PRODUCED BY ELECTRO-CHEMICAL ACTION. Ill 



velocity. Does not the necessity of this condition show why these 

 laj'ers, 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 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 dif- 

 ferent 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 ^vith 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 verj^ 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 li(|uid film does not affect the phsenomenon, 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 phsenomenon. 



Below the violet tlie 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 tiie experiment of the humid films no changes are visible, — none at 



