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.176 Argilläceous Genui. Of Clays in general. The confiituent parts of clays are argill and filiceous Sones, ftotti the fize öf^ v tö of an inch. All other ingredients (except water), and there are many, are extraneous to its compo- fition, The proportion of thefe 2 ingredients to each other is variable; fometimes the argill exceeds, but moft commonly the filiceous ingredient pre- dominates. When the argill exceeds, its propor tion to filex fcarce ever reaches, but fometimes approaches, the ratio of 4 to 1; but moie fre- quently fubfills in the ratio of 3 to 1, or 2 to 1, or 1,5 to r, or approaching flill nearer to equali- ty. When the filiceous ingredient exceeds, which is by far the commor,eft cafe, it is found at nioft, and very rarely in the proportion of 6 to 1, more commonly in that of 4 to. 1; and flill oftener in that of 3 to 1, or2to 1, more rarely it approach- ts an equality. Thefe proportions produce different efledts. Thus the clays, in which the filiceous ingredient enters in the proportion of from 3 or 4 to 1, are beft for porcelain, thofe in which the argill ex- ceeds are beft fov coarfepottery; and particularly for the glafs-houfe crucibles, as being lefs acted on by alkalis The kind of filiceous ftone which conftitutes the fandy part of clay, fliould alfo be attended to ; I believe it to be more frequently quartz, * 14 Ann. Cby. 143. 3 Neu. Entdeck. 13. 35. but
