Page:Scientific Memoirs, Vol. 1 (1837).djvu/123

Rh velocity. Does not the necessity of this condition show why these layers, in order to produce the desired effect, should be brought into contact with the metallic surface by the agency either of fire or electricity? The humid way is perhaps too tedious in all cases; it gradually oxidizes the surfaces of the metals, but never covers them with that thin and extended veil, the application of which requires a rapidity unattainable in this circumstance.

Nature presents in the specular iron a beautiful instance of the coloration which we have been considering. The ordinary colour of this ore is an iron gray; yet the faces of its crystals often display beautiful tints of every kind. They commence, in general, with the blue (No. 13) of the second, and go on as far as the reds (37 and 38) of the third order. These colours change as those of the scale do, and are so very like them that I thought they might be successfully imitated. I was not mistaken: a crystal of specular iron coloured by nature could not be distinguished from one coloured by the application of the electro-chemical process. There is no doubt as to the origin of these crystals; they are produced by fire, and it is that which has given them their colour by covering their surfaces with thin layers analogous to the electro-chemical. The humid way would have produced a very different effect: it would have destroyed their metallic brilliancy, and corroded their surfaces by the ordinary process of oxidation.

Singular Property of some Tints of the Scale.

A drop of alcohol is let fall on the violet (No. 11), and spread so as to cover part of the colour. The part thus made wet is no longer the same: we see instead of it a feeble tint resembling that of coffee mixed with milk; but the other part remains unchanged. The comparison can be instantaneously made, and the difference between the two tints is so striking, that we are at a loss to conceive how a transparent and very limpid film of alcohol can produce such a change in the violet colour on which it is placed. The alcohol gradually evaporates, and the colour recovers its former brilliancy.

Water, oil, and the different saline solutions produce the same effect; the thickness of the liquid film does not affect the phænomenon, and the colour undergoes the same change whether it be a thin film or a considerable mass. When transparent solids, such as glass, crystals, &c., are laid over the violet colour, it suffers no change. The liquids with which the plate is overspread adhere to its surface, so that this condition seems necessary to the production of the phænomenon.

Below the violet the indigo No. 12 and the blue No. 13, and (yet lower down) the red No. 10, the ochres Nos. 8 and 9 are subject to very marked variations. In the other colours of the scale when submitted to the experiment of the humid films no changes are visible, — none at