Part 13 (1/2)
When Davy saw these metallic globules burst through the crust of fusing potash, we are told by one of his biographers, ”he could not contain his joy, he actually bounded about the room in ecstatic delight; and some little time was required for him to compose himself sufficiently to continue the experiment.”
This was the culminating point of the researches in which he had been continuously engaged for about six years. His interest and excitement were intense; the Bakerian Lecture was written ”on the spur of the occasion, before the excitement of the mind had subsided,” yet, says his biographer--and we may well agree with him--”yet it bears proof only of the maturest judgment; the greater part of it is as remarkable for experimental accuracy as for logical precision.” But ”to every action there is an equal and opposite reaction:” immediately after the delivery of the lecture, Davy was prostrated by a severe attack of illness, which confined him to bed for nine weeks, and was very nearly proving fatal.
That the phenomenon just described was really the decomposition of potash, and the production of the metal of which this substance is an oxygenized compound, was proved by obtaining similar results whether plates of silver, copper, or gold, or vessels of plumbago, or even charcoal, were used to contain the potash, or whether the experiment was conducted in the air, or in a gla.s.s vessel from which air had been exhausted, or in gla.s.s tubes wherein the potash was confined by mercury. The decomposition of potash was followed within a few days by that of soda, from which substance metallic globules were obtained which took fire when exposed to the air.
But the a.n.a.lysis of potash and soda was not sufficient for Davy; he determined to accomplish the synthesis of these substances. For this purpose he collected small quant.i.ties of the newly discovered metals, by conducting the electrolysis of potash and soda under experimental conditions such that the metals, as soon as produced, were plunged under the surface of naphtha, a liquid which does not contain oxygen, and which protected them from the action of the surrounding air.
A weighed quant.i.ty of each metal was then heated in a stream of pure dry oxygen, the products were collected and weighed, and it was found that solutions of these products in water possessed all the properties of aqueous solutions of potash and soda.
The new metals were now obtained in larger quant.i.ty by Davy, and their properties carefully determined by him; they were named _pota.s.sium_ and _sodium_ respectively. They were shown to possess all those properties which were generally accepted as characteristic of metal, except that of being heavy. The new metals were extremely light, lighter than water. For some time it was difficult to convince all chemists that a metal could be a very light substance. We are a.s.sured that a friend of Davy, who was shown pota.s.sium for the first time, and was asked what kind of substance he supposed it to be, replied, ”It is metallic, to be sure;” ”and then, balancing it on his finger, he added in a tone of confidence, 'Bless me, how heavy it is!'”
Davy argued that since the alkalis, potash and soda, were found to be oxygen compounds of metals, the earths would probably also be found to be metallic oxides. In the year 1808 he succeeded in decomposing the three earths, lime, baryta and strontia, and in obtaining the metals _calcium_, _barium_ and _strontium_, but not in a perfectly pure condition, or in any quant.i.ty. He also got evidence of the decomposition of the earths silica, alumina, zirconia and beryllia, by the action of powerful electric currents, but he did not succeed in obtaining the supposed metallic bases of these substances.
So far Davy's discoveries had all tended to confirm the generally accepted view which regarded alkalis and earths as metallic oxides. But we found that the outcome of these views was to regard all salts--and among these, of course, common salt--as oxygen compounds.[11] Acids were oxygen compounds, bases were oxygen compounds, and as salts were produced by the union of acids with bases, they, too, must necessarily be oxygen compounds.
Berthollet had thrown doubt on the universality of Lavoisier's name ”oxygen,” _the_ acidifier, but he had not conclusively proved the existence of any acid which did not contain oxygen.
The researches of Davy naturally led him to consider the prevalent views regarding acids, bases and salts.
Muriatic (or as we now call it hydrochloric) acid had long been a stumbling-block to the thorough-going Lavoisierian chemists. Oxygen could not be detected in it, yet it ought to contain oxygen, because oxygen is the acidifier. Of course, if muriatic acid contains oxygen, the salts--muriates--produced by the action of this acid on alkalis and earths must also contain oxygen. Many years before this time the action of muriatic acid on manganese ore had been studied by the Swedish chemist Scheele, who had thus obtained a yellow-coloured gas with a very strong smell. Berthollet had shown that when a solution of this gas in water is exposed to sunlight, oxygen is evolved and muriatic acid is produced. The yellow gas was therefore supposed to be, and was called, ”oxidized muriatic acid,” and muriatic acid was itself regarded as composed of oxygen and an unknown substance or _radicle_.
In 1809 Gay-Lussac and Thenard found that one volume of hydrogen united with one volume of the so-called oxidized muriatic acid to form muriatic acid; the presence of hydrogen in this acid was therefore proved.
When Davy began (1810-11) to turn his attention specially to the study of salts, he adopted the generally accepted view that muriatic acid is a compound of oxygen and an unknown radicle, and that by the addition of oxygen to this compound oxidized muriatic acid is produced. But unless Davy could prove the presence of oxygen in muriatic acid he could not long hold the opinion that oxygen was really a const.i.tuent of this substance. He tried to obtain direct evidence of the presence of oxygen, but failed. He then set about comparing the action of muriatic acid on metals and metallic oxides with the action of the so-called oxidized muriatic acid on the same substances. He showed that salt-like compounds were produced by the action of oxidized muriatic acid either on metals or on the oxides of these metals, oxygen being evolved in the latter cases; and that the same compounds and water were produced by the action of muriatic acid on the same metallic oxides.
These results were most easily and readily explained by a.s.suming the so-called oxidized muriatic acid to be an elementary substance, and muriatic acid to be a compound of this element with hydrogen. To the new element thus discovered--for he who establishes the elementary nature of a substance may almost be regarded as its discoverer--Davy gave the name of _chlorine_, suggested by the yellow colour of the gas (from Greek, = _yellow_). He at once began to study the a.n.a.logies of chlorine, to find by experiment which elements it resembled, and so to cla.s.sify it. Many metals, he found, combined readily with chlorine, with evolution of heat and light.
It acted, like oxygen, as a supporter of combustion; it was, like oxygen, attracted towards the negative pole of the voltaic battery; its compound with hydrogen was an acid; hence said Davy chlorine, like oxygen, is a supporter of combustion and also an acidifier.
But it was very hard to get chemists to adopt these views. As Bacon says, ”If false facts in Nature be once on foot, what through neglect of examination, the countenance of antiquity, and the use made of them in discourse, they are scarce ever retracted.”
Chemists had long been accustomed to systems which pretended to explain all chemical facts. The phlogistic theory, which had tyrannized over chemistry, had been succeeded by the Lavoisierian chemistry, which recognized one acidifier, and this also the one supporter of combustion. To ascribe these properties to any element other than oxygen appeared almost profane.
But when Davy spoke of chlorine as an acidifier, he did not use this word in the same sense as that in which it was employed by the upholders of the oxygen theory of acids; he simply meant to express the fact that a compound containing chlorine as one of its const.i.tuents, but not containing oxygen, was a true acid. When Gay-Lussac attempted to prove that hydrogen is an _alkalizing principle_, Davy said, ”This is an attempt to introduce into chemistry a doctrine of occult qualities, and to refer to some mysterious and inexplicable energy what must depend upon a peculiar corpuscular arrangement.” And with regard to Gay-Lussac's strained use of a.n.a.logies between hydrogen compounds and alkalis, he says, ”The subst.i.tution of a.n.a.logy for fact is the bane of chemical philosophy; the legitimate use of a.n.a.logy is to connect facts together, and to guide to new experiments.”
But Davy's facts were so well established, and his experiments so convincing, that before two or three years had pa.s.sed, most chemists were persuaded that chlorine was an element--_i.e._ a substance which had never been decomposed--and that muriatic acid was a compound of this element with hydrogen.
Berzelius was among the last to adopt the new view. Wohler tells us that in the winter of 1823, when he was working in the laboratory of Berzelius, Anna, while was.h.i.+ng some basins, remarked that they smelt strongly of oxidized muriatic acid: ”Now,” said Berzelius, ”listen to me, Anna. Thou must no longer say 'oxidized muriatic acid,' but 'chlorine;' that is better.”
This work on chlorine was followed up, in 1813, by the proof that the cla.s.s of acidifiers and supporters of combustion contains a third elementary substance, viz. iodine. As Davy's views regarding acids and salts became developed, he seems to have more and more opposed the a.s.sumption that any one element is especially to be regarded as the acidifying element; but at the same time he seems to admit that most, if not all, acids contain hydrogen. Such oxides as sulphur trioxide, nitrogen pentoxide, etc., do not possess acid properties except in combination with water. But he of course did not say that all hydrogen compounds are acids; he rather regarded the possession by a substance of acid properties as dependent, to a great extent, on the nature of the elements other than hydrogen which it contained, or perhaps on the arrangement of all the elements in the particles of the acid. He regarded the hydrogen in an acid as capable of replacement by a metal, and to the metallic derivative--as it might be called--of the acid, thus produced, he gave the name of ”salt.” An acid might therefore be a compound of hydrogen with one other element--such were hydrochloric, hydriodic, hydrofluoric acids--or it might be a compound of hydrogen with two or more elements, of which one might or might not be oxygen--such were hydrocyanic acid and chloric or nitric acid. If the hydrogen in any of these acids were replaced by a metal a salt would be produced. A salt might therefore contain no oxygen, _e.g._ chloride or iodide of pota.s.sium; but in most cases salts did contain oxygen, _e.g._ chlorate or nitrate of pota.s.sium.
Acids were thus divided into oxyacids (or acids which contain oxygen) and acids containing no oxygen; the former cla.s.s including most of the known acids. The old view of salts as being compounds of acids (_i.e._ oxides of the non-metallic elements) and bases (_i.e._ oxides of metals) was overthrown, and salts came to be regarded as metallic derivatives of acids.
From this time, these terms--acids, salts, bases--become of less importance than they formerly were in the history of chemical advance.
In trying to explain Davy's electro-chemical theory I have applied the word _affinity_ to the mutual action and reaction between two substances which combine together to form a chemical compound. It is now necessary that we should look a little more closely into the history of this word _affinity_.
Oil and water do not mix together, but oil and potash solution do; the former may be said not to have, and the latter to have, an affinity one for the other. When sulphur is heated, the yellow odourless solid, seizing upon oxygen in the air, combines with it to produce a colourless strongly smelling gas. Sulphur and oxygen are said to have strong affinity for each other.
If equal weights of lime and magnesia be thrown into diluted nitric acid, after a time it is found that some of the lime, but very little of the magnesia, is dissolved. If an aqueous solution of lime be added to a solution of magnesia in nitric acid, the magnesia is precipitated in the form of an insoluble powder, while the lime remains dissolved in the acid.
It is said that lime has a stronger affinity for nitric acid than magnesia has. Such reactions as these used to be cited as examples of _single elective affinity_--single, because one substance combined with one other, and elective, because a substance seemed to choose between two others presented to it, and to combine with one to the exclusion of the other.
But if a neutral solution of magnesia in sulphuric acid is added to a neutral solution of lime in nitric acid, sulphate of lime and nitrate of magnesia are produced. The lime, it was said, leaves the nitric and goes to the sulphuric acid, which, having been deserted by the magnesia, is ready to receive it; at the same time the nitric acid from which the lime has departed combines with the magnesia formerly held by the sulphuric acid. Such a reaction was said to be an instance of _double affinities_.