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salt is at once seen to consist of a collection of smaller solid particles, with intervening spaces; but the porosity of such bodies as water, spirits of wine, or iron, is not so obvious, although the existence of the property is not less certain. The Tig. I. porosity of spirit and of water may be shown as follows :— Take a long narrow tube with a couple of bulbs blown in it, and furnished with an accurately fitting stopper, as represented in fig. i; fill the tube and lower bulb with water, then carefully and completely fill up the upper bulb and neck with spirit of wine, and insert the stopper. The structure of the apparatus, and the different densities of the two liquids, prevent them from mixing; but on turn

King the tube upside down and back again three or four times, so as to mix the spirit and the water thoroughly, and theu holding the instrument with the bulbs downwards, an empty space will be seen in the tube after they have been thus mixed, showing that they now occupy less space than before; that their particles are in fact closer together. Proofs of porosity are afforded even by the metals; for example, many of them become more compact by hammering, as is the case with platinum; and all of them, not excepting platinum and gold, two of the densest forms of matter, however cold they may be, shrink into a smaller space when rendered still colder. The ultimate particles therefore cannot be in contact.

(4 a) Divisibility of Matter.—We have no means of determining the real size of the ultimate particles of matter, although, as will be seen hereafter, there are strong grounds for believing that the divisibility of matter, extreme as it is, has its assigned and definite limits. Experience, however, shows that whatever be the form of matter selected for experiment, its divisibility may be manifested to an extent which indicates further divisibility beyond our powers of conception. In the ordinary process of making gold leaf, for example, the gold is hammered out so thin that 280,000 leaves would be required to make up the thickness of an inch, and a single grain of gold is hammered out until it covers a square space seven inches in the side. Each square inch of this may be cut into 100 strips, and each strip into 100 pieces, each of which is distinctly visible to the unaided eye. A single grain of gold may thus, by mechanical means, be subdivided into 49 x 100 x 100 = 490,000 visible pieces. But this is not all; if attached to a piece of glass, this gold leaf may be subdivided still further; io,coo parallel lines may be ruled in


the space of one single inch, so that a square inch of gold leaf, weighing ^ of a grain, may be cut into io,coo times 10,000, or 100,000,000 pieces, or an entire grain into 4,900,000,000 fragments—each of which is visible by means of the microscope. Yet ire are quite sure that we have not even approached the possible limits of subdivision, because, in coating silver wire, the covering of gold is far thinner than the gold leaf originally attached to it, since in drawing down the gilt wire the gold continues to become thinner and thinner each time, in proportion as the silver wire itself is reduced in thickness.

When a substance is dissolved in any liquid, the subdivision is carried still further, and the particles are rendered so minute as to escape our eyesight even when aided by the most powerful magnifiers.

(5) Varieties of Attraction.—Mere mechanical subdivision, or even the more perfect separation of the particles of a compound body, by the process of solution, does not put us in possession of the simple substances from which the compound is formed. A piece of loaf-sugar may be reduced, by trituration, to an impalpable powder, but every particle of that powder will still be sugar; it may be dissolved in water, but each drop of the liquid will still contain sugar, unaltered except in appearance. Sugar is composed of three elements—carbon, hydrogen, and oxygen; but trituration or solution in water will not enable us to extract any of these substances from it. The molecule, or minutest particle of sugar which can exist, is still a compound body, and still contains its constituent carbon, hydrogen, and oxygen.

The existence of a body as a solid in one continuous mass is owing to cohesion—an attraction of considerable intensity, but which varies in degree in different bodies, and by this variation produces varieties in the toughness, hardness, and brittleness of bodies. But the cause which unites the various chemical elements—such as the carbon, hydrogen, and oxygen of sugar— to form a new compound, endowed with properties entirely different from those of any of its constituents, is of a .different nature from cohesion, and of a more subtle kind. Chemical attraction (or affinity, as it is often, but not very philosophically termed) is the cause which unites the elements into compound bodies. It is exerted between the smallest or ultimate particles of one element, and the corresponding particles of the other elements with which it is associated in the particular compound under examination. These ultimate particles are often spoken of "atoms, a term derived from the Greek arofioq, 'indivisible,' which


implies that the particles admit of no further subdivision. We may, in fact, contrast the effects of chemical attraction with those of cohesion, by stating that the molecule of a body is formed by the union of atoms under the influence of chemical attraction, whilst the mass is formed by the union of molecules under the influence of cohesion.

The separation of a body into its constituents is the business of chemical analysis (from ava, 'up,' or 'backwards,' and \vaig, 'separation'), and it has for its object, first, the determination of the nature of the components—this is qualitative analysis; secondly, the determination of their quantity—this being quantitative analysis. The successful performance of these operations of analysis requires an extensive acquaintance with the principles and the facts of the science, combined with considerable skill in manipulation, or the management of chemical apparatus and processes.

On the other hand, chemical synthesis (from avv, 'together/ OeotQ, 'putting'), the production of new compounds by the union of their elements, is an operation the reverse of analysis, and constitutes another equally important portion of the labours of the chemist.

(6) General Characters of Acids, Alkalies, and Salts.—It will assist the comprehension of our remarks on chemical attraction if we allude briefly to the general characters of three very important classes of compounds—viz., acids, alkalies, and salts.

The term acid was originally applied to certain substances which are soluble in water, have a sour taste, and exert such an action on vegetable blue colours as to change them to red. For example, tincture of litmus, which is of a blue colour, is exceedingly sensitive to the action of an acid: paper stained with this tincture is in frequent use by the chemist for detecting the presence of acids.

