tions are far surpassed by those of the thin laminæ. If we imagine one of the colours of the laminæ combined with another, we have the impression of a new tint. The combinations that may be obtained in this way are almost innumerable, and, it will be said, need well be so, in order to match the variety which nature exhibits. Such is our opinion too; but we shall not attempt to conceal the difficulty presented by the fact, that several of the natural colours, especially those of the metallic substances, have but a very slight resemblance to the colours of thin plates, among which it were vain to seek, for example, either the yellow of gold or the red of copper. The colours of the plates which approach them most nearly are found among the first seven or eight tints of the scale. The gold might be placed among the blond colours, and the copper among the tawny; but the difference is still so striking that it would be unwarrantable, before it is accounted for, to put entire and implicit confidence in the principle of the laminæ.
This principle requires, as a primary condition, that the integrant molecules of bodies should be transparent. It is true that almost all bodies reduced to a certain degree of tenuity are permeable to light; but it is equally true that the existence of a single body perfectly opaque and yet exhibiting a colour, would render it necessary to look for another principle of coloration besides Newton's, which is applicable only to diaphanous substances.
In my Memoirs on the electro-chemical appearances, I have shown that they are not exclusively produced by one of the poles of the pile. The appearances which constitute the chromatic scale are due to the electro-negative elements of the solution (oxygen and acid), which being transferred by the current to the positive pole, are there spread out into thin transparent films, from which all the colours of the scale arise. The electro-positive elements (such as hydrogen and the metallic bases) are, on the contrary, transferred to the negative pole, and there deposited in layers which never produce the colours of thin plates. Here it is impossible to mistake in any case, but more particularly in respect to the solutions of certain salts with a base of gold or of copper, which produce negative appearances invariably of the same colour as the metallic base. It cannot be said in this case that the substance has not been brought to the degree of tenuity necessary to render it transparent. The electro-chemical layers commence with the first degree of attenuation at the positive as well as the negative pole. If the layers of the positive pole produce the ordinary colours of the plates, while the oppo- site pole completely fails to present any other than that of the metallic base, it necessarily follows, either that these bases are perfectly opaque, or at least that their transparency is so imperfect as to render it impossible to apply the general laws to them, unless with very important restrictions. Indeed we have here a decisive proof that the colours
