90 .OYSTERS, AND ALL ABOUT THEM. 



ing, the grooves and the colours resulting from them will 

 still be apparent. Still further : if the surface have anv 

 scratches or dents, the bottoms and sides are grooved just 

 as if the surface had been level. 



It follows from these facts that any mechanical contri- 

 vance by which similar grooves can be produced, must give 

 rise to similar results ; and this has been strikingly con- 

 firmed by Mr. Barton, of the Royal Mint. This gentleman 

 constructed an engine by which he can engrave, on the 

 surface of steel and other metals, lines so exquisitely 

 minute that from 2,000 to 10,000 are included in a single 

 inch. Such surfaces, when viewed on a cloudy day, pre- 

 sent but few appearances of colour ; but, when the light of 

 the sun or of gas falls upon them, an extremely brilliant 

 display of colours is the result : every gradation of tint is 

 exhibited, and a change is produced by every motion, 

 either of the object or the source of the illumination. 



Aquafortis or nitric acid, and the cutting engines of the 

 lapidary will respectively separate mother-of-pearl, which is 

 of a scaly, laminated structure, into a number of thin films, 

 each surface of which presents the same beautifully coloured 

 appearance. These are applied to a large variety of orna- 

 mental purposes, in the same way as tortoiseshell or the 

 veneers of fancy woods, among which are handles for 

 pen-knives, combs, buttons, spoons, and the inlaying of 

 work-boxes and dressing-cases. 



Devices or patterns are often drawn upon the films or 

 plates with opaque turpentine varnish. The shell is then 

 repeatedly brushed over with strong nitric acid by which 

 the parts, which have not been covered with the varnish, 

 are eaten away. An oil of turpentine being employed to 

 wash off the varnish, the device which had been traced 



