One of the earliest questions asked by an intelligent child is : "What is this made of?";
"What is that made of?" - And the answer is generally more or less satisfactory.
For example, if the question relates to butter, the reply may be, " From milk." It may be explained, besides, that when milk is beaten up, or churned, the butter separates, leaving skim-milk behind. But the question has not been answered. The child may ask, "Was the butter in the milk before it was churned? Or has it been made out of the milk by the churning?". Possibly the person to whom the question is addressed may know that the milk contained the butter in the state of fine globules, and that the process of churning breaks up the globules, and causes them to stick together. The original question has not really been answered ; and indeed it is not an easy one to reply to. Precisely such questions suggested themselves to the people of old, and they led to many speculations.
The Chemical Elements
One of these speculations was that things which we see around us were built up out of elements, just as a word is built up out of letters. Indeed, the word elements, which is the Latin word for element, is probably derived from the letters l, m, and n, and involves that idea. The ancient Greeks surmised that there were four such elements earth, water, air, and fire. But as it was obvious that some things, for instance gold and silver, did not contain either water or air, the word element was often used to signify, not the constituent of a thing, but rather a property of a thing ; and it might have been said that gold partook of the properties of earth and water, because, like earth, it is not altered by being heated, and yet it can take a fluid form like water if heated hot enough. Hence the old word " element" had a double meaning; it was sometimes used in the sense of " constituent," and sometimes more in the sense of " property."
If a child is given a mechanical toy, his wish to see how it works generally leads to his taking it to bits. This is unfortunately only too easy ; but it is seldom that he succeeds in putting it together again. Now, if we inquire what a piece of wood or stone is made of, we can, after a fashion, take them to bits ; we may pull the wood into fibres, or we may crush the stone, and pick out the pieces that appear to differ from each other in colour, if they are large enough. But the fibres have much the same appearance as the piece of wood, and the fragments of stone, though somewhat different from each other, are still pieces of stone. The question is still to be answered, of what do wood and stone consist? It is evident that some plan must be tried by which the wood and stone will be unbuilt, as it were, and by which they will yield their constituents.
It had long been noticed that many things are greatly changed when heated. A piece of wood takes fire and burns ; some kinds of stone melt ; some metals, such as lead and iron, change into earthy-coloured powders. Surely these changes ought to lead to some knowledge of the nature of wood, stone, and metals. It was long, however, before it was recognized that the presence or absence of air made a difference in the result of heating substances. When attention was drawn to this difference, a new sug gestion was adopted. It was, that things, besides consist ing of or sharing the properties of earth, water, air, and fire, also consist of, or at least are like, salt, sulphur, and mercury. Salt dissolves when put into water ; so do many other things. These things must either contain a kind of salt to account for this property; or they must at least share the property of salt, in so far as they dissolve. Similarly, other things, especially metals, must either contain or share the property of mercury, seeing that they shine with the same kind of lustre ; and many things resemble sulphur in so far as they burn and produce a smell in burning. And it was often imagined that when things burn, the sulphur which they contain flies away and disappears, just as ordinary sulphur, when set on fire, burns away completely, leaving nothing behind. About the middle of the seventeenth century, Johann Joachim Becher, a German alchemist, altered somewhat the conception that substances contain, or are like, salt, sulphur, and mercury ; he imagined all things existing on the surface of the globe to contain three earths, namely the mercurial, the glassy, and the fatty, the last implying the property of being able to burn. And in. The early years of the eighteenth century, Becher's pupil, George Ernest Stahl, who was Professor of Medicine in Jena, and later in Halle, two small German towns, made an important addition to the ancient theories, namely, that it was possible to restore the "sulphur," or the " fatty earth," as Becher called it, to things which had been deprived of it by burning, by heating them with other substances rich in that constituent:
We have seen, how the idea of an "element" as a constituent of compounds gradually became more defined. As fresh discoveries were made, it was found that certain substances could not further be decomposed, yielding new constituents. But it is not easy always to determine whether or no a substance is an element. For certain compounds are very stable, that is, are very difficult to decompose ; and it has happened several times that such com pounds were mistaken for elements. A remarkable instance is a copper-coloured body, found in the debris left in the hearth of an old iron furnace, which was for long supposed to be the element titanium ; more careful investigation, however, proved it to be a compound of titanium with nitrogen and carbon.
Methods of Preparing Elements
There are three methods by which elements have been prepared, and all elements have been made by one of these methods. They are :
(i) Separation of the Element by Means of an Electric Current
We have already seen that the com pound 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 dis charge 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 sodiom ion 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.
( 2 ) Separation of an Element from a Compound by Heat
There appears to be little doubt that at a suffi ciently high temperature all compounds would be decom posed 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 decom posed. But certain compounds, like silica or quartz, for example, are so stable that they resist the highest tempera ture 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 358, 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.
(3) Separation of an Element from a Compound by Displacement
On heating one element with a com pound 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 com bination 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 ions and sections, I and Na. On adding to it a solution of chlorine in water, in which there are certainly many non-ionised chlorine molecules, C1 , molecular iodine, I I is set free, while ionised chlorine, Cl, goes into solution. The free iodine forms a brown solu tion, 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 dis placed 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 :
1. Free hydrogen at a red heat which displaces elements from their oxides or chlorides.
2. 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.
3. Metallic sodium, which displaces elements from their chlorides or fluorides.
4. Metallic magnesium, which displaces elements from their chlorides or oxides.
5. Metallic aluminium, which displaces elements from their oxides.
6. Metallic iron, which displaces elements from their
sulphides.
7. Fluorine, which displaces oxygen from water ; chlorine in sunlight, which acts slowly in the same way ; chlorine displaces bromine, and bromine, iodine.
8. 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 some times difBcult to decide whether or not an element has really been set free. Experiments on its compounds are often required to decide the question.
Classification of Elements
For long it had been noted that certain elements displayed a marked similarity with each other. Thus the metals sodium and potassium, discovered by Sir Humphry Davy, are both white, soft, easily oxidisable metals, forming soluble salts with almost all acids ; these salts resemble each other in colour, in crystalline form, and in other properties. The subsequently discovered metals, lithium, rubidium, and caesium, have also a strong resemblance to potassium and sodium. Their atomic weights also increase progressively ; thus we have the series, Li = 7, Na= 23, K = 39-i, Rb = 85, and Cs = 133.
Similar series had been noticed with calcium, strontium, and barium ; magnesium, zinc, and cadmium ; and so on. It was not until 1863 that John Newlands called attention to the fact that if the elements be arranged in the order of their atomic weights a curious fact becomes notice able. It is that, omitting hydrogen, the first, the eighth, the fifteenth, and, in short, all elements may be so arranged that the " difference between the number of the lowest member of a group and that immediately above it is 7 ;
