Of Simple Earths. 9 Earth of Alum, or Argilh This earth forms the bafis of common alum, and hence is by many called aluminous earth; it is alfo the true clayey part of common clay ; and hence, to diftinguifh it from the other hete- rogeneous parts, it is called argill. It is feldom found nearly pure, and never abfolutely fo; the pureft is extricated from alum by precipitadon with the cauftic volatile alkali; it is very white, fmooth, and undtuous; its fpecific gravity, when thoroughly dry, and free from air, is 2,00 ; it is very diffufible, but not more foluble in water than pure magnefia. . It is combinable with mofl acids, but (except in the adt of precipitation) with great difficulty; with the vitriolic it forms alum; with the nitrous 30 uT natiC h difficuitl y cr yftallizers. i keated 't hardens and diminifhes in u i ut it is infufible in the ftrongeft heat of our furnaces: the heat, however, given out by pure air appears to affedt it, and difpofe it to tulion, more than it does any of the before-men- tioned earths, Microcofmic falt feems the flux bell fuited to g 1 > it diuolves it with confiderable effer- w S° rax is nearl y as povverful, and pro- duces lefs effervefcence; but alkalis adf nearly as on the preceding earths. ! ble o7 aäing on'aigiU. 3 " ^ In .ts Uiual ftate of drynefs it is capable ol abforb.ng Ilme , its wcight of P“ e ° , loffcmg any to drop out, and rctains it, Xr expofec