals, two different atoms or equivalents, each conbining with the same weight of chlorine, &c., but themselves possessing a different weight. For instance, some have even said that the iron of a ferrous was not the same metal as the iron of a ferric salt: in the first case it would have the usual equivalent or atomic weight of 28, and in the second, the atomic weight 18.66, that is, rds of the first atom. The two molecules are respectively called ferrosum (Fe) and ferricum (fe), and are each equivalent to 1 atom of hydrogen, being each capable of combining with 1 atom of chlorine. Viewed

From the above Table it will be seen that the conventional expression for that salt which contains the greater number of equivalents of the basic, and the less number of equivalents of the acid element, is -ous salt, the other being termed the -ic salt.

But when we pass beyond the cases just cited, and examine the characters of compounds in which the basic element is combined with a yet larger quantity of the acid-radical, we find that the new compound is no longer neutral, but partakes of the acid character of the preponderating element. Thus, when in the union of chromium or iron with oxygen, the oxygen amounts to two equivalents, we find the new body possessing powerfully acid properties, and capable of uniting as a compound acid- (i. e. salt-) radical with hydrogen or the metals, thus

HCro, chromic acid. KCro, chromate of potassium.

HFeO, ferric acid. KFeO, ferrate of potassium. It must not be supposed that two equivalents is the largest quantity of oxygen that a compound acid-radical can contain: permanganic acid, for instance, is the hydrogen salt of a radical containing four equivalents of oxygen, thus HMn, 0, permanganic acid. KMnO, permanganate of potassium.

A precisely similar result occurs when, on the other hand, there is a preponderating number of equivalents of basic element in a compound. The compound radical ammonium, containing one equivalent of nitrogen and four of hydrogen, partakes in a striking manner of the basic properties of the hydrogen, which

in the light of this theory, the ferrous chloride remains FeCl, while the ferric chloride becomes feCl; and so on with the various oxides.

To the beginner this may perhaps be elucidated thus :

FeCl and Fe, Cl, are not comparable, but 3(FeCl) or Fe, Cl, is the true chemical representative of Fe, Clz.

Therefore, Fe, is equal in saturating power to Feg, for Fe,=28 x3, and Fe,=28 X2 are each equal to Clz=35.5x3:

Therefore, say some, the metals are different and have different equivalents, which may be obtained by dividing each formula by 3,—

[ocr errors]
[merged small][ocr errors]
« PreviousContinue »