The term alkali is of Arabic origin: it was given in the first instance to carbonate of soda, or sodic carbonate, which was then obtained from the ashes of sea-weeds; but it is now extended to a class of substances possessing many qualities exactly the reverse of those which belong to the acids. An alkali is soluble in water, and produces a liquid soapy to the touch, and of a peculiar, nauseous taste; it restores the blue colour to vegetable infusions which have been reddened by an acid; it turns many of these blue colours into green, as in the cases of the solutions of red cabbage and of syrup of violets; and it gives a brown colour to vegetable yellows, such as those of turmeric and rhubarb. Litmus paper


which has been feebly reddened by an acid affords a ready test of the presence of an alkali, and is more sensitive than paper stained with turmeric or with rhubarb, which is also in common use for the same purpose. These different test papers, as they are called, show whether an acid or an alkali predominates in a solution.

Vinegar or acetic acid, oil of vitriol or sulphuric acid, muriatic or hydrochloric acid, aquafortis or nitric acid, are familiar instances of the class of acids. Potash, soda, and hartshorn or ammonia, are instances of well-known alkalies.

Many of the acids and all the alkalies are remarkable for their great chemical activity. Nitric acid attacks copper quickly and violently, with brisk effervescence, and the copious escape of red fumes, whilst a blue liquid is formed from the action. Sulphuric acid shows similar energy, if mixed with water and placed in contact with iron or zinc. Moreover, both these acids, when not much diluted with water, produce speedy destruction of the texture of nearly all animal and vegetable matters. The solvent action of potash, or of soda, is not less marked. Either of these alkalies destroys the skin if allowed to remain upon it; and also gradually dissolves portions of earthenware, or of glaze from the vessels which contain it, and the solution, if suffered to fall upon a painted surface, quickly removes the paint. But the most remarkable property of acids and alkalies is the power which they have of acting upon each other, and destroying or neutralizing the chemical activity which distinguishes them when separate.

Some of these properties of acids and alkalies may be submitted to experiment by means of a coloured vegetable solution, such, for example, as the purplish liquid prepared by slicing a red cabbage and boiling it with water. If a quantity of this infusion be divided into two portions, and to the one be added a quantity of diluted sulphuric acid, a red liquid is obtained; and if to the other a solution of caustic potash be added, a liquid of a green colour is formed: then, on gradually pouring the alkaline into the acid solution, stirring the mixture constantly, the green colour of the portions first added instantly disappears, and the whole liquid remains red; as more and more of the alkali is added, the red passes by degrees into purple, and on continuing to add the alkaline solution, a point is attained when the liquid has a clear blue tint: at this moment there is neither potash nor sulphuric acid in excess in the liquid, the two having chemically acted upon one another, and the characteristic properties of both have disappeared. On evaporating the solution at a gentle heat, a solid


crystalline substance, resulting from the chemical action of the sulphuric acid upon the potash, is obtained. This substance is the salt called sulphate of potash, or potassic sulphate. For the present it may be sufficient to state, that any compound produced by the action of an acid on an alkali, or rather, that is the result of the action of an acid upon a base, is termed a salt. Other modes of forming salts will be mentioned hereafter.

It must not be supposed that all acids closely resemble those which have just been mentioned, and which are freely soluble in water; some acids, like the citric, tartaric, and oxalic, may be readily obtained in crystals: other acids are not at all soluble in water; as, for example, metastannic acid, the white substance obtained by the action of nitric acid upon tin. The leading character of an acid, in a chemical sense, is its power of reacting with alkalies to form salts; and this character is possessed by various bodies not familiarly regarded as acids. Of course if an acid be insoluble, it has no sourness, and is without action on vegetable bluesy

There are no insoluble alkalies, but there are substances

* Most chemists, following Gerhardt's practice of limiting the title of acid to a particular class of substances which contain hydrogen, now regard all true acids as salts of hydrogen. Formerly many bodies, such as silica or white arsenic, were regarded as acids, though, if we adopt the foregoing definition, they are not really so until they have combined with water. Such bodies, since they contain no hydrogen, are now distinguished as anhydrides; the substance familiarly known as carbonic acid must, upon this principle, be termed carbonic anhydride.

The characters of acids and of salts, and their various modifications, will be more fully discussed hereafter, when the general properties of the metals and of their compounds are considered.

It may, however, be useful in this place to quote the definitions of the terms acid, base, and salt given by Dr. Frankland in his ' Lecture Notes for Chemical Students':—' An acid may be defined as a compound containing one or more atoms of hydrogen which become displaced by a metal when the latter is presented to the compound in the form of a hydrate. The hydrogen capable of being so displaced may be conveniently termed displaceable hydrogen. An acid containing one such atom is said to be monobasic, two such atoms, dibasic, Ac. Acids of a greater basicity than unity are frequently termed polybasic acids'

'The term base is applied to three classes of compounds, all of which are converted into salts by the action of acids. These are :—1st. Certain compounds of metals with oxygen, such as sodic oxide (Na 0), zinc oxide (Zn O), &c. 2nd. Certain compounds of metals with the compound radical hydroxyl (HO), such as sodic hydrate (Na (HO)). zincic hydrate (Zn (H0)„), &c. 3rd. Certain compounds of nitrogen, phosphorus, arsenic, and antimony, such as ammonia (NHt).'

'By the mutual action of an acid and a base upon each other, a salt is produced, (see following note).

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