ple substances are really simple. Each yields a spectrum having lines varying in number from two to eighty—lines every one of which implies the intercepting of ethereal undulations of a certain order by something oscillating in unison or in harmony with them. Were iron absolutely elementary, it is not conceivable that its atom could intercept ethereal undulations of eighty different orders: though it does not follow that its molecule contains as many separate atoms as there are lines in its spectrum, it must clearly be a complex molecule. The evidence thus gained points to the conclusion that, out of some primordial unit, the so-called elements arise by compounding and recompounding; just as by the compounding and recompounding of elements there arise oxides, and acids, and salts. And this hypothesis is entirely in harmony with the phenomena of allotropy. Various so-called elementary substances have several forms under which they present quite different properties. The semitransparent, colorless, extremely active substance commonly called phosphorus may be so changed as to become opaque, dark red, and inert. Like changes are known to occur in some gaseous, non-metallic elements, as oxygen; and also in metallic elements, as antimony. These total changes of properties, brought about without any changes to be called chemical, are interpretable only as due to molecular arrangements; and, by showing that difference of property is produced by difference of arrangement, they support the inference otherwise to be drawn, that the properties of different elements result from differences of arrangement produced by the compounding and recompounding of ultimate homogeneous units. Thus Mr. Martineau's objection, which at best would imply a turning of our ignorance of the nature of elements into positive knowledge that they are simple, is, in fact, to be met by two sets of evidences, which distinctly imply that they are compound.
Mr. Martineau next alleges that a fatal difficulty is put in the way of the General Doctrine of Evolution by the existence of a chasm between the living and the not-living. He says: "But with all your enlargement of data, turn them as you will, at the end of every passage which they explore, the door of life is closed against them still." Here again our ignorance is employed to play the part of knowledge: the fact that we do not know distinctly how an alleged transition has taken place is transformed into the fact that no transition has taken place. We have over again the mode of argument which until lately was thought conclusive—because the genesis of each species of creature had not been explained, therefore each species must be specially created. Merely noting this, however, I go on to remark that scientific discovery is day by day narrowing the chasm, or, to use Mr. Martineau's metaphor, "opening the door." Not many years since, it was held as certain that the chemical compounds distinguished as organic could not be formed artificially. Now, more than a thousand organic compounds have been formed artificially. Chemists have discovered the art of
