Part 13 (2/2)
The chemical changes were caused, it was said, by the simultaneous affinity of lime for sulphuric acid, which was greater than its affinity for nitric acid, and the affinity of magnesia for nitric acid, which was greater than its affinity for sulphuric acid.
If a number of salts were mixed, each base--supposing the foregoing statements to be correct--would form a compound with that acid for which it had the greatest affinity. It should then be possible to draw up tables of affinity. Such tables were indeed prepared. Here is an example:--
_Sulphuric Acid._
Baryta. Lime.
Strontia. Ammonia.
Potash. Magnesia.
Soda.
This table tells us that the affinity of baryta for sulphuric acid is greater than that of strontia for the same acid, that of strontia greater than that of potash, and so on. It also tells that potash will decompose a compound of sulphuric acid and soda, just as soda will decompose a compound of the same acid with lime, or strontia will decompose a compound with potash, etc.
But Berthollet showed in the early years of this century that a large quant.i.ty of a body having a weak affinity for another will suffice to decompose a small quant.i.ty of a compound of this other with a third body for which it has a strong affinity. He showed, that is, that the formation or non-formation of a compound is dependent not only on the so-called affinities between the const.i.tuents, but also on the relative quant.i.ties of these const.i.tuents. Berthollet and other chemists also showed that affinity is much conditioned by temperature; that is, that two substances which show no tendency towards chemical union at a low temperature may combine when the temperature is raised. He, and they, also proved that the formation or non-formation of a compound is much influenced by its physical properties.
Thus, if two substances are mixed in solution, and if by their mutual action a substance can be produced which is insoluble in the liquids present, that substance is generally produced whether the affinity between the original pair of substances be strong or weak.
The outcome of Berthollet's work was that tables of affinity became almost valueless. To say that the affinity of this body for that was greater than its affinity for a third body was going beyond the facts, because the formation of this or that compound depended on many conditions much more complex than those connoted by the term ”affinity.” Yet the conception of affinity remained, although it could not be applied in so rigorous a way as had been done by the earlier chemists. If an element, A, readily combines with another element, B, under certain physical conditions, but does not, under the same conditions, combine with a third element, C, it may still be said that A and B have, and A and C have not, an affinity for each other.
This general conception of affinity was applied by Berzelius to the atoms of elements. Affinity, said Berzelius, acts between unlike atoms, and causes them to unite to form a compound atom, unlike either of the original atoms; cohesion, on the other hand, acts between like atoms, causing them to hold together without producing any change in their properties. Affinity varies in different elements. Thus the affinity of gold for oxygen is very small; hence it is that gold is found in the earth in the metallic state, while iron, having a great affinity for oxygen, soon rusts when exposed to air, or when buried in the earth. Pota.s.sium and sodium have great affinities for oxygen, chlorine, etc.; yet the atoms of pota.s.sium and sodium do not themselves combine. The more any elements are alike chemically the smaller is their affinity for each other; the more any elements are chemically unlike the greater is their mutual affinity; but this affinity is modified by circ.u.mstances. Thus, said Berzelius, if equal numbers of atoms of A and B, having equal or nearly equal affinity for C, mutually react, compound atoms, AC and BC, will be produced, but atoms of A and B will remain. The amounts of AC and BC produced will be influenced by the greater or less affinity of A and B for C; but if there be a greater number of A than of B atoms, a greater amount of AC than of BC will be produced. In these cases all the reacting substances and the products of the actions are supposed to be liquids; but BC, if a solid substance, will be produced even if the affinity of A for C is greater than that of B for C.
In some elements, Berzelius taught, affinity slumbers, and can be awakened only by raising the temperature. Thus carbon in the form of coal has no affinity for oxygen at ordinary temperatures; it has remained for ages in the earth without undergoing oxidation; but when coal is heated the affinities of carbon are awakened, combination with oxygen occurs, and heat is produced.
