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14 FORMATION OF CHEMICAL COMPOUNDS. [7.

as the solid which furnished it:—Into a glass flask A (fig. 2), of

the capacity of about 250 cubic inches, or 4 litres, and which is

provided with a brass cap and stop-cock, introduce 10 or 12 grains

(o"6 or o*7 gramme) of gun-cotton; at

FlCK 2' tach the flask to the air-pump, exhaust it

completely, and afterwards weigh it. Then

set fire to the cotton by means of a

voltaic current sent through the wires,

a, b, which are insulated from each other

and from the cap of the instrument, by

passing through a varnished cork. The

cotton will entirely disappear with a

brilliant flash, but the dash, if weighed

again,will be found to beof the same weight

as it was before the cotton was fired.

v. There are two modes of forming chemical compounds; the simplest is that in which the two substances unite directly together, as when hydrogen burns in air, and, by direct union with oxygen, produces water; or when an acid and an alkali, such as hydrochloric acid and ammonia, combine and produce a salt. This mode of combination usually prevails between bodies which have a powerful tendency to unite. The other mode is still more common: it occurs when one of the ingredients of a compound is displaced by another substance, and a new compound is the result. The instances already specified, in which one metal precipitates another from its solution, are cases in which new bodies are produced by the displacement of one of the substances in a compound previously formed. This method of forming compounds by displacement, or substitution, is one of great importance; and the study of its various modes of action is rapidly contributing to the discovery of many subtle processes concerned in the chemistry of organized beings.

vi. Chemical combination, in a large proportion of cases, does not commence spontaneously. A heap of charcoal may remain unaltered in the air for years; but, if a few fragments of it be made red hot, chemical action will begin at the spot to which the heat is applied, and it will continue until the whole mass is burned; that is, the chemical action between the oxygen of the air and the charcoal will continue as long as any charcoal is left. In other instances, however, the chemical effects begin more readily. A piece of phosphorus begins to be oxidized slowly the instant it comes into the atmosphere, and in warm weather it

/] FORMATION OF CHEMICAL COMPOUNDS. 15

often bursts into a blaze. Again zinc ethyl, a liquid containing zinc, carbon, and hydrogen, inflames on being brought in contact with the air at ordinary temperatures, though at low temperatures oxidation takes place less rapidly and without inflammation. In all these cases, therefore, heat is necessary to start the action; the difference being one of degree and not of kind.

vii. Whenever substances unite directly with each other, heat is emitted, and the more rapidly the union is effected, the larger is the quantity of heat emitted in a given time, until, in some cases, it rises so high as to produce ignition and combustion; light and heat are abundantly evolved when the temperature rises high enough, since all substances, when heated beyond a certain point, become luminous.

When compounds are formed by substitution, the liberation of heat is usually much less, and is sometimes not perceptible without special contrivances.

viii. The physical state of one or of both the bodies which enter into combination is frequently altered by the operation of chemical attraction. Two solids may become converted into a liquid; two liquids, or even two gases, may become solid. Differences of state are therefore not in all cases due to differences of temperature; differences in the chemical arrangement of the particles are equally important in bringing about physical differences of condition.

The foregoing leading characters, by which chemical attraction is distinguished from other sources, may be thus summed

Chemical attraction is an action of extreme energy, which acts only on the minutest particles of matter, and at distances too small to be perceptible. Under its influence the elementary bodies, though comparatively few in number, arrange themselves into the unmberless compounds which constitute the different forms of matter in the three great kingdoms of nature; and it is important to observe that the proportions in which they unite are fixed and invariable. Chemical attraction operates between the particles of dissimilar kinds of matter and by its exertion produces new properties in the resulting compound. It exists between different kinds of matter with different but definite degrees of intensity. As a result of its operations, no destruction of matter occurs in the materials submitted to its influence; there is consequently no loss of weight, but only change of form. The act of combination may either occur instantly on mixture, or it may be indefinitely postponed till some other action, such as heat, con

16 LAW OF DEFINITE PROPORTIONS. [7.

curs to commence the change. Compounds may be formed either by the direct union of their ingredients, or by the displacement of one substance by a different one in a compound previously formed; and lastly, heat and light, in amount proportioned to the rapidity of the action, are generally emitted in cases of the direct union of the constituents.

(8) Laws of Combination.—The relative proportion in which the different elements unite is regulated by fixed laws. These laws, which form the basis of chemical science, are three in number, and they regulate the mode of combination of every known chemical compound. They are usually termed the laws of chemical combination.

(9) The first of these laws, the law of Definite Proportions, may be stated in very few words; it is as follows:—In every chemical compound the nature and the proportions of the constituent elements are definite and invariable. For instance, ioo parts of water contain 88^9 of oxygen and iii of hydrogen, or the weight of the oxygen is always exactly eight times that of the hydrogen. Whether water be derived from the snows of high mountains, from rain clouds, from dew, or from direct chemical action, as when the hydrogen of a burning lamp or candle unites with the oxygen of the air, its composition is uniform and certain. So also a piece of flint, or of rock crystal, in whatever part of the world it be found, will, on analysis, yield in every 100 parts 46"7 of silicon and 53*3 of oxygen. So also hydrochloric acid gas, however obtained, always contains in Joo parts 97*26 of chlorine and 274 of hydrogen. In fact, experiment shows that all true chemical compounds which have been submitted to analysis have a composition equally definite. It is this law of definite proportions which gives value to analysis, by affording certainty and uniformity to its results. Mere mechanical intermixture is at once distinguished from true chemical combination by the absence of all regularity in the proportions of the bodies that have been mingled: and in the same manner chemical attraction stands strongly contrasted with that kind of adhesion which produces the solution of solids in a liquid, or the intermixture of two liquids like spirit of wine and water with each other.

(10) The second law of combination is the law of Multiple Proportions. It frequently happens that the same pair of elementary bodies unites together in more than one proportion. The compounds so obtained are usually very different from each other; but there is always a regularity in the plan upon which these

10.] LAW OF MULTIPLE PROPORTIONS. 17

compounds are formed, and the ratios of the two elements in each are very simply related. This law may be thus stated :— When two elements, A and B, unite together in more ratios than one, if we compare together quantities of each of the resulting compounds which contains the same amount of A, the quantities of B will bear a very simple relation to each other; such as

A + B, A + 3B, A + 5B, &c;

or, A + 2B, A + 4B, A + 6B, &c.

or, 2A4-3B, 2A+5B, 3A+7B, &c;

Water, for instance, is a compound of oxygen and hydrogen; in ico parts there are, as already mentioned, 88-9 of oxygen and in of hydrogen. But there is another compound of oxygen and hydrogen known to chemists, termed hydric peroxide, or hydroxyl. By analysis it has been found that 100 parts of this body contain 94'1 of oxygen and 5*9 of hydrogen. Now, on comparing together the quantities of oxygen which in these two compounds are united with an equal quantity, say 2 grammes of hydrogen, it is evident that in water, for 2 grms. of hydrogen there are 16 of oxygen,

since in : 88-9 :: 3 : 16

and by a similar process it is seen that in hydric peroxide, for 2 grms. of hydrogen 32 grms. (or 16 x 2) of oxygen are present:—

59 ■■ 94"i :: a : 32

the quantity of oxygen combined with the hydrogen in the peroxide being just double of that combined with the same quantity of hydrogen in water.

Mercury in like manner forms two compounds with chlorine— viz., calomel, and corrosive sublimate. In calomel 200 grammes of mercury are combined with 35*5 of chlorine; whilst in corrosive sublimate the 200 of mercury are united with 71 grammes of chlorine, or with just twice as much.

A similar simple ratio between the quantities of the combining elements is found to hold good in every series of compounds formed by the union of two elements with each other. A certain quantity of one of the elements combines with a certain quantity of the other: in the next compound with twice as much as in the first; in the next with three times as much; in the next with four times as much, and so on.

An excellent example of this regularity is afforded by the series of compounds which nitrogen forms with oxygen. They are

18 LAW OP EQUIVALENT PROPORTIONS. [iO.

five in number, and contain in ioo parts of each, the following ratio of their constituents, the oxygen increasing in amount from the first to the fifth :—

[table]

Nitrous oxide
Nitric oxide ....
Nitrous anhydride .
Peroxide of nitrogen'
Nitric anhydride

Now on comparing quantities of each of these different compounds which contain equal quantities of nitrogen, and in the order in which the compounds stand in the foregoing table, it will be seen that the oxygen increases in quantity in the proportion of 1, 2, 3, 4, and 5. In the nitrous oxide the quantity of nitrogen combined with 16 parts of oxygen is 28, and taking this amount of nitrogen in each case, we obtain by proportion the following series:—

[table]

the quantity of the oxygen increasing progressively, in the proportion of 16, twice 16, 3 times 16, 4 times 16, and 5 times 16.

In some cases the ratio in which the elements unite is rather less simple, two parts of one element combining with 3, 5, or 7 of the other.

This important law was first clearly established by Dalton.

(11) The third law of combination is usually known as the Law of Equivalent Proportions. It may be stated as follows:— Each elementary substance, in combining with other elements, or in displacing others from their combinations, does so in a fixed ratio, which may be represented numerically.

This principle of equivalent ratio may be illustrated by reference to the experiments upon the displacement of the metals from solutions of their nitrates (p. 12), or still more simply from solutions of their chlorides, by the introduction of some other metal, the attraction of which for the chlorine is stronger than that of the metal with which it is already combined. When, for instance, a slip of copper is introduced into a solution of mercuric chloride, the two metals change places, owing to the stronger

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