18 Of the Formation of titones. Calces of iron are formed of iron, combirled with different proportions of pure air, and fre- quently of water alfo, and fixed air. One hundred parts metallic iron are capable of taking up 66 or 70 of pure air. When 100 parfs iron contain but of this air, the compound is ftill magnetic* SECTION II. 0/ the Formation of Stony Subßances. The flighteft acquaintance with the ftony fub- ftanees that come underour infpeflion is fuffieienc to convince us they were once in a foft or liquid ftate. Many of them have the appearance of having been cryftallized, like falts, in feparating from their folvent whatever that may have been; and thefe cryftals difeover all the variations to which faline cryftals are liable. Many of them inclofe organized fubftances which they could not have admitted but when in a foft or liquid ftate; many of them appear to have been formed round mineral fubftances, asthefe, having received their fhape and form from the operations of art could never have introduced tbemfelves into then?! The bare mention of thefe circumftances is fuf- ficient, as this point has, I believe, never been difputed. The only difficulty that at prefent occurs is to afcertain the nature of that fluid which