There are three methods by which elements have been prepared, and all elements have been made by one of these methods. They are:—
We have already seen that the compound must be ionised, and this is attained only by dissolving it in water or some other appropriate solvent, or by fusing it. It is the act of solution or of fusion which ionises the compound ; and the effect of the current is to direct the ions towards one or other electrode, and discharge them ; they then assume the form of the free element. It is necessary, in order that this method shall succeed, that the discharged ion shall not act on the solvent, nor on the electrode. It is impossible, for instance, to deposit sodium from an aqueous solution of any of its salts, for no sooner is the sodion discharged than it is attacked by the water ; hydrogen is evolved in equivalent amount to the sodium, and sodium hydroxide is produced, in which the sodium has taken the place of one of the hydrogen atoms in water; its formula is therefore NaOH. Chlorine, too, cannot be produced by the electrolysis of a chloride, if the anode is of iron, for example, for it at once unites with the iron, and forms a chloride of that metal instead of coming off as an element.
There appears to be little doubt that at a sufficiently high temperature all compounds would be decomposed into their elements: in the sun, which possesses a temperature much higher than can be reached by any means at our disposal, it is probable that all compounds are decomposed. But certain compounds, like silica or quartz, for example, are so stable that they resist the highest temperature which we can give them, without any change, except fusion and volatilisation. There is, moreover, another reason why this process often fails to isolate an element. The compound may be decomposed on heating, but its constituents may re-unite on cooling, unless one of them is more volatile than the other, and removes itself from the sphere of action. For these reasons this process of obtaining elements is of somewhat limited application. But it forms the most convenient method of preparing oxygen ; for example, if oxide of mercury be heated, it decomposes into gaseous oxygen, the boiling-point of which lies far below atmospheric temperature, —182°; while the mercury, which boils at 3580, although it volatilises at the temperature requisite to effect the decomposition of the oxide, condenses in the flask or tube in which the oxide is heated. Sulphide of gold, too, can be separated into gold and sulphur on being heated; for while sulphur boils at 446°, the boiling-point of gold is probably not much below 2000°.
On heating one element with a compound of another element, it not infrequently happens that the element in combination is displaced and liberated, while the other element takes its place in the compound. This is doubtless an ionic phenomenon ; one element—that in combination—being ionised, and hence electrically charged, exchanges its charge with the added element, which in its turn becomes ionised. A solution of iodide of sodium, for example, contains iodions and sodious, I and Na. On adding to it a solution of chlorine in water, in which there are certainly many non-ionised chlorine molecules, Cl0, molecular iodine, I—I is set free, while ionised chlorine, CI, goes into solution. The free iodine forms a brown solution, or, if much is present, a black precipitate. Again, when metallic sodium is heated with magnesium chloride to a red heat, globules of metallic magnesium are set free, while the sodium enters into combination with the chlorine. It may be supposed that on fusion the magnesium chloride contains some ions of chlorine and magnesium ; the non-ionised sodium takes the charge of the ionised magnesium, while the latter metal is liberated in an non-ionised state. But it may be objected that only those magnesium ions which exist as such should exchange their charges with the sodium ; that is true ; but when they have done so others become ionised and undergo a similar change; for if the temperature be kept constant, the ratio between the number of the ionised atoms of magnesium and the non-ionised atoms of magnesium in the chloride must remain constant, so that when the magnesium ions are replaced by sodium ions, other molecules of magnesium chloride become ionised to keep up the balance.
The element carbon is most frequently used to displace other elements. In its case, little or nothing is known of the electrical actions ; but if analogy may be taken as a guide, its action may be attributed to a similar exchange of electric charge between the displaced element and the carbon. But here the carbon, as soon as it unites with the oxygen which was previously in combination with the displaced element, escapes in the form of gas, and the oxide of carbon is certainly not an ionised compound.
An essential condition for the preparation of elements by the method of displacement is that the element which it is proposed to prepare in the free state shall not itself combine with the element which is used to displace it. Thus, chlorine cannot be used to displace either carbon or sulphur from the compound of carbon with sulphur, bisulphide of carbon, since it itself combines with both the carbon and the sulphur, yielding chloride of sulphur together with chloride of carbon. In general, however, this difficulty does not occur.
The elements which are generally used for the displacement of others from their compounds are :—
Free Hydrogen At A Red Heat which displaces elements from their oxides or chlorides.
Ions Of Hydrogen, on the point of being discharged electrically, or hydrogen " in the nascent state," i.e. hydrogen being set free from its compounds by the action of a metal; it also displaces elements from their oxides or chlorides, or, in general, from their salts.
Metallic Sodium, which displaces elements from their chlorides or fluorides.
Metallic Magnesium, which displaces elements from their chlorides or oxides.
Metallic Aluminium, which displaces elements from their oxides.
Metallic Iron, which displaces elements from their sulphides.
Fluorine, which displaces oxygen from water ; chlorine in sunlight, which acts slowly in the same way ; chlorine displaces bromine, and bromine,iodine.
Carbon, which is the most generally employed agent for replacing other elements ; it combines with oxygen, forming carbonic oxide or carbonic anhydride gases, and liberating the element with which the oxygen was combined.
The question of cost or of convenience often decides as to which of these methods is used. In the sequel, only the more generally used plan will be described. It must be remembered, too, that the employment of these processes does not always lead to the isolation of the element; in many cases a compound is produced, containing less of the element which it was intended to remove ; and it is sometimes difficult to decide whether or not an element has really been set free. Experiments on its compounds are often required to decide the question.