But why is it that certain elementary atoms exhibit affinity for certain others? It depends, said Berzelius, on the electrical states of these atoms. According to the Berzelian theory, every elementary atom has attached to it a certain quant.i.ty of electricity, part of which is positive and part negative. This electricity is acc.u.mulated at two points on each atom, called respectively the positive pole and the negative pole; but in each atom one of these electricities so much preponderates over the other as to give the whole atom the character of either a positively or a negatively electrified body. When two atoms combine chemically the positive electricity in one neutralizes the negative electricity in the other. As we know that similar electricities repel, and opposite electricities attract each other, it follows that a markedly positive atom will exhibit strong affinity for a markedly negative atom, less strong affinity for a feebly negative, and little or no affinity for a positively electrified atom; but two similarly electrified atoms may exhibit affinity, because in every positive atom there is some negative electricity, as in every negative atom there is some positive electricity. Thus, in the atoms of copper and zinc positive electricity predominates, said Berzelius, but the zinc atoms are more positive than those of copper; hence, when the metals are brought into contact the negative electricity of the copper atoms is attracted and neutralized by the positive electricity of the zinc atoms, combination takes place, and the compound atom is still characterized by a predominance of positive electricity.
Hence Berzelius identified ”electrical polarity” with chemical affinity.
Every atom was regarded by him as _both_ positively _and_ negatively electrified; but as one of these electricities was always much stronger than the other, every atom regarded as a whole appeared to be _either_ positively _or_ negatively electrified. Positive atoms showed affinity for negative atoms, and _vice versa_. As a positive atom might become more positive by increasing the temperature of the atom, so might the affinity of this atom for that be more marked at high than at low temperatures.
Now, if two elementary atoms unite, the compound atom must--according to the Berzelian views--be characterized either by positive or negative electricity. This compound atom, if positive, will exhibit affinity for other compound atoms in which negative electricity predominates; if negative, it will exhibit affinity for other positively electrified compound atoms. If two compound atoms unite chemically, the complex atom so produced will, again, be characterized by one or other of the two electricities, and as it is positive or negative, so will it exhibit affinity for positively or negatively electrified complex atoms. Thus Berzelius and his followers regarded every compound atom, however complex, as essentially built up of two parts, one of which was positively and the other negatively electrified, and which were held together chemically by virtue of the mutual attractions of these electricities; they regarded every compound atom as a _dual_ structure. The cla.s.sification adopted by Berzelius was essentially a dualistic cla.s.sification. His system has always been known in chemistry as _dualism_.
Berzelius divided compound atoms (we should now say molecules) into three groups or orders--
_Compound atoms of the first order_, formed by the immediate combination of atoms of two, or in organic compounds of three, elementary substances.
_Compound atoms of the second order_, formed by the combination of atoms of an element with atoms of the first order, or by the combination of two or more atoms of the first order.
_Compound atoms of the third order_, formed by combination of two or more atoms of the second order.
When an atom of the third order was decomposed by an electric current, it split up, according to the Berzelian teaching, into atoms of the second order--some positively, others negatively electrified. When an atom of the second order was submitted to electrolysis, it decomposed into atoms of the first order--some positively, others negatively electrified.
Berzelius said that a base is an electro-positive oxide, and an acid is an electro-negative oxide. The more markedly positive an oxide is, the more basic it is; the more negative it is, the more is it characterized by acid properties.
One outcome of this teaching regarding acids and bases was to overthrow the Lavoisierian conception of oxygen as the acidifying element. Some oxides are positive, others negative, said Berzelius; but acids are characterized by negative electricity, therefore the presence of oxygen in a compound does not always confer on that compound acid properties.
We have already seen that silica was regarded by most chemists as a typical earth; but Berzelius found that in the electrolysis of compounds of silica, this substance appeared at the positive pole of the battery--that is, the atom of silica belonged to the negatively electrified order of atoms.
Silica was almost certainly an oxide; but electro-negative oxides are, as a cla.s.s, acids; therefore silica was probably an acid. The supposition of the acid character of silica was amply confirmed by the mineralogical a.n.a.lyses and experiments of Berzelius. He showed that most of the earthy minerals are compounds of silica with electro-positive metallic oxides, and that silica plays the part of an acid in these minerals; and in 1823 he obtained the element silicon, the oxide of which is silica. On this basis Berzelius reared a system of cla.s.sification in mineralogy which much aided the advance of that branch of natural science.
By the work of Berzelius and Davy the Lavoisierian conception of acid has now been much modified and extended; it has been rendered less rigid, and is therefore more likely than before to be a guide to fresh discoveries.
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