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But it has become a maxim, in which all chemists have been contented to acquiesce, that a body is to be regarded as simple, because we are not able to shew that it is compound. Yet a juster maxim would be, that a body is to be regarded as compound, when we are not able to prove it to be simple ; for the reasons for believing a body to be derived from others more simple than itself, are founded on far stronger analogies than any which have yet conducted us to the opposite conclusion. If it can be shewn that the bodies which we term simple, cannot be placed under an order of natural products, different from those which we know to be compound, we are not entitled to infer that a body is simple, because we have been unable to prove it compound by the agents of the laboratory.
It is surprising that the rule, that a body is to be regarded as simple, because it has not been decomposed, should have been so generally admitted into chemistry, seeing that various chemists have from time to time maintained the opinion, that some at least of the simple bodies ought to be regarded as compound. Thus, Baron Berzelius has given the strongest reasons for believing nitrogen to be a compound body, and has expressed his doubts even as to the composition of the metals. Sir Humphry Davy, in the earlier period of his chemical inquiries, was conducted to the opinion that the bodies sulphur and phosphorus, which give off hydrogen under the influence of voltaic action, might be compound. He even expressed the opinion that all the simple bodies might be compound, and resolvable into hydrogen and some unknown base. He never, however, pursued his own hypothesis to its consequences; and at length he seems to have abandoned it altogether; for no chemist applied more rigidly the rule, that a body was to be held to be simple which could not be decomposed. Upon this principle he established the
simpleconstitution of nitrogen, which he suspected to be compound, but which did not yield to the agents of decomposition employed by him, and of chlorine, which, mainly upon his high authority, chemists gradually consented to admit amongst the simple bodies. Having proved, by a series of brilliant experiments, that certain substances, the earths and alkalies, which had before been held to be simple, were really compound, he endeavoured to prove, that another body, Iodine, which presented all the characters of a compound body, was really simple. This substance, when first discovered by a manufacturer of saltpetre in Paris, was believed by every one be a new compound. M. Gay-Lussac, and almost at the same time Sir Humphry Davy, examined it with rigid care. M. Gay-Lussac contented himself with giving an account, the most minute that has yet been given, of its properties. Davy, finding it to resist all the agents which he employed to decompose it, pronounced it to be a simple body, according, as he himself expresses it, “ to the just logic of chemical philosophy.” The conclusion was acquiesced in by all chemists; but the question was not the more determined, whether this conclusion was arrived at by the rules of a sound logic or by the admission of an erroneous dogma.
The substance in question has many of the characters of chlorine, both in its own actions and in those of its compounds, and it is never found in the natural state but associated with chlorine, or where chlorine may have been. It scarcely differs from chlorine in its properties, more than one body sometimes differs from itself, according to the mode in which it is prepared ; and yet, in opposition to analogies of this kind, iodine was at once received into the list of simple bodies, and thus admitted to be not only distinct from chlorine, inasmuch as it was equally a simple product of nature, but to differ, with respect to its molecular constitution, from the whole class of bodies which we term compound, inasmuch as it was simple, and they were derivative.
Of the bodies termed simple, there are now reckoned 55, some of them derived from a few minerals of the rarest occurrence, and some of them found only in minute quantity, associated with some other substance, which they resemble in their general characters. The latest discovered, Lantanum, was derived from two or three mineral species, all resembling one another, found in the primary rocks of Scandinavia and Greenland, and of the rarest occurrence. But this substance, having resisted the usual agents of the laboratory, was at once pronounced to be simple. Nay, more, from the same minerals which yielded lantanum, had been derived another body, Cerium, which had long been admitted into every system of chemistry, as a simple body. These two substances resemble one another so nearly, that they are only to be distinguished by some slight differences in fusibility and minor characters, not so great as one body not unfrequently presents, according to the temperature to which it is exposed, and present too many points of resemblance, to allow us to infer that they are distinct from one another, any more than that they are distinct from other bodies which we term compound. Their rarity in the mineral kingdom, and their occurrence in masses of primary rock, which have been subjected, in the revolutions of the earth, to conditions of pressure and temperature, which we cannot produce by art, supply us with abundant reasons for believing them to be the products of pre-existing matter, and not natural elements distinct from one another, and from other bodies, in their molecular constitution. The opposite conclusion arrived at, should convince us that the rule which we have adopted is unsound, and is arrived at, not by “ the just logic of chemical philosophy,” but by a
chemical dogma, which ought long ere now to have been banished from the science into which it has been introduced.
In all our inquiries into the chemical nature of bodies, we naturally seek to resolve one substance into others more simple than itself. This is analysis, to which we add synthesis as the highest proof that can be given of analytical conclusions. All the great results which we have yet obtained with regard to the nature of bodies, have been arrived at by following this natural train of investigation. We derive one substance from another, until we arrive at roots or elements which we cannot further decompose. But this proves, in no degree, that the law of continuity is broken at the point where our means of further analysis fail. If we have not the experiments of the laboratory to direct us, we have our reasoning powers. We have induction and analogy, and no one will contend that we ought to reject the aid which these afford us in judging of the unknown from the known.
It is a rule in physics, that we are not to assign more causes for the explanation of an effect, than are necessary to explain it. Now, to explain the varieties of material bodies, as produced by combination one with another, we resort to the supposition of 55 roots or elements. But the supposition of the existence of two distinct roots would equally solve the problem. If we suppose two bodies to exist, A and B, each with its own molecules, we can conceive any
number of combinations to be formed. For if A combines with B, there is formed a third body with its own particles A+B. But A+B may combine with A, and then there is formed a fourth body A+B+ A, or with B, and there is formed a fifth body, A+B+B; and A + B + A may combine with A + B + B, and this new compound again with A or B, and so on through any number of combinations. It suffices, therefore, to account for every possible combination, that we assume the existence of two bodies.
Nor need our generalization stop even at this point. We may admit the possibility of the production of all material bodies from the same order of molecules, that is, from a single body. We know that there are substances as identical in composition as the nicest processes of the laboratory can determine, and yet different in their properties. But bodies different in properties are different bodies, for bodies are only distinguishable from one another by their properties. Similar molecules, therefore, may combine, and form different bodies. The difference between the bodies produced
be ascribed to a difference in the modes of combination, or to the action of unknown forces on the ultimate particles of matter, or to any other cause ; but our conclusion is not affected by our hypothesis as to the cause. Having two distinct bodies, however, produced, it is manifest that we can conceive any number of combinations to be formed, that is, any number of bodies to be produced.
All material bodies have a common class of characters; and it is a perfectly natural supposition, that they are all derived from a common order of particles or molecules ; and that all the differences which we find to exist between bodies, arise from the different relations of these particles or molecules to one another, and not from a difference in their own nature. This, at least, is a natural idea, and must have suggested itself, in some form or another, to most persons who have pursued this class of inquiries. That matter has been derived from pre-existing particles, was an idea perhaps coeval with the earliest period of philosophical inquiry. When Democritus and the earlier atomists of Greece assumed the existence of pristine elements, they followed out a perfectly natural train of thought, however they may have disfigured the hypothesis by their own bold imaginations.