“PRODUCED BY ELECTRO-CHEMICAL ACTION. 111 
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 elec- 
tricity? 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 un- 
attainable in this circumstance. 
Nature presents in the specular iron a beautiful instance of the co- 
loration 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 onas 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 dif- 
ferent effect: it would have destroyed their metallic brilliancy, and 
corroded their surfaces by the ordinary process of oxidation. 
1 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 com- 
parison can be instantaneously made, and the difference between the 
two tints is so striking, that we are at a loss to conceive how a trans- 
parent 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 phenomenon, and 
the colour undergoes the same change whether it be a thin film ora 
considerable mass. When transparent solids, such as glass, erystals, 
&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 phenomenon. . 
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 
