It is only certain kinds of atoms that will combine at all with other kinds; and when they do so combine, they will only unite in absolutely fixed proportions, so that chemists have been able to assign to every kind of element its own combining proportion. The substances that will combine will do so in these proportions, or in proportions of any _even multiple_ of the number, and in no other. Thus fourteen parts of nitrogen will combine with sixteen of oxygen; and we have several substances in nature, called nitrous oxide, nitric oxide, nitric di-oxide, &c., which illustrate this, in which fourteen parts of nitrogen combine with sixteen oxygen or fourteen nitrogen with a multiple of sixteen oxygen, or a multiple of fourteen nitrogen combine with sixteen oxygen, and so on. See now where we have got to. When we had spoken of a tiny fragment of primal matter--a drop of water, for instance--it seemed as if there was no more to be said; but no, we found ourselves able to give a whole history of the molecules of which the substance consists; and when we had considered the molecule, we found a further beautiful and intricate order of _atoms_ inside the molecule, as it were. And there is no reason to suppose that science has yet revealed all that is possible to be known about atoms and molecules; so that if further wonders should be evoked, the argument will grow and grow in cumulative force. Let me sum up the conclusion to be drawn from these facts in a quotation from a discourse of Sir John F.W. Herschel. "When we see," says that eminent philosopher, "a great number of things precisely alike, we do not believe this similarity to have originated except from _a common principle independent of them_; and that we recognize this likeness, chiefly by the _identity of their deportment under similar circumstances_ strengthens rather than weakens the conclusion. "A line of spinning jennies, or a regiment of soldiers dressed exactly alike and going through precisely the same evolutions, gives us no idea of independent existence: we must see them act out of concert before we can believe them to have independent wills and properties not impressed on them from without. "And this conclusion, which would be strong even if there were only two individuals precisely alike in _all_ respects and _for ever_, acquires irresistible force when their number is multiplied beyond the power of imagination to conceive. "If we mistake not, then, the discoveries alluded to effectually destroy the ideas of an _eternal_ self-existent matter by giving to each of its atoms the essential characters at once of a _manufactured_ article and of a _subordinate agent_."