Chemical Attraction.

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hollow, as in the former case. The same is seen if we dip a rod or slip of glass in mercury, there is a rounding of the liquid edge next the glass and a depression as if it were repelled by it. This is of course not the case. The explanation is simply that the adhesion to the glass is too weak to affect the shape which the cohesion imposes on the mercury.

It has been calculated that, as a general rule, if the adhesion of the liquid and solid be greater than half the cohesion of the liquid, · the solid will be wetted by it, and the liquid will rise in a tube of this substance; but if less than one half, the liquid cannot wet the solid, and there will be a depression of the liquid within a fine tube.

89. Certain modifications or peculiar manifestations of adhesion and capillarity exhibited in absorption, exhalation, imbibition, osmose, and diffusion of liquids and gases, will be dealt with when we have considered the properties of liquids and gases.

90. So much for the varied manifestations of molecular attraction. They come within the experience of every-day life, and are of the utmost importance in the multitude of human avocations.

But we have not yet done with attraction. When we pass to the confines of material existence we can still trace the operation of a similar principle, by which the ultimate parts of substance spontaneously group themselves in endless variety. This interatomic attraction is the living power of the material universe, and is the final source to which it is possible to trace any change in the world about us.

"Interatomic or Chemical attraction."

91. There are some points of resemblance between interatomic or chemical attraction and the forms of attraction already considered; the points of difference, however, are much more decided.

In the first place, the chemical affinity (as it is usually, though not quite correctly, designated) between atoms does not appear in any way to correspond to their gravity, but exhibits the most singular and unexpected variations of degree.

Secondly, it differs widely from cohesion or adhesion, inasmuch as these are more powerful between similar than between altogether dissimilar kinds of matter, while chemical attraction is stronger the more unlike the natures of the atoms. Between some atoms there

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Chemical Attraction.

seems to be absolutely no attraction at all, between others it is so violent that they appear eager to rush together.

Thus, sulphuric acid will not unite with gold, but it will very readily combine with iron or copper.

An acid of one kind does not combine chemically with an acid of another kind, but it will unite instantly with an alkali.

In all these cases of chemical union the properties of the compound are different from those of its constituents, while in a mechanical mixture each constituent retains its properties, so that they may be entirely separated from each other by mechanical processes. In a chemical compound such a separation cannot take place except by the exercise of chemical attraction in another form.

In

As an illustration, gunpowder is a mixture of nitre, sulphur, and charcoal. The nitre may be separated from it by solution in water, the sulphur by solution in benzole, and the charcoal remains. spite of mixture each substance retains its special properties. When a heat of 540° is applied to the mixture, the three consti"tuents are resolved into various chemical compounds of gases and vapours. The elements are the same, and the weight of the products is equal to the weight of the gunpowder; but the properties of these substances are entirely changed. The nitre, sulphur, and charcoal no longer exist as such in the products.

92. The distinguishing feature of chemical attraction is, therefore, that it destroys, or at least masks, the individuality of the atoms or molecules united.

This draws the line between adhesion and chemical attraction. When sugar is dissolved in water, every drop has at once the properties of water and of sweetness combined; the one is not lost or merged in the other, because the union is merely collateral; it is not one of fusion. If, again, we mix sand with powdered soda they may lie side by side unchanged for centuries; but the chemical union of the two produced by melting them under a strong heat turns them into transparent glass, a substance altogether different from either of the constituents.

Sulphuric acid unites with copper, and produces a beautiful transparent blue salt (commonly known as blue vitriol). By heating this blue salt to a moderate temperature it becomes perfectly white, and on adding to it colourless water it becomes again blue. Thus colour is imparted under chemical attraction by combining two colourless substances.

Changes in the Properties of Bodies.

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93. It is impossible to predict the effect of the chemical union of any two elements, although we may know all about them individually. Often the result is most surprising. Thus, the study of atomic actions is wholly an EXPERIMENTAL one.

Hence it follows that the properties of a chemical compound cannot be inferred from the properties of its constituents. Its physical condition, whether as a gas, liquid, or solid, and its chemical and physiological characters can be determined only by experiment.

Nitrogen and hydrogen are two comparatively inert gases, while carbon is an innoxious black solid. The combination of these three bodies produces a highly poisonous liquid-prussic acid. Hydrogen has no smell, and sulphur only a slight smell if rubbed ; when chemically combined, these substances produce a most offensive-smelling gas-the sulphide of hydrogen. Carbon, hydrogen, oxygen, and nitrogen are innoxious agents, and have no taste. When chemically combined in certain proportions they produce strychnia, remarkable for its intensely bitter taste and highly poisonous properties. Iron manifests magnetism most powerfully, and oxygen is the most magnetic of gases, yet these two bodies, when chemically united in the proportions of two of iron to three of oxygen, produce a compound in which no trace of magnetism can be discovered.

