Of Volcanos. 39 Every volcano hath tberefore neceffarily an aperture, or rnouth, through which this ficry matter is difcharged; this aperture is called its crater, generally it is placed in the fummit of the mountain, but fometimes on its fides. The aperture on the fummit is, however, moft fre- quently fo called, as it is more commonly open, and is formed of materials that, after havmg been ejedted, have fallen on the borders of the aperture and in fucccffion of time have formed a fort of chimney all around. The lateral aperture is commonly blocked np by the laft poruons of the liquified matter that have flown through it. The crater is always of confiderable extent, o.ten of many miles in circumference. The ejedtive power of volcanos is now gene rally believed to proceed partly from air, paffing from a concrete into an elaftic llare ; partly, and indeed principally, front water and b.tumen, converted into fteam; and often alIo from the decompofition of fteam, the caufe of this decom- pofition is eafily underftood by thofe who are acquainted with the dil'coveries of Mr. Lavoifier and Dr. Prieftiey. The liquified matter that tflues from volcanos is i a general called lava, from the Gothic Upc, or Laufen, to run. The heat comnuinicated by volcanic fire, to the immenfe mai's of materials on which it opcrates, fcarce ever equals 12.0 o Wedgewood, and ftiil more rarcly exceeds that degree; this is evident from its efledts; tor, limcftones are frequentlv ejected from \o canos Uncalcined *, and yet their calcmation requues only a hcat of 120°, and ihorls, which are fuliolo * Dolont. Notes on Bergm. p. 77.