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SECT. XII.

Black glass and enamel.

T HERE is a species of blackness, as we have for-merly seen, which results, in certain circumstances,from the simple deepness or concentration of other colours.Thus many vegetable juices and infusions, yellow, reddish,blue, &c. on being evaporated to the thick consistence ofan. extract,, look black ; and these black masses, whenspread thin or diluted with water, exhibit again the ori-ginal colours of the liquors. Something of the same kindseems to happen in glass and enamel. Smalt or zaffre,which in a certain proportion give a blue colour to vitre-ous bodies, if employed in a larger quantity make themblack.. Manganese,, which in a little quantity gives apurplish tinge, in a large one gives a black. Preparationsof iron,, whose colour in glass, in a dilute state, is some-times yellow and sometimes greenish or bluish, are alwaysof a dark brown or black when the glass is over-dosedwith them : hence many of the ferrugineous earths andstones melt into a black glass, as the coloured clays, severalslates, and the stone called whynn stone, with which someof the streets of London have been lately paved. Blackglasses or enamels made on this principle have however,,like the concentrated vegetable liquids,.one imperfection;that though of a deep black colour when in masses of anyconsiderable thickness, yet when spread thin they alwaysbetray some of the original colour, or of the particularhue which they would have if the colouring matter wasin less quantity. The most perfect black is obtained byadding a mixture of two or more of the above darkeningmaterials : instead of taking colourless glass or enamel forthe basis, it will be of advantage to use fragments of dif-ferent