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burnt, op the soot caught. The soots produced in com-mon chimneys, from different kinds of wood, resinousand not resinous, dry and green, do not differ near somuch from one another, as those which are producedfrom one kind of wood, in a common chimney and in themore confined way of burning above-mentioned.

III. Black metallic calces.

The mineral cobalt , roasted till its arsenical parts are dis-sipated, becomes black; and being then melted with in-flammable fluxes, yields a regulus, which likewise assumesa black colour by calcination. The same reguluS is ob-tained from the artificial zaffre , whose basis is the roastedcobalt, and which is employed for tinging glass blue; asalso from the deep blue glass itself, called, when ground,by the painters/;W/, and by the laundresses powder-blue.The cobalt, more valuable for these important products thanfor the property which occasions it to be here taken noticeof, and which has hitherto been afforded ^hiefly by Saxony,has of late, by the encouragement of the Society for pro-moting arts, been discovered in our own country: infurther searches for it, the property here mentioned maybe of great assistance, those minerals, and those only, whichcalcine black, promising to be useful cobalts. Calces ofiron, whether red, yellow, or of other colours, on beingbrought into fusion by the addition of vitreous bodies, givealways a black colour to the glass if the quantity of iron isconsiderable. From copper also a black colour may be pro-duced by fire, and applied to the staining and embellishingof certain stones, of which an account will be given here-after towards the end of this history. I have not observedthat any of the other metallic bodies are changed black inany circumstances by simple heat.

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