It is to be remarked that this change of properties is only temporary; when the chemical union is destroyed the bodies resume their original properties, and magnetic iron and oxygen may be re-obtained from the non-magnetic compound iron-pyrites.

Sugar is an innocent substance. By dissolving it in nitric acid of a certain strength it is not recovered as sugar, but re-appears in the form of crystalline prisms of that powerful poison, oxalic acid.

The constituents of gunpowder are not explosive or in any way dangerous if kept apart. When the nitre, sulphur, and charcoal are mixed in certain proportions and well incorporated in the state of the finest powder, they form, by the application of heat, a dangerously explosive substance; each solid cubic inch of gunpowder being suddenly converted into 2000 cubic inches of gaseous matter.

Glycerine is a perfectly innocent syrupy liquid obtained in the manufacture of soap. It has no explosive properties. When dropped into a mixture of nitric and sulphuric acids, it combines with some of the elements of the former and produces a brownish

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Chemical Attraction.

coloured liquid-nitro-glycerine, which explodes by concussion so violently as to destroy everything in contact with or near it.

So with all the other physical or chemical properties of compounds; they must be ascertained by actual trial, they can never be determined from a knowledge of the elements.

Sugar is but a union in certain proportions of pure carbon (or charcoal) with the elements of water.

Sulphur and quicksilver, when heated together, form the beautiful red pigment known as vermilion.

Sulphur and iron, combined at some remote period, have produced those gold-like cubes called iron pyrites which are seen in slates. Lead, with oxygen absorbed from the air or other source, forms the red lead used by painters.

94. One most important exception there is to the statement that the effects of the action of unknown elementary substances are incalculable, and it is this:

Atomic actions in no way interfere with the weights of the elements: the weight of any compound is just the sum of the weights of its constituent particles.

Unnecessary and self-evident as this statement may appear, it was long after the dawn of philosophy ere its true import was recognised, and now it may be said that on this simple hypothesis is built the whole structure of modern chemistry.

It is from this that the chemist infers the indestructibility of matter itself, and it is from this that he infers the independent existence of sixty-five different elementary substances.

Provided with an extremely delicate balance or means of detecting changes of weight, the chemist causes different chemical agents to react on each other within a vessel or envelope, which is unaltered by his operations, or he causes matter to be dissipated into vapour and pass from view altogether, and finding not the smallest change of weight to follow, he concludes that the quantity of matter is unchanged, and unchangeable by any kind of chemical action.

So, again, assuming that an increase of weight always indicates an addition of matter, and a loss of weight always a withdrawal of matter, he concludes that iron, gold, oxygen, and hydrogen, are elementary substances. For by no process of heating or chemical action can he separate an ounce of iron, say, into two quite different substances whose weights shall be together an ounce; and when an apparent decomposition takes place, as in the rusting of iron, he

Laws of Combination.

39

finds on weighing that, the mass is heavier than before, and concludes that the rust is really a more complex and not a more simple substance than iron itself.

95. The most striking feature of chemical combination is that the union cannot be made, as in mechanical mixtures and solutions, to take place in any proportions. We may dissolve a small piece of sugar in a cup of tea, or we may dissolve several large lumps. The only difference will be that each drop in the latter case will be much sweeter. Ultimately we may have a limit, when a drop refuses to become more sweet, that is to say, when the adhesion between the liquid and the sugar just balances the cohesion of the latter.

In real chemical combinations, however, the limits are quite definite. Thus, if we mix three measures of hydrogen with one of oxygen, and apply a light, we find that all the oxygen disappears, but that an unemployed measure of hydrogen remains. The two will combine to form water vapour in no other proportion than that of two of hydrogen to one of oxygen, whatever quantities of each there may be.

So the proportion of elements in any compound is always the same: it owes all its qualities, its very existence, to the union of the elements in certain special proportions. There is no accidental or indifferent mixture, with a corresponding gradation of properties.

96. Sometimes, indeed, two elements will combine in different ways under different circumstances; but the transition of proportions and of physical properties is always abrupt and at the same time perfectly definite.

As a general rule, when two elements may unite in several ways, they invariably follow definite proportions. A single measure of the one combines with one, two or more of the other, or two measures of the first combine with one, three, or five of the second, and so on. We never have a complicated numerical relation between the combining measures.

97. Further, there is an equal simplicity of relationship between the measures in which two elements combine, and those in which each of them combines with a third. Two measures of hydrogen, as we have seen, go with one of oxygen to form water; now, if we try how these unite with a third element, say carbon, or sulphur, we find that a measure of carbon unites either with two of oxygen, or with four of hydrogen, producing in the former case carbonic acid gas, and in the latter marsh gas (so called because it is copiously produced by decomposing vegetable matter in boggy or marshy places); or again, one of oxygen unites with one of carbon